Номер 24, страница 157, часть 1 - гдз по физике 9 класс учебник Генденштейн, Булатова

Авторы: Генденштейн Л. Э., Булатова А. А., Корнильев И. Н., Кошкина А. В.

Тип: Учебник

Издательство: Бином

Год издания: 2016 - 2026

Часть: 1

Цвет обложки: бирюзовый

ISBN: 978-5-9963-3062-1

Часть 1. Механические явления. Глава III. Законы сохранения в механике. Параграф 18. Закон сохранения энергии в механике - номер 24, страница 157.

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                    "field_allowed" => null
                    "field_reserve_field" => null
                    "field_link_to_source" => null
                    "field_tasks_count" => "1111"
                    "field_priority" => null
                    "field_default_folder" => "/fizika_09/gendenshtejn-u/"
                    "field_isbn" => "978-5-9963-3062-1"
                    "field_cover" => array:2 [ …2]
                    "field_cover_alts" => array:2 [ …2]
                    "field_covers" => array:2 [ …2]
                    "field_popular_book" => "0"
                    "field_recommended_books" => Illuminate\Database\Eloquent\Collection {#1104 …2}
                    "field_new_book" => Illuminate\Database\Eloquent\Collection {#1105 …2}
                    "field_old_book" => Illuminate\Database\Eloquent\Collection {#1106 …2}
                    "field_url" => "/9-klass/fizika/gendenshtejn-uchebnik"
                    "field_cover_color" => Illuminate\Database\Eloquent\Collection {#1107 …2}
                    "field_metatags_title" => null
                    "field_metatags_description" => null
                    "field_h1" => null
                    "field_description_top" => null
                    "field_description_bottom" => null
                    "breadcrumbs" => array:3 [ …3]
                  ]
                  #changes: []
                  #casts: []
                  #classCastCache: []
                  #attributeCastCache: []
                  #dateFormat: null
                  #appends: []
                  #dispatchesEvents: []
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                  #relations: []
                  #touches: []
                  +timestamps: true
                  +usesUniqueIds: false
                  #hidden: []
                  #visible: []
                  #fillable: []
                  #guarded: array:1 [
                    0 => "*"
                  ]
                }
              ]
              #escapeWhenCastingToString: false
            }
            "branch_parent" => Illuminate\Database\Eloquent\Collection {#1109
              #items: []
              #escapeWhenCastingToString: false
            }
          ]
          #original: array:24 [
            "id" => 1433206
            "created_at" => "2026-04-10 13:58:26"
            "updated_at" => null
            "field_display_title" => null
            "field_branch_order" => "1"
            "field_url" => null
            "field_branch_type" => Illuminate\Database\Eloquent\Collection {#1076}
            "field_page_start" => "6"
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            "field_display_branch_in_title" => "1"
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            "field_display_branch_page" => "1"
            "field_branch_title_in_content" => "0"
            "field_navigation_title" => null
            "field_metatags_title" => null
            "field_metatags_description" => null
            "field_h1" => null
            "field_description_top" => null
            "field_description_bottom" => null
            "field_branch_cover" => []
            "field_branch_covers" => []
            "book" => Illuminate\Database\Eloquent\Collection {#1077}
            "branch_parent" => Illuminate\Database\Eloquent\Collection {#1109}
          ]
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          #attributeCastCache: []
          #dateFormat: null
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          +timestamps: true
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          #hidden: []
          #visible: []
          #fillable: []
          #guarded: array:1 [
            0 => "*"
          ]
        }
        1 => App\Models\Branch {#1110
          #connection: "mysql"
          #table: "branches"
          #primaryKey: "id"
          #keyType: "int"
          +incrementing: true
          #with: []
          #withCount: []
          +preventsLazyLoading: false
          #perPage: 15
          +exists: true
          +wasRecentlyCreated: false
          #escapeWhenCastingToString: false
          #attributes: array:24 [
            "id" => 1433207
            "created_at" => "2026-04-10 13:58:26"
            "updated_at" => null
            "field_display_title" => "Механические явления"
            "field_branch_order" => null
            "field_url" => null
            "field_branch_type" => Illuminate\Database\Eloquent\Collection {#1111
              #items: []
              #escapeWhenCastingToString: false
            }
            "field_page_start" => "6"
            "field_branch_display" => "0"
            "field_branch_expanded" => "0"
            "field_display_branch_in_title" => "1"
            "field_display_task_interval" => "0"
            "field_display_branch_page" => "1"
            "field_branch_title_in_content" => "0"
            "field_navigation_title" => null
            "field_metatags_title" => null
            "field_metatags_description" => null
            "field_h1" => null
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            "field_description_bottom" => null
            "field_branch_cover" => []
            "field_branch_covers" => []
            "book" => Illuminate\Database\Eloquent\Collection {#1112
              #items: array:1 [
                0 => App\Models\Book {#1113
                  #connection: "mysql"
                  #table: "books"
                  #primaryKey: "id"
                  #keyType: "int"
                  +incrementing: false
                  #with: []
                  #withCount: []
                  +preventsLazyLoading: false
                  #perPage: 15
                  +exists: true
                  +wasRecentlyCreated: false
                  #escapeWhenCastingToString: false
                  #attributes: array:50 [
                    "id" => 2980
                    "created_at" => "2026-04-10 13:58:26"
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                    "field_subject" => Illuminate\Database\Eloquent\Collection {#1114 …2}
                    "field_class" => Illuminate\Database\Eloquent\Collection {#1116 …2}
                    "field_publisher" => Illuminate\Database\Eloquent\Collection {#1118 …2}
                    "field_author" => Illuminate\Database\Eloquent\Collection {#1120 …2}
                    "field_author_foreign" => Illuminate\Database\Eloquent\Collection {#1125 …2}
                    "field_book_type" => Illuminate\Database\Eloquent\Collection {#1126 …2}
                    "field_country" => Illuminate\Database\Eloquent\Collection {#1128 …2}
                    "field_city" => Illuminate\Database\Eloquent\Collection {#1130 …2}
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                    "field_umk" => Illuminate\Database\Eloquent\Collection {#1133 …2}
                    "field_level_of_education" => Illuminate\Database\Eloquent\Collection {#1134 …2}
                    "field_standart_of_education" => Illuminate\Database\Eloquent\Collection {#1135 …2}
                    "field_publication_number" => null
                    "field_publication_type" => Illuminate\Database\Eloquent\Collection {#1136 …2}
                    "field_under_the_edition" => Illuminate\Database\Eloquent\Collection {#1137 …2}
                    "field_under_the_edition_degree" => null
                    "field_cover_description" => null
                    "field_publication_year" => "2016"
                    "field_publication_year_until" => null
                    "field_part" => "1, 2"
                    "field_part_writing" => "Часть"
                    "field_second_foreign_language" => null
                    "field_for_whom" => "Учебное издание"
                    "field_allowed" => null
                    "field_reserve_field" => null
                    "field_link_to_source" => null
                    "field_tasks_count" => "1111"
                    "field_priority" => null
                    "field_default_folder" => "/fizika_09/gendenshtejn-u/"
                    "field_isbn" => "978-5-9963-3062-1"
                    "field_cover" => array:2 [ …2]
                    "field_cover_alts" => array:2 [ …2]
                    "field_covers" => array:2 [ …2]
                    "field_popular_book" => "0"
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                    "field_old_book" => Illuminate\Database\Eloquent\Collection {#1141 …2}
                    "field_url" => "/9-klass/fizika/gendenshtejn-uchebnik"
                    "field_cover_color" => Illuminate\Database\Eloquent\Collection {#1142 …2}
                    "field_metatags_title" => null
                    "field_metatags_description" => null
                    "field_h1" => null
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                    "breadcrumbs" => array:3 [ …3]
                  ]
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                    "id" => 2980
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                    "field_subject" => Illuminate\Database\Eloquent\Collection {#1114 …2}
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                    "field_author" => Illuminate\Database\Eloquent\Collection {#1120 …2}
                    "field_author_foreign" => Illuminate\Database\Eloquent\Collection {#1125 …2}
                    "field_book_type" => Illuminate\Database\Eloquent\Collection {#1126 …2}
                    "field_country" => Illuminate\Database\Eloquent\Collection {#1128 …2}
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                    "field_level_of_education" => Illuminate\Database\Eloquent\Collection {#1134 …2}
                    "field_standart_of_education" => Illuminate\Database\Eloquent\Collection {#1135 …2}
                    "field_publication_number" => null
                    "field_publication_type" => Illuminate\Database\Eloquent\Collection {#1136 …2}
                    "field_under_the_edition" => Illuminate\Database\Eloquent\Collection {#1137 …2}
                    "field_under_the_edition_degree" => null
                    "field_cover_description" => null
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                    "field_part" => "1, 2"
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                    "field_default_folder" => "/fizika_09/gendenshtejn-u/"
                    "field_isbn" => "978-5-9963-3062-1"
                    "field_cover" => array:2 [ …2]
                    "field_cover_alts" => array:2 [ …2]
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                    "field_h1" => null
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                    "field_description_bottom" => null
                    "breadcrumbs" => array:3 [ …3]
                  ]
                  #changes: []
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                  #touches: []
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                    0 => "*"
                  ]
                }
              ]
              #escapeWhenCastingToString: false
            }
            "branch_parent" => Illuminate\Database\Eloquent\Collection {#1144
              #items: array:1 [
                0 => App\Models\Branch {#1145
                  #connection: "mysql"
                  #table: "branches"
                  #primaryKey: "id"
                  #keyType: "int"
                  +incrementing: true
                  #with: []
                  #withCount: []
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                  #perPage: 15
                  +exists: true
                  +wasRecentlyCreated: false
                  #escapeWhenCastingToString: false
                  #attributes: array:24 [
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                    "created_at" => "2026-04-10 13:58:26"
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                    "field_branch_type" => Illuminate\Database\Eloquent\Collection {#1146 …2}
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                    "field_branch_covers" => []
                    "book" => Illuminate\Database\Eloquent\Collection {#1148 …2}
                    "branch_parent" => Illuminate\Database\Eloquent\Collection {#1180 …2}
                  ]
                  #original: array:24 [
                    "id" => 1433206
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                    "updated_at" => null
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                    "field_url" => null
                    "field_branch_type" => Illuminate\Database\Eloquent\Collection {#1146 …2}
                    "field_page_start" => "6"
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                    "field_branch_covers" => []
                    "book" => Illuminate\Database\Eloquent\Collection {#1148 …2}
                    "branch_parent" => Illuminate\Database\Eloquent\Collection {#1180 …2}
                  ]
                  #changes: []
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                  #attributeCastCache: []
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                  #touches: []
                  +timestamps: true
                  +usesUniqueIds: false
                  #hidden: []
                  #visible: []
                  #fillable: []
                  #guarded: array:1 [
                    0 => "*"
                  ]
                }
              ]
              #escapeWhenCastingToString: false
            }
          ]
          #original: array:24 [
            "id" => 1433207
            "created_at" => "2026-04-10 13:58:26"
            "updated_at" => null
            "field_display_title" => "Механические явления"
            "field_branch_order" => null
            "field_url" => null
            "field_branch_type" => Illuminate\Database\Eloquent\Collection {#1111}
            "field_page_start" => "6"
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            "field_branch_covers" => []
            "book" => Illuminate\Database\Eloquent\Collection {#1112}
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          ]
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          #attributeCastCache: []
          #dateFormat: null
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          #touches: []
          +timestamps: true
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          #hidden: []
          #visible: []
          #fillable: []
          #guarded: array:1 [
            0 => "*"
          ]
        }
        2 => App\Models\Branch {#1181
          #connection: "mysql"
          #table: "branches"
          #primaryKey: "id"
          #keyType: "int"
          +incrementing: true
          #with: []
          #withCount: []
          +preventsLazyLoading: false
          #perPage: 15
          +exists: true
          +wasRecentlyCreated: false
          #escapeWhenCastingToString: false
          #attributes: array:24 [
            "id" => 1433223
            "created_at" => "2026-04-10 13:58:26"
            "updated_at" => null
            "field_display_title" => "Законы сохранения в механике"
            "field_branch_order" => "III"
            "field_url" => null
            "field_branch_type" => Illuminate\Database\Eloquent\Collection {#1182
              #items: array:1 [
                0 => App\Models\Term {#1183
                  #connection: "mysql"
                  #table: "terms"
                  #primaryKey: "id"
                  #keyType: "int"
                  +incrementing: false
                  #with: []
                  #withCount: []
                  +preventsLazyLoading: false
                  #perPage: 15
                  +exists: true
                  +wasRecentlyCreated: false
                  #escapeWhenCastingToString: false
                  #attributes: array:7 [
                    "id" => 29
                    "created_at" => "2026-04-10 13:58:26"
                    "updated_at" => null
                    "name" => "Глава"
                    "field_cases" => array:6 [ …6]
                    "field_page_check_not_needed" => null
                    "field_short_name" => null
                  ]
                  #original: array:7 [
                    "id" => 29
                    "created_at" => "2026-04-10 13:58:26"
                    "updated_at" => null
                    "name" => "Глава"
                    "field_cases" => array:6 [ …6]
                    "field_page_check_not_needed" => null
                    "field_short_name" => null
                  ]
                  #changes: []
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                  #attributeCastCache: []
                  #dateFormat: null
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                  #touches: []
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                  #hidden: []
                  #visible: []
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                  #guarded: array:1 [
                    0 => "*"
                  ]
                }
              ]
              #escapeWhenCastingToString: false
            }
            "field_page_start" => "118"
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            "field_branch_title_in_content" => "0"
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            "field_metatags_description" => null
            "field_h1" => null
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            "field_description_bottom" => null
            "field_branch_cover" => []
            "field_branch_covers" => []
            "book" => Illuminate\Database\Eloquent\Collection {#1184
              #items: array:1 [
                0 => App\Models\Book {#1185
                  #connection: "mysql"
                  #table: "books"
                  #primaryKey: "id"
                  #keyType: "int"
                  +incrementing: false
                  #with: []
                  #withCount: []
                  +preventsLazyLoading: false
                  #perPage: 15
                  +exists: true
                  +wasRecentlyCreated: false
                  #escapeWhenCastingToString: false
                  #attributes: array:50 [
                    "id" => 2980
                    "created_at" => "2026-04-10 13:58:26"
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                    "field_subject" => Illuminate\Database\Eloquent\Collection {#1186 …2}
                    "field_class" => Illuminate\Database\Eloquent\Collection {#1188 …2}
                    "field_publisher" => Illuminate\Database\Eloquent\Collection {#1190 …2}
                    "field_author" => Illuminate\Database\Eloquent\Collection {#1192 …2}
                    "field_author_foreign" => Illuminate\Database\Eloquent\Collection {#1197 …2}
                    "field_book_type" => Illuminate\Database\Eloquent\Collection {#1198 …2}
                    "field_country" => Illuminate\Database\Eloquent\Collection {#1200 …2}
                    "field_city" => Illuminate\Database\Eloquent\Collection {#1202 …2}
                    "field_series" => Illuminate\Database\Eloquent\Collection {#1204 …2}
                    "field_umk" => Illuminate\Database\Eloquent\Collection {#1205 …2}
                    "field_level_of_education" => Illuminate\Database\Eloquent\Collection {#1206 …2}
                    "field_standart_of_education" => Illuminate\Database\Eloquent\Collection {#1207 …2}
                    "field_publication_number" => null
                    "field_publication_type" => Illuminate\Database\Eloquent\Collection {#1208 …2}
                    "field_under_the_edition" => Illuminate\Database\Eloquent\Collection {#1209 …2}
                    "field_under_the_edition_degree" => null
                    "field_cover_description" => null
                    "field_publication_year" => "2016"
                    "field_publication_year_until" => null
                    "field_part" => "1, 2"
                    "field_part_writing" => "Часть"
                    "field_second_foreign_language" => null
                    "field_for_whom" => "Учебное издание"
                    "field_allowed" => null
                    "field_reserve_field" => null
                    "field_link_to_source" => null
                    "field_tasks_count" => "1111"
                    "field_priority" => null
                    "field_default_folder" => "/fizika_09/gendenshtejn-u/"
                    "field_isbn" => "978-5-9963-3062-1"
                    "field_cover" => array:2 [ …2]
                    "field_cover_alts" => array:2 [ …2]
                    "field_covers" => array:2 [ …2]
                    "field_popular_book" => "0"
                    "field_recommended_books" => Illuminate\Database\Eloquent\Collection {#1211 …2}
                    "field_new_book" => Illuminate\Database\Eloquent\Collection {#1212 …2}
                    "field_old_book" => Illuminate\Database\Eloquent\Collection {#1213 …2}
                    "field_url" => "/9-klass/fizika/gendenshtejn-uchebnik"
                    "field_cover_color" => Illuminate\Database\Eloquent\Collection {#1214 …2}
                    "field_metatags_title" => null
                    "field_metatags_description" => null
                    "field_h1" => null
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                    "breadcrumbs" => array:3 [ …3]
                  ]
                  #original: array:50 [
                    "id" => 2980
                    "created_at" => "2026-04-10 13:58:26"
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                    "field_subject" => Illuminate\Database\Eloquent\Collection {#1186 …2}
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                    "field_publisher" => Illuminate\Database\Eloquent\Collection {#1190 …2}
                    "field_author" => Illuminate\Database\Eloquent\Collection {#1192 …2}
                    "field_author_foreign" => Illuminate\Database\Eloquent\Collection {#1197 …2}
                    "field_book_type" => Illuminate\Database\Eloquent\Collection {#1198 …2}
                    "field_country" => Illuminate\Database\Eloquent\Collection {#1200 …2}
                    "field_city" => Illuminate\Database\Eloquent\Collection {#1202 …2}
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    "content" => Illuminate\Database\Eloquent\Collection {#1370
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            "edition" => Illuminate\Database\Eloquent\Collection {#1389
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                0 => App\Models\Edition {#1381
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            "task" => array:2 [
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            "text" => "<p><strong>24.</strong> На рисунке 18.7 изображён график зависимости потенциальной энергии свободно падающего без начальной скорости тела массой 200 г.</p><p>Рис. 18.7</p><p><strong>а)</strong> С какой высоты падало тело?</p><p><strong>б)</strong> Чему равна полная механическая энергия тела через 1,5 с после начала падения?</p><p><strong>в)</strong> Чему равны кинетическая и потенциальная энергия тела через 1,5 с после начала падения?</p><p><strong>г)</strong> На какой высоте находилось тело через 1,5 с после начала падения?</p><p><strong>д)</strong> Чему равна скорость тела через 1,5 с после начала падения?</p>"
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            "task" => array:2 [
              "refs" => "1433809"
              "type" => "task"
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            "text" => "<p><strong>24.</strong> На рисунке 18.7 изображён график зависимости потенциальной энергии свободно падающего без начальной скорости тела массой 200 г.</p><p>Рис. 18.7</p><p><strong>а)</strong> С какой высоты падало тело?</p><p><strong>б)</strong> Чему равна полная механическая энергия тела через 1,5 с после начала падения?</p><p><strong>в)</strong> Чему равны кинетическая и потенциальная энергия тела через 1,5 с после начала падения?</p><p><strong>г)</strong> На какой высоте находилось тело через 1,5 с после начала падения?</p><p><strong>д)</strong> Чему равна скорость тела через 1,5 с после начала падения?</p>"
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            "text" => "<p><strong>Дано:</strong></p><p>$m = 200 \text{ г} = 0.2 \text{ кг}$</p><p>$v_0 = 0 \text{ м/с}$ (свободное падение без начальной скорости)</p><p>Из графика: начальная потенциальная энергия $E_{p0} = 90 \text{ Дж}$ при $t_0 = 0 \text{ с}$</p><p>Ускорение свободного падения $g \approx 10 \text{ м/с}^2$</p><p><strong>Найти:</strong></p><p>а) $\text{H}$ - ?</p><p>б) $\text{E}$ через 1,5 с - ?</p><p>в) $E_k$ и $E_p$ через 1,5 с - ?</p><p>г) $\text{h}$ через 1,5 с - ?</p><p>д) $\text{v}$ через 1,5 с - ?</p><p><strong>Решение:</strong></p><p><strong>а) С какой высоты падало тело?</strong><br>В начальный момент времени ($t=0$) тело покоилось на максимальной высоте $\text{H}$. Вся его механическая энергия была представлена потенциальной энергией. Формула потенциальной энергии: $E_p = mgh$. В начальный момент времени: $E_{p0} = mgH$. Отсюда можем выразить начальную высоту $\text{H}$:<br>$H = \frac{E_{p0}}{mg} = \frac{90 \text{ Дж}}{0.2 \text{ кг} \cdot 10 \text{ м/с}^2} = \frac{90}{2} = 45 \text{ м}$.</p><p><strong>Ответ:</strong> тело падало с высоты 45 м.</p><p><strong>б) Чему равна полная механическая энергия тела через 1,5 с после начала падения?</strong><br>Так как тело падает свободно и сопротивление воздуха не учитывается, действует закон сохранения полной механической энергии. Это означает, что полная механическая энергия тела в любой момент времени остается постоянной и равной ее значению в начальный момент. В начальный момент времени кинетическая энергия $E_{k0} = 0$ (тело покоилось), а потенциальная энергия $E_{p0} = 90 \text{ Дж}$. Полная механическая энергия $E = E_{k0} + E_{p0} = 0 + 90 \text{ Дж} = 90 \text{ Дж}$. Эта величина не изменяется со временем.</p><p><strong>Ответ:</strong> полная механическая энергия через 1,5 с равна 90 Дж.</p><p><strong>в) Чему равны кинетическая и потенциальная энергия тела через 1,5 с после начала падения?</strong><br>Найдем высоту тела $\text{h}$ через 1,5 с после начала падения. При свободном падении без начальной скорости высота изменяется по закону: $h(t) = H - \frac{gt^2}{2}$.<br>$h(1.5) = 45 \text{ м} - \frac{10 \text{ м/с}^2 \cdot (1.5 \text{ с})^2}{2} = 45 - \frac{10 \cdot 2.25}{2} = 45 - 11.25 = 33.75 \text{ м}$.<br>Теперь можем найти потенциальную энергию в этот момент времени:<br>$E_p(1.5) = mgh(1.5) = 0.2 \text{ кг} \cdot 10 \text{ м/с}^2 \cdot 33.75 \text{ м} = 2 \cdot 33.75 = 67.5 \text{ Дж}$.<br>Кинетическую энергию найдем из закона сохранения энергии: $E = E_k + E_p$.<br>$E_k(1.5) = E - E_p(1.5) = 90 \text{ Дж} - 67.5 \text{ Дж} = 22.5 \text{ Дж}$.</p><p><strong>Ответ:</strong> потенциальная энергия равна 67,5 Дж, кинетическая энергия равна 22,5 Дж.</p><p><strong>г) На какой высоте находилось тело через 1,5 с после начала падения?</strong><br>Высоту тела в этот момент времени мы уже рассчитали в предыдущем пункте. $h(t) = H - \frac{gt^2}{2}$<br>$h(1.5) = 45 \text{ м} - \frac{10 \text{ м/с}^2 \cdot (1.5 \text{ с})^2}{2} = 33.75 \text{ м}$.<br>Также можно найти высоту, зная потенциальную энергию $E_p(1.5) = 67.5 \text{ Дж}$:<br>$h(1.5) = \frac{E_p(1.5)}{mg} = \frac{67.5 \text{ Дж}}{0.2 \text{ кг} \cdot 10 \text{ м/с}^2} = \frac{67.5}{2} = 33.75 \text{ м}$.</p><p><strong>Ответ:</strong> тело находилось на высоте 33,75 м.</p><p><strong>д) Чему равна скорость тела через 1,5 с после начала падения?</strong><br>Скорость тела можно найти, зная его кинетическую энергию $E_k(1.5) = 22.5 \text{ Дж}$.<br>Формула кинетической энергии: $E_k = \frac{mv^2}{2}$.<br>Выразим скорость $\text{v}$: $v = \sqrt{\frac{2E_k}{m}}$.<br>$v(1.5) = \sqrt{\frac{2 \cdot 22.5 \text{ Дж}}{0.2 \text{ кг}}} = \sqrt{\frac{45}{0.2}} = \sqrt{225} = 15 \text{ м/с}$.<br>Проверим результат по формуле скорости для свободного падения: $v(t) = gt$.<br>$v(1.5) = 10 \text{ м/с}^2 \cdot 1.5 \text{ с} = 15 \text{ м/с}$. Результаты совпадают.</p><p><strong>Ответ:</strong> скорость тела равна 15 м/с.</p>"
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            "text" => "<p><strong>Дано:</strong></p><p>$m = 200 \text{ г} = 0.2 \text{ кг}$</p><p>$v_0 = 0 \text{ м/с}$ (свободное падение без начальной скорости)</p><p>Из графика: начальная потенциальная энергия $E_{p0} = 90 \text{ Дж}$ при $t_0 = 0 \text{ с}$</p><p>Ускорение свободного падения $g \approx 10 \text{ м/с}^2$</p><p><strong>Найти:</strong></p><p>а) $\text{H}$ - ?</p><p>б) $\text{E}$ через 1,5 с - ?</p><p>в) $E_k$ и $E_p$ через 1,5 с - ?</p><p>г) $\text{h}$ через 1,5 с - ?</p><p>д) $\text{v}$ через 1,5 с - ?</p><p><strong>Решение:</strong></p><p><strong>а) С какой высоты падало тело?</strong><br>В начальный момент времени ($t=0$) тело покоилось на максимальной высоте $\text{H}$. Вся его механическая энергия была представлена потенциальной энергией. Формула потенциальной энергии: $E_p = mgh$. В начальный момент времени: $E_{p0} = mgH$. Отсюда можем выразить начальную высоту $\text{H}$:<br>$H = \frac{E_{p0}}{mg} = \frac{90 \text{ Дж}}{0.2 \text{ кг} \cdot 10 \text{ м/с}^2} = \frac{90}{2} = 45 \text{ м}$.</p><p><strong>Ответ:</strong> тело падало с высоты 45 м.</p><p><strong>б) Чему равна полная механическая энергия тела через 1,5 с после начала падения?</strong><br>Так как тело падает свободно и сопротивление воздуха не учитывается, действует закон сохранения полной механической энергии. Это означает, что полная механическая энергия тела в любой момент времени остается постоянной и равной ее значению в начальный момент. В начальный момент времени кинетическая энергия $E_{k0} = 0$ (тело покоилось), а потенциальная энергия $E_{p0} = 90 \text{ Дж}$. Полная механическая энергия $E = E_{k0} + E_{p0} = 0 + 90 \text{ Дж} = 90 \text{ Дж}$. Эта величина не изменяется со временем.</p><p><strong>Ответ:</strong> полная механическая энергия через 1,5 с равна 90 Дж.</p><p><strong>в) Чему равны кинетическая и потенциальная энергия тела через 1,5 с после начала падения?</strong><br>Найдем высоту тела $\text{h}$ через 1,5 с после начала падения. При свободном падении без начальной скорости высота изменяется по закону: $h(t) = H - \frac{gt^2}{2}$.<br>$h(1.5) = 45 \text{ м} - \frac{10 \text{ м/с}^2 \cdot (1.5 \text{ с})^2}{2} = 45 - \frac{10 \cdot 2.25}{2} = 45 - 11.25 = 33.75 \text{ м}$.<br>Теперь можем найти потенциальную энергию в этот момент времени:<br>$E_p(1.5) = mgh(1.5) = 0.2 \text{ кг} \cdot 10 \text{ м/с}^2 \cdot 33.75 \text{ м} = 2 \cdot 33.75 = 67.5 \text{ Дж}$.<br>Кинетическую энергию найдем из закона сохранения энергии: $E = E_k + E_p$.<br>$E_k(1.5) = E - E_p(1.5) = 90 \text{ Дж} - 67.5 \text{ Дж} = 22.5 \text{ Дж}$.</p><p><strong>Ответ:</strong> потенциальная энергия равна 67,5 Дж, кинетическая энергия равна 22,5 Дж.</p><p><strong>г) На какой высоте находилось тело через 1,5 с после начала падения?</strong><br>Высоту тела в этот момент времени мы уже рассчитали в предыдущем пункте. $h(t) = H - \frac{gt^2}{2}$<br>$h(1.5) = 45 \text{ м} - \frac{10 \text{ м/с}^2 \cdot (1.5 \text{ с})^2}{2} = 33.75 \text{ м}$.<br>Также можно найти высоту, зная потенциальную энергию $E_p(1.5) = 67.5 \text{ Дж}$:<br>$h(1.5) = \frac{E_p(1.5)}{mg} = \frac{67.5 \text{ Дж}}{0.2 \text{ кг} \cdot 10 \text{ м/с}^2} = \frac{67.5}{2} = 33.75 \text{ м}$.</p><p><strong>Ответ:</strong> тело находилось на высоте 33,75 м.</p><p><strong>д) Чему равна скорость тела через 1,5 с после начала падения?</strong><br>Скорость тела можно найти, зная его кинетическую энергию $E_k(1.5) = 22.5 \text{ Дж}$.<br>Формула кинетической энергии: $E_k = \frac{mv^2}{2}$.<br>Выразим скорость $\text{v}$: $v = \sqrt{\frac{2E_k}{m}}$.<br>$v(1.5) = \sqrt{\frac{2 \cdot 22.5 \text{ Дж}}{0.2 \text{ кг}}} = \sqrt{\frac{45}{0.2}} = \sqrt{225} = 15 \text{ м/с}$.<br>Проверим результат по формуле скорости для свободного падения: $v(t) = gt$.<br>$v(1.5) = 10 \text{ м/с}^2 \cdot 1.5 \text{ с} = 15 \text{ м/с}$. Результаты совпадают.</p><p><strong>Ответ:</strong> скорость тела равна 15 м/с.</p>"
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№24 (с. 157)
Условие. №24 (с. 157)
скриншот условия
Физика, 9 класс Учебник, авторы: Генденштейн Лев Элевич, Булатова Альбина Александрова, Корнильев Игорь Николаевич, Кошкина Анжелика Васильевна, издательство Бином, Москва, 2016, бирюзового цвета, Часть 1, страница 157, номер 24, Условие
Физика, 9 класс Учебник, авторы: Генденштейн Лев Элевич, Булатова Альбина Александрова, Корнильев Игорь Николаевич, Кошкина Анжелика Васильевна, издательство Бином, Москва, 2016, бирюзового цвета, Часть 1, страница 157, номер 24, Условие (продолжение 2)

24. На рисунке 18.7 изображён график зависимости потенциальной энергии свободно падающего без начальной скорости тела массой 200 г.

Рис. 18.7

а) С какой высоты падало тело?

б) Чему равна полная механическая энергия тела через 1,5 с после начала падения?

в) Чему равны кинетическая и потенциальная энергия тела через 1,5 с после начала падения?

г) На какой высоте находилось тело через 1,5 с после начала падения?

д) Чему равна скорость тела через 1,5 с после начала падения?

Решение. №24 (с. 157)

Дано:

$m = 200 \text{ г} = 0.2 \text{ кг}$

$v_0 = 0 \text{ м/с}$ (свободное падение без начальной скорости)

Из графика: начальная потенциальная энергия $E_{p0} = 90 \text{ Дж}$ при $t_0 = 0 \text{ с}$

Ускорение свободного падения $g \approx 10 \text{ м/с}^2$

Найти:

а) $\text{H}$ - ?

б) $\text{E}$ через 1,5 с - ?

в) $E_k$ и $E_p$ через 1,5 с - ?

г) $\text{h}$ через 1,5 с - ?

д) $\text{v}$ через 1,5 с - ?

Решение:

а) С какой высоты падало тело?
В начальный момент времени ($t=0$) тело покоилось на максимальной высоте $\text{H}$. Вся его механическая энергия была представлена потенциальной энергией. Формула потенциальной энергии: $E_p = mgh$. В начальный момент времени: $E_{p0} = mgH$. Отсюда можем выразить начальную высоту $\text{H}$:
$H = \frac{E_{p0}}{mg} = \frac{90 \text{ Дж}}{0.2 \text{ кг} \cdot 10 \text{ м/с}^2} = \frac{90}{2} = 45 \text{ м}$.

Ответ: тело падало с высоты 45 м.

б) Чему равна полная механическая энергия тела через 1,5 с после начала падения?
Так как тело падает свободно и сопротивление воздуха не учитывается, действует закон сохранения полной механической энергии. Это означает, что полная механическая энергия тела в любой момент времени остается постоянной и равной ее значению в начальный момент. В начальный момент времени кинетическая энергия $E_{k0} = 0$ (тело покоилось), а потенциальная энергия $E_{p0} = 90 \text{ Дж}$. Полная механическая энергия $E = E_{k0} + E_{p0} = 0 + 90 \text{ Дж} = 90 \text{ Дж}$. Эта величина не изменяется со временем.

Ответ: полная механическая энергия через 1,5 с равна 90 Дж.

в) Чему равны кинетическая и потенциальная энергия тела через 1,5 с после начала падения?
Найдем высоту тела $\text{h}$ через 1,5 с после начала падения. При свободном падении без начальной скорости высота изменяется по закону: $h(t) = H - \frac{gt^2}{2}$.
$h(1.5) = 45 \text{ м} - \frac{10 \text{ м/с}^2 \cdot (1.5 \text{ с})^2}{2} = 45 - \frac{10 \cdot 2.25}{2} = 45 - 11.25 = 33.75 \text{ м}$.
Теперь можем найти потенциальную энергию в этот момент времени:
$E_p(1.5) = mgh(1.5) = 0.2 \text{ кг} \cdot 10 \text{ м/с}^2 \cdot 33.75 \text{ м} = 2 \cdot 33.75 = 67.5 \text{ Дж}$.
Кинетическую энергию найдем из закона сохранения энергии: $E = E_k + E_p$.
$E_k(1.5) = E - E_p(1.5) = 90 \text{ Дж} - 67.5 \text{ Дж} = 22.5 \text{ Дж}$.

Ответ: потенциальная энергия равна 67,5 Дж, кинетическая энергия равна 22,5 Дж.

г) На какой высоте находилось тело через 1,5 с после начала падения?
Высоту тела в этот момент времени мы уже рассчитали в предыдущем пункте. $h(t) = H - \frac{gt^2}{2}$
$h(1.5) = 45 \text{ м} - \frac{10 \text{ м/с}^2 \cdot (1.5 \text{ с})^2}{2} = 33.75 \text{ м}$.
Также можно найти высоту, зная потенциальную энергию $E_p(1.5) = 67.5 \text{ Дж}$:
$h(1.5) = \frac{E_p(1.5)}{mg} = \frac{67.5 \text{ Дж}}{0.2 \text{ кг} \cdot 10 \text{ м/с}^2} = \frac{67.5}{2} = 33.75 \text{ м}$.

Ответ: тело находилось на высоте 33,75 м.

д) Чему равна скорость тела через 1,5 с после начала падения?
Скорость тела можно найти, зная его кинетическую энергию $E_k(1.5) = 22.5 \text{ Дж}$.
Формула кинетической энергии: $E_k = \frac{mv^2}{2}$.
Выразим скорость $\text{v}$: $v = \sqrt{\frac{2E_k}{m}}$.
$v(1.5) = \sqrt{\frac{2 \cdot 22.5 \text{ Дж}}{0.2 \text{ кг}}} = \sqrt{\frac{45}{0.2}} = \sqrt{225} = 15 \text{ м/с}$.
Проверим результат по формуле скорости для свободного падения: $v(t) = gt$.
$v(1.5) = 10 \text{ м/с}^2 \cdot 1.5 \text{ с} = 15 \text{ м/с}$. Результаты совпадают.

Ответ: скорость тела равна 15 м/с.

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