Номер 4, страница 99, 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 II. Динамика. Параграф 11. Силы трения - номер 4, страница 99.

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                    "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: []
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                  #attributeCastCache: []
                  #dateFormat: null
                  #appends: []
                  #dispatchesEvents: []
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                  #touches: []
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                  #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_h1" => null
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            "field_branch_cover" => []
            "field_branch_covers" => []
            "book" => Illuminate\Database\Eloquent\Collection {#1077}
            "branch_parent" => Illuminate\Database\Eloquent\Collection {#1109}
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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
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            "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}
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                    "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_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"
                    "field_part_writing" => "Часть"
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                    "field_for_whom" => "Учебное издание"
                    "field_allowed" => null
                    "field_reserve_field" => null
                    "field_link_to_source" => null
                    "field_tasks_count" => "1111"
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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]
                    "field_covers" => array:2 [ …2]
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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}
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                    "breadcrumbs" => array:3 [ …3]
                  ]
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                    "field_subject" => Illuminate\Database\Eloquent\Collection {#1114 …2}
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                    "field_author_foreign" => Illuminate\Database\Eloquent\Collection {#1125 …2}
                    "field_book_type" => Illuminate\Database\Eloquent\Collection {#1126 …2}
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                    "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
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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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                    "breadcrumbs" => array:3 [ …3]
                  ]
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                    0 => "*"
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                }
              ]
              #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: []
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                  #perPage: 15
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                  #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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                    "book" => Illuminate\Database\Eloquent\Collection {#1148 …2}
                    "branch_parent" => Illuminate\Database\Eloquent\Collection {#1180 …2}
                  ]
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                    "field_branch_type" => Illuminate\Database\Eloquent\Collection {#1146 …2}
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                    "book" => Illuminate\Database\Eloquent\Collection {#1148 …2}
                    "branch_parent" => Illuminate\Database\Eloquent\Collection {#1180 …2}
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                  #changes: []
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                  +usesUniqueIds: false
                  #hidden: []
                  #visible: []
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                  #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}
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            "field_branch_covers" => []
            "book" => Illuminate\Database\Eloquent\Collection {#1112}
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          #hidden: []
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          #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" => 1433214
            "created_at" => "2026-04-10 13:58:26"
            "updated_at" => null
            "field_display_title" => "Динамика"
            "field_branch_order" => "II"
            "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: []
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                    0 => "*"
                  ]
                }
              ]
              #escapeWhenCastingToString: false
            }
            "field_page_start" => "59"
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            "field_navigation_title" => null
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            "field_metatags_description" => null
            "field_h1" => null
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            "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}
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                    "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
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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}
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                    "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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                    "field_branch_type" => Illuminate\Database\Eloquent\Collection {#1218 …2}
                    "field_page_start" => "6"
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        3 => App\Models\Branch {#1288
          #connection: "mysql"
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            "id" => 1433220
            "created_at" => "2026-04-10 13:58:26"
            "updated_at" => null
            "field_display_title" => "Силы трения"
            "field_branch_order" => "11"
            "field_url" => null
            "field_branch_type" => Illuminate\Database\Eloquent\Collection {#1289
              #items: array:1 [
                0 => App\Models\Term {#1290
                  #connection: "mysql"
                  #table: "terms"
                  #primaryKey: "id"
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                  #attributes: array:7 [ …7]
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            "field_page_start" => "96"
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            "book" => Illuminate\Database\Eloquent\Collection {#1291
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                0 => App\Models\Book {#1292
                  #connection: "mysql"
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                  #escapeWhenCastingToString: false
                  #attributes: array:50 [ …50]
                  #original: array:50 [ …50]
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                  #guarded: array:1 [ …1]
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            "branch_parent" => Illuminate\Database\Eloquent\Collection {#1323
              #items: array:1 [
                0 => App\Models\Branch {#1324
                  #connection: "mysql"
                  #table: "branches"
                  #primaryKey: "id"
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                  #attributes: array:24 [ …24]
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            "field_display_title" => "Силы трения"
            "field_branch_order" => "11"
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            "field_branch_type" => Illuminate\Database\Eloquent\Collection {#1289}
            "field_page_start" => "96"
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    "content" => Illuminate\Database\Eloquent\Collection {#1432
      #items: array:2 [
        0 => App\Models\Element {#1442
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            "id" => 1696360
            "created_at" => "2026-04-10 13:58:26"
            "updated_at" => null
            "edition" => Illuminate\Database\Eloquent\Collection {#1451
              #items: array:1 [
                0 => App\Models\Edition {#1443
                  #connection: "mysql"
                  #table: "editions"
                  #primaryKey: "id"
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                  +incrementing: false
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                  #attributes: array:21 [ …21]
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            "task" => array:2 [
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            "text" => "<p><strong>4.</strong> Лежащему на столе бруску сообщили скорость 2 м/с, после чего он скользил по столу до остановки 80 см.</p><p><strong>а)</strong> С каким по модулю ускорением двигался брусок?</p><p><strong>б)</strong> Чему равен коэффициент трения между бруском и столом?</p>"
            "img" => array:1 [
              0 => array:5 [
                "name" => "4-1.jpg"
                "alt" => null
                "width" => "1556"
                "height" => 319
                "path" => "/media/fizika_09/gendenshtejn-u/0-11/4-1.webp?ts=1759348229"
              ]
            ]
          ]
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            "id" => 1696360
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            "edition" => Illuminate\Database\Eloquent\Collection {#1451}
            "task" => array:2 [
              "refs" => "1433619"
              "type" => "task"
            ]
            "text" => "<p><strong>4.</strong> Лежащему на столе бруску сообщили скорость 2 м/с, после чего он скользил по столу до остановки 80 см.</p><p><strong>а)</strong> С каким по модулю ускорением двигался брусок?</p><p><strong>б)</strong> Чему равен коэффициент трения между бруском и столом?</p>"
            "img" => array:1 [
              0 => array:5 [
                "name" => "4-1.jpg"
                "alt" => null
                "width" => "1556"
                "height" => 319
                "path" => "/media/fizika_09/gendenshtejn-u/0-11/4-1.webp?ts=1759348229"
              ]
            ]
          ]
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          #guarded: array:1 [
            0 => "*"
          ]
        }
        1 => App\Models\Element {#1449
          #connection: "mysql"
          #table: "elements"
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          +incrementing: true
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            "id" => 1780430
            "created_at" => "2026-04-10 13:58:26"
            "updated_at" => null
            "edition" => Illuminate\Database\Eloquent\Collection {#1459
              #items: array:1 [
                0 => App\Models\Edition {#1450
                  #connection: "mysql"
                  #table: "editions"
                  #primaryKey: "id"
                  #keyType: "int"
                  +incrementing: false
                  #with: []
                  #withCount: []
                  +preventsLazyLoading: false
                  #perPage: 15
                  +exists: true
                  +wasRecentlyCreated: false
                  #escapeWhenCastingToString: false
                  #attributes: array:21 [ …21]
                  #original: array:21 [ …21]
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            "task" => array:2 [
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            "text" => "<p><strong>Дано:</strong></p><p>Начальная скорость, $v_0 = 2$ м/с</p><p>Конечная скорость, $v = 0$ м/с (так как брусок остановился)</p><p>Пройденный путь, $S = 80$ см</p><p><i>Перевод в систему СИ:</i></p><p>$S = 80 \text{ см} = 0.8 \text{ м}$</p><p><strong>Найти:</strong></p><p>а) Модуль ускорения, $a - ?$</p><p>б) Коэффициент трения, $\mu - ?$</p><p><strong>Решение:</strong></p><p><strong>а) С каким по модулю ускорением двигался брусок?</strong></p><p>Поскольку движение бруска является равнозамедленным, мы можем использовать формулу для пути, не зависящую от времени:</p><p>$S = \frac{v^2 - v_0^2}{2a}$</p><p>Из этой формулы выразим ускорение $\text{a}$:</p><p>$a = \frac{v^2 - v_0^2}{2S}$</p><p>Теперь подставим числовые значения из условия задачи:</p><p>$a = \frac{(0 \text{ м/с})^2 - (2 \text{ м/с})^2}{2 \cdot 0.8 \text{ м}} = \frac{-4 \text{ м}^2/\text{с}^2}{1.6 \text{ м}} = -2.5 \text{ м/с}^2$</p><p>Знак "минус" указывает на то, что ускорение направлено против начальной скорости, то есть движение является замедленным. По условию задачи требуется найти модуль ускорения.</p><p>$|a| = 2.5 \text{ м/с}^2$</p><p><strong>Ответ:</strong> модуль ускорения бруска равен $2.5 \text{ м/с}^2$.</p><p><strong>б) Чему равен коэффициент трения между бруском и столом?</strong></p><p>На брусок в горизонтальном направлении действует только сила трения скольжения $F_{тр}$, которая и сообщает ему отрицательное ускорение. Запишем второй закон Ньютона для бруска в проекции на горизонтальную ось, направленную по движению:</p><p>$-F_{тр} = ma$</p><p>Сила трения скольжения вычисляется по формуле $F_{тр} = \mu N$, где $\mu$ — коэффициент трения, а $\text{N}$ — сила нормальной реакции опоры.</p><p>В вертикальном направлении сила тяжести $mg$ уравновешивается силой нормальной реакции опоры $\text{N}$, так как вертикального движения нет. Следовательно, $N = mg$.</p><p>Подставим выражения для сил во второй закон Ньютона:</p><p>$-\mu mg = ma$</p><p>Сократим массу $\text{m}$ в обеих частях уравнения:</p><p>$-\mu g = a$</p><p>Отсюда выразим коэффициент трения $\mu$:</p><p>$\mu = -\frac{a}{g}$</p><p>Подставим значение ускорения $\text{a}$ из пункта а) и примем ускорение свободного падения $g \approx 10 \text{ м/с}^2$:</p><p>$\mu = -\frac{-2.5 \text{ м/с}^2}{10 \text{ м/с}^2} = 0.25$</p><p><strong>Ответ:</strong> коэффициент трения между бруском и столом равен $0.25$.</p>"
          ]
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            "edition" => Illuminate\Database\Eloquent\Collection {#1459}
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            "text" => "<p><strong>Дано:</strong></p><p>Начальная скорость, $v_0 = 2$ м/с</p><p>Конечная скорость, $v = 0$ м/с (так как брусок остановился)</p><p>Пройденный путь, $S = 80$ см</p><p><i>Перевод в систему СИ:</i></p><p>$S = 80 \text{ см} = 0.8 \text{ м}$</p><p><strong>Найти:</strong></p><p>а) Модуль ускорения, $a - ?$</p><p>б) Коэффициент трения, $\mu - ?$</p><p><strong>Решение:</strong></p><p><strong>а) С каким по модулю ускорением двигался брусок?</strong></p><p>Поскольку движение бруска является равнозамедленным, мы можем использовать формулу для пути, не зависящую от времени:</p><p>$S = \frac{v^2 - v_0^2}{2a}$</p><p>Из этой формулы выразим ускорение $\text{a}$:</p><p>$a = \frac{v^2 - v_0^2}{2S}$</p><p>Теперь подставим числовые значения из условия задачи:</p><p>$a = \frac{(0 \text{ м/с})^2 - (2 \text{ м/с})^2}{2 \cdot 0.8 \text{ м}} = \frac{-4 \text{ м}^2/\text{с}^2}{1.6 \text{ м}} = -2.5 \text{ м/с}^2$</p><p>Знак "минус" указывает на то, что ускорение направлено против начальной скорости, то есть движение является замедленным. По условию задачи требуется найти модуль ускорения.</p><p>$|a| = 2.5 \text{ м/с}^2$</p><p><strong>Ответ:</strong> модуль ускорения бруска равен $2.5 \text{ м/с}^2$.</p><p><strong>б) Чему равен коэффициент трения между бруском и столом?</strong></p><p>На брусок в горизонтальном направлении действует только сила трения скольжения $F_{тр}$, которая и сообщает ему отрицательное ускорение. Запишем второй закон Ньютона для бруска в проекции на горизонтальную ось, направленную по движению:</p><p>$-F_{тр} = ma$</p><p>Сила трения скольжения вычисляется по формуле $F_{тр} = \mu N$, где $\mu$ — коэффициент трения, а $\text{N}$ — сила нормальной реакции опоры.</p><p>В вертикальном направлении сила тяжести $mg$ уравновешивается силой нормальной реакции опоры $\text{N}$, так как вертикального движения нет. Следовательно, $N = mg$.</p><p>Подставим выражения для сил во второй закон Ньютона:</p><p>$-\mu mg = ma$</p><p>Сократим массу $\text{m}$ в обеих частях уравнения:</p><p>$-\mu g = a$</p><p>Отсюда выразим коэффициент трения $\mu$:</p><p>$\mu = -\frac{a}{g}$</p><p>Подставим значение ускорения $\text{a}$ из пункта а) и примем ускорение свободного падения $g \approx 10 \text{ м/с}^2$:</p><p>$\mu = -\frac{-2.5 \text{ м/с}^2}{10 \text{ м/с}^2} = 0.25$</p><p><strong>Ответ:</strong> коэффициент трения между бруском и столом равен $0.25$.</p>"
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№4 (с. 99)
Условие. №4 (с. 99)
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 99, номер 4, Условие

4. Лежащему на столе бруску сообщили скорость 2 м/с, после чего он скользил по столу до остановки 80 см.

а) С каким по модулю ускорением двигался брусок?

б) Чему равен коэффициент трения между бруском и столом?

Решение. №4 (с. 99)

Дано:

Начальная скорость, $v_0 = 2$ м/с

Конечная скорость, $v = 0$ м/с (так как брусок остановился)

Пройденный путь, $S = 80$ см

Перевод в систему СИ:

$S = 80 \text{ см} = 0.8 \text{ м}$

Найти:

а) Модуль ускорения, $a - ?$

б) Коэффициент трения, $\mu - ?$

Решение:

а) С каким по модулю ускорением двигался брусок?

Поскольку движение бруска является равнозамедленным, мы можем использовать формулу для пути, не зависящую от времени:

$S = \frac{v^2 - v_0^2}{2a}$

Из этой формулы выразим ускорение $\text{a}$:

$a = \frac{v^2 - v_0^2}{2S}$

Теперь подставим числовые значения из условия задачи:

$a = \frac{(0 \text{ м/с})^2 - (2 \text{ м/с})^2}{2 \cdot 0.8 \text{ м}} = \frac{-4 \text{ м}^2/\text{с}^2}{1.6 \text{ м}} = -2.5 \text{ м/с}^2$

Знак "минус" указывает на то, что ускорение направлено против начальной скорости, то есть движение является замедленным. По условию задачи требуется найти модуль ускорения.

$|a| = 2.5 \text{ м/с}^2$

Ответ: модуль ускорения бруска равен $2.5 \text{ м/с}^2$.

б) Чему равен коэффициент трения между бруском и столом?

На брусок в горизонтальном направлении действует только сила трения скольжения $F_{тр}$, которая и сообщает ему отрицательное ускорение. Запишем второй закон Ньютона для бруска в проекции на горизонтальную ось, направленную по движению:

$-F_{тр} = ma$

Сила трения скольжения вычисляется по формуле $F_{тр} = \mu N$, где $\mu$ — коэффициент трения, а $\text{N}$ — сила нормальной реакции опоры.

В вертикальном направлении сила тяжести $mg$ уравновешивается силой нормальной реакции опоры $\text{N}$, так как вертикального движения нет. Следовательно, $N = mg$.

Подставим выражения для сил во второй закон Ньютона:

$-\mu mg = ma$

Сократим массу $\text{m}$ в обеих частях уравнения:

$-\mu g = a$

Отсюда выразим коэффициент трения $\mu$:

$\mu = -\frac{a}{g}$

Подставим значение ускорения $\text{a}$ из пункта а) и примем ускорение свободного падения $g \approx 10 \text{ м/с}^2$:

$\mu = -\frac{-2.5 \text{ м/с}^2}{10 \text{ м/с}^2} = 0.25$

Ответ: коэффициент трения между бруском и столом равен $0.25$.

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