Please Help with questions on Newtons Third Law

Question

Draw a Free Body Diagram (FBD) for each object for each of the following below. Label informatively.

1. Two boxes, A and B, are touching each other and are at rest on a horizontal, frictionless surface. Box A has a mass of 25.0 kg and box B a mass of 15.0 kg. A person applies a force of 30.0 N on box A which, in turn, pushes on box B. There should be two FBDs.

2. A 4.00 kg oak block on a horizontal, rough oak surface table is attached by a light string that passes over a light, frictionless pulley to a hanging 2.00 kg object. The magnitude of the force of friction on the 4.00 kg block is 11.8 N. There should be two FBDs.

3. Block X and Y are attached to each other by a light rope and can slide along a horizontal surface. Block X has a mass of 10.0 kg and block Y a mass of 5.00 kg. An applied force of 36.0 N acts on block X. The friction on block X and Y are 8.00 N and 4.00 N. There should be two FBDs.

Answer 1

[I’m using g rounded to 10 m/s² to label weights and normal forces. Length scales are not necessarily proportional, especially in using Word to draw figures!]

1.     In addition to applied force 30 N on A, there are equal & opposite contact forces F_c between the two blocks.

                                   F_N = 250 N                                       F_N = 150 N

                                       ^

                                       |                                                         ^

                                       |                                                         |

                         F_c <---- A -----> 30 N                                 B --> F_c

                                       |                                                         |

                                       |                                                         v

                                       v

                                   F_g = 250 N                                       F_g = 150 N

2.    The 4 kg block has tension toward pulley. There is equal & opposite tension force between A and B. In static situation T = 20 N, but this exceeds the friction on the 4kg block so there is acceleration.

                                   F_N = 40 N                                             T

                                       ^

                                       |                                                         ^

                                       |                                                         |

          F_f = 11.8 N <---- A -----> T                                       B

                                       |                                                         |

                                       |                                                         v

                                       v

                                   F_g = 40 N                                          F_g = 20 N

[#3 to follow, else I exceed character limit]