Newton’s 3rd Law - Example Problems of Interacting Objects

In this video we solve force problems that involve interacting objects where we use Newton’s 3rd Law to identify action-reaction pairs. We draw free-body diagrams, use Newton’s 2nd law to sum forces in all directions and then solve for either an unknown force or acceleration of the objects. 0:00 - Problem 1: At the top of a 25 degree ramp, two blocks begin sliding down together. Block A has a mass of 7 kg and a coefficient of friction of 0.25. Block B has a mass of 12 kg and a coefficient of friction of 0.20. Find the acceleration of the blocks down the ramp. 9:39 - Problem 2: Two boxes A and B are connected by a massless cable while they slide together at constant speed down a 25 degree ramp. Box A has a mass of 2.5 kg and a coefficient of friction of 0.30 and box B has a mass of 3.5 kg and a coefficient of friction of 0.20. Find the tension in the cable. 18:31 - Problem 3: A 6 kg block sits on top of a 4 kg block. The coefficient of static friction between the blocks is 0.50. The coefficient kinetic friction between the bottom and the floor is 0.25. Find the acceleration if the top block is pulled with the maximum amount of force without it sliding across the bottom block. 25:54 - Problem 4: A small 1.5 kg block is sitting on top of another 2.5 kg block resting on a flat surface. The top block is attached to a wall by a cable while the lower block is pulled in the other direction by a 24 N force. The coefficient of kinetic friction between the two blocks and between the lower surface and the bottom block is 0.35. Find the tension in the cable attaching the top block to the wall and the acceleration of the bottom block. 33:31 - Problem 5: Three blocks are attached by cords that loop over frictionless pulleys as shown. The coefficient of friction between the table and middle block is 0.35. Find the acceleration of the block on the table. 45:43 - Problem 6: Find the force needed to keep the block on the ramp from sliding up or down if all surfaces are frictionless.