Question

A small robot (mass = 2kg) is hovering in the air when the wind pushes it towards a pole with a speed of 6 m/s. The robot rebounds with a speed of 1.5 m/s, and the collision lasts for 0.5 s. Determine the average force exerted by the pole on the robot when they collide.

Answer 1

momentum inbound = 2 * 6 = 12 kg m/s
momentum outbound = 2 * -1.5 = -3 kg m/s
change in momentum = -3 - 12 = -15 kg m/s

average force = rate of change of momentum = change in momentum / time
= -15 kg m /s / .5 s = -30 kg m/s^2 = -30 Newtons

Answer 2

Is the velocity always taken to be negative when we consider the rebound of the object?

Answer 3

I defined it going + direction to start. You can draw your coordinate system otherwise. However they asked for the force, not the magnitude of the force, so it has to have a direction (lousy problem statement)