Momentum help???

Question

A 17000 kg freight car is rolling along a track at 1.7 m/s.

Calculate the time needed for a force of 1890 N to stop the car. Answer in units of s.

Answer 1

Actually, this is a force, not momentum, problem.

F = m(delv/delt); so we are looking for delt which is the time to reduce v0 = 1.7 mps to v1 = 0 mps. You have F = 1,890 kg-m/sec^2; so you have everything you need so solve for delt = m delv/F; where m = 17,000 kg, delv = (0 - 1.7 mps), and F = -1,890 kg-m/sec^2 (where the minus sign shows a decelerating force). Just eyeballing the numbers, looks to me that delt is about 15 seconds or so. You can do the math.

So here's the physics. This is a force, not momentum problem. Why? Because you are dealing with mass changing velocity and when velocity changes, that uses force. Momentum deals with fixed velocities, usually before and after an impact for example. However force and momentum are related in that the change in momentum over time is called force; that is, dp/dt = mdv/dt = ma; where dp/dt is the change in momentum over time and dv/dt is called acceleration (a).