What is the speed?

Question

. A boxcar traveling at 20 m/s approaches a string of three identical boxcars sitting stationary on the track. The moving boxcar collides and links with the stationary cars, and the four move together along the track. What is the speed of the
four cars immediately after the collision?
(a) 0.15 m/s
(b) 0.20 m/s
(c) 5.0 m/s
(d) 6.7 m/s

Answer 1

This problem relates to the idea of conservation of momentum - the total momentum of the system before and after the collision does not change.

momentum = mass* velocity.

so initial mass * initial velocity = final mass * final velocity, or
mi*vi = mf*vf

We wnat to know final velocity, so can rearrange the formula
vf = mi*vi/mf

we are not told the masses of the box cars, but we are told that they are identical. So, we can say that the fist box car has a mass of "mi" and therefore, the four combined have a mass of "4mi"

So then our equation becomes
vf = mi*20 / 4mi and the mi's cancel and we are left with
vf = 20/4 = 5 m/s :)

Answer 2

(c) 5.0 m/s

20 x Mass = 5 x (4 x Mass)