Question

A railroad flatcar of weight 7840 N can roll without friction along a straight horizontal track. Initially, a man of weight 557 N is standing on the car, which is moving to the right with speed 25 m/s; see the figure. What is the change in velocity of the car if the man runs to the left so that his speed relative to the car is 5.0 m/s?

Answer 1

PICTURE:

SOLUTION:

Given that,

Velocity of the car before the man started running, v=25 m/s
Weight of the car, W=7840 N

Weight of the man, w=557 N
Velocity of the car once the man is running , u= ?
Relative velocity of the man and car, Vr= 5.0 m/s

From conservation of momentum,

v(W + w) = uW + (u - Vr)w

(25 m/s)(7840 N + 557 N)=(u)(7840 N)+(u-5 m/s)(557 N)

209925 kgm/s =7840 u + 557 u -2785 kgm/s

                   u=25.332 m/s

change in velocity (u-v) = 0.332 m/s