What will be the car's new speed?

Question

a 12600kg railroad car travels on a level frictionless track with constant speed of 18m/s. a 5350 kg load, initially at rest, is dropped onto the car. what will be the car's new speed?

How do I do this problem. where do i start??

Answer 1

p=mv
p=12600*18=226800

V=p/m
v=226800/(12600+5350)
v=12.6

Answer 2

Use conservation of momentum. The initial mass times the initial velocity (both given) will be the same as the final mass times the final velocity. You can calculate the final mass by just adding the weight of the load to the initial mass, so the final velocity will be your only unknown.