# physics

posted by bill

Two identical cars hit each other and lock bumpers. In each of the following cases, what are the speeds of the cars immediately after coupling bumpers? (a) A car moving with a speed of 90 km/h approaches a stationary car; (b) two cars approach each other with speeds of 90 km/h and 120 km/h, respectively; (c) two cars travel in the same direction with speeds of 90 km/h and 120 km/h, respectively.

1. Damon

These are all conservation of momentum problems, but easy because the cars stick together after the crash so the final mass is (2m)and they are both at the same final speed v
so
m u1 + m u2 = 2m v
or
u1 + u2 = 2v for all cases
for example for part b
u1 = 90
u2 = -120
so for part b
90 - 120 = 2 v
v = -15 answer for part b
You should be able to do parts a and c now.

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