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# Physics question?

A 2560 kg van runs into the back of a 829 kg compact car at rest. They move off together at 8.5 m/s. Assuming the friction with the road can be negligible, find the initial speed of the van.

### 3 Answers

- oubaasLv 71 month ago
8.5 = Vx*2560/(2560+829)

Vx = 8.5*(2560+829)/2560 = 11.25 m/sec (11 with just two sign. digits)

- Anonymous1 month ago
there are so many variables in such a collision it is impossible to calculate and friction is NEVER negligible

- husoskiLv 71 month ago
Conservation of momentum:

m₁v₁ = (m₁ + m₂)v₂

On the left is the initial momentum of mass m₁ = 2560 kg at speed v₁ (to be determined). On the right is the final momentum of the combined mass m₁ + m₂ = (2560 + 829) kg traveling at v₂ = 8.5 m/s. Plug and solve:

2560v₁ = (2560 + 829)(8.5)

v₁ = (3389)(8.5) / 2560 ~~ 11 m/s

I rounded to 2 significant digits, since that's all the precision you have in 8.5 m/s; but there's an argument for quoting 11.3 m/s since 1 part in 113 is closer to the desired 1 part in 85 than is 1 part in 11.