Math Is Fun Forum

hannah_106

Member

Registered: 2007-10-17

Posts: 3

help momentum question?

A railway truck of mass 4 x 10exp4 Kg moves at 3ms-1 towards a stationary truck of mass 2 x 10exp4 Kg. the trucks collide and join calculate the total momentum of the two before the collision. What is the total momentum of the two trucks after the collision? Find the speed at which the joined trucks move off after their collision. Compare the kinetic energy after the collision with that before the collision.
Momentum= mass x velocity

4 x 10exp4 x 3ms-1 = 120000 Kg ms-1

2 x 10exp4 x 0ms-1 = 0 Kg ms-1 ( by stationary I take it, it means 0 ?)

Total momentum = 120000 + 0 = 120000

Total momentum after collision is equal to total momentum before collision if you exclude all other factors?

Speed ?
KE = ½mv2

If this is all wrong, which I am pretty sure it is, can you please show your full working.

Thanks.

mathsyperson

Moderator

Registered: 2005-06-22

Posts: 4,900

Re: help momentum question?

Everything you wrote is fine.
From where you stopped, you needed to work out the speed of the combined trucks.

Their combined momentum is 120000 kgm/s, due to it being conserved from before the collision.
Their combined mass is 40000 + 20000 = 60000kg, and so their combined momentum is 120000/60000 = 2m/s.

The KE formula you posted is correct, so you just need to work out (and then compare) the KE of the moving railway truck before it collides, and of the combined trucks after the collision.

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Why did the vector cross the road?
It wanted to be normal.