# Homework Help: Speed of two bodies that sticks together after collision

1. Nov 24, 2013

### coconut62

1. The problem statement, all variables and given/known data

Two railway trucks of masses m and 3m move towards each other in opposite directions with speeds 2v and v respectively. These trucks collide and stick together after the collision. What is the speed of the trucks after the collision?

2. Relevant equations

Conservation of momentum: m1v1 + m2v2 = (m1+m2)Vtotal

3. The attempt at a solution

m(2v) + 3m(-v) = (m+3m)V

V= -4/v
∴ speed = 4/v

Answer given: 5v/4

Apparently they added the 2mv and 3mv together for the initial momentum. Is the book wrong or am I wrong?

2. Nov 24, 2013

### ehild

Correct so far.

Wrong, do it again.

The book is wrong.

ehild

3. Nov 24, 2013

### coconut62

From " m(2v) + 3m(-v) = (m+3m)V "

2mv -3mv = 4mV

-mv = 4mV

V= -v/4

Speed does not have a direction, so answer is v/4. Why wrong?

4. Nov 24, 2013

### ehild

You wrote speed=4/v in the original post

ehild

5. Nov 24, 2013

### haruspex

That would be the right answer if m had speed v and 3m had speed 2v. Are you sure you quoted the question correctly?

6. Nov 24, 2013

### coconut62

Yes. I quoted it correctly.

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