# Particle momentum problem

1. Mar 11, 2006

### frozen7

6. Two particles, each of mass m, are connected by a light inextensible string of length 2l. Initially they lie on a smooth horizontal table at points A and B distant l apart. The particle at A is projected across the table with velocity u. Find the speed with which the second particle begins to move if the directions of u is.
(a) along AB,

mu = (m+m)v
v = u/2

But the given answer is u/2 and mu/2. Can anyone explain to me why there are 2 answers? Thanks.

2. Mar 11, 2006

### Staff: Mentor

Only the first answer is a speed; the other is a momentum.

3. Mar 11, 2006

### Hootenanny

Staff Emeritus
I can't see the answer of $\frac{1}{2}mu$ being correct, because that would leave the velocity having units of $m\cdot kg \cdot s^{-1}$, which doesn't happen.:yuck:

4. Mar 11, 2006

### frozen7

Thanks..I am so stupid...

5. Mar 11, 2006

### Hootenanny

Staff Emeritus
It really is bad practise by a book to ask for a velocity then give an answer in momentum. I've just had some problems with a textbook myself ...