# Momentum of falling mass problem

1. Mar 8, 2006

### rachael

6 An object of mass 0.80 kg travelling horizontally with a
speed of 30 m s–1 hits a hanging metal plate of mass
12.0 kg and rebounds with a speed of 16 m s–1. What is
the speed with which the plate moves forward?

i used p=mu+mu but the answer was not correct why is that?

2. Mar 8, 2006

### finchie_88

Did you try (0.8*30) - (0.8*(-16)) = 12v ? If you did, and that didn't work, maybe there is a coefficient of resistuation (sorry about the spelling) between the object and the plate, so $$\frac{Speed.of.sparation}{Speed.of.approach} = e$$

3. Mar 8, 2006

### Hootenanny

Staff Emeritus
Find the total momementum of the system before the collision. This momentum must remain constant. You can calculate the momentum of the object, but remember this is travelling in the oppoisite direction to the plate so:
$$P_{plate} - P_{object} = P_{initial}$$

4. Mar 9, 2006

### rachael

thank you ........