# Homework Help: Simple mechanics have got stuck!

1. Dec 17, 2007

### Zoe-b

1. The problem statement, all variables and given/known data
I am trying to work out the force which a mass applies when it drops from a known height onto another object.

2. Relevant equations
F=ma
suvat:
v^2 = u^2 + 2as
F= change in momentum/time
and possibly KE= 1/2mv^2
3. The attempt at a solution

I don't think that F=ma applies exactly as this does not take into account where the mass is dropped from at all.. the force must be larger if the mass is travelling at a greater speed, surely?? This is the force the mass would apply if it was not moving..

I can work out the KE by finding the velocity at the moment of contact, but I don't know how to transfer this into a force. This is from a practical experiment so I don't actually have any time measurements =/ although I could estimate them by using suvat.

Thanks for any help.

Last edited: Dec 17, 2007
2. Dec 17, 2007

### chickendude

The force which gravity applies on the mass is simply mg (the mass times gravity)

The force which the the mass applies on whatever object it hits can be shown by F=ma, but a is not gravity

a is the change in velocity over the change in time

the change in velocity is v_final - v_initial
It would be really big if it say... bounced.
It would be smaller if it stuck to the floor
It would be even smaller if it broke through the floor

The change in time is the amount of time the mass is in contact with the floor (the amount of time it was accelerating)

Basically, you need more information. Something about how long the mass takes to decelerate when it hits the floor (or how elastic the floor and the mass is)

3. Dec 17, 2007

### Zoe-b

can't believe I didn't realise that.. hehe. The mass isn't actually hitting the floor, its hitting the end of a pencil.. long story! But I have some other experiments that tell me roughly the efficiency of the collision (in terms of percentage energy transferred).. so I reckon I'll be able to get a before and after speed and work from that. Thanks!