# Simple physics problem I can't get

1. Nov 2, 2004

ok you have a rock of mass 22.8 KG
It has an initial KE of 28 Joules
This rock is thrown upwards with an initial velocity of 1.56721 m/s
It peak height is .122807 ( i hope thats right )
What is the height of the rock at half its initial speed?
If someone could just give me the right formula or head me in the right direction that would be great. :D

2. Nov 2, 2004

### Staff: Mentor

One way is to use conservation of energy. As it rises, it loses KE (figure out how much) and gains PE.

Another way is to use kinematics: $v_f^2 = v_i^2 + 2ad$

3. Nov 2, 2004