# Homework Help: Block question

1. Jun 9, 2006

### c4iscool

Where do I start?
A 10kg block being held at rest above the ground is released. The block begins to fall under only the effect of gravity. At the instant that the block is 2.0 meters above the ground, the speed of the block is 2.5m/sec. The block was initially released at a height of how many meters.

2. Jun 9, 2006

### neutrino

Hint: Energy conservation.

3. Jun 9, 2006

### c4iscool

is that the equation m1vf^2-1/2mvi^2, if it is how would u find the height it was released at? or are you talking about potential energy?

Last edited: Jun 9, 2006
4. Jun 9, 2006

### neutrino

It's $$mgh_1 + \frac{1}{2}mv_1^2 = mgh_2 + \frac{1}{2}mv_2^2$$

The left side (initial) variables are at the instant when the block was at the "top", and those on the right (final) are the instant when the block is at the given height of 2m. Therefore, it is Initial energy = Final energy.

5. Jun 9, 2006

### c4iscool

(10kg)(9.8m/sec)h+(1/2)10kg(0^2)=10kg(9.8m/sec)(2m)+(1/2)10kg(2.5m/sec)^2

6. Jun 9, 2006

### c4iscool

i should have it from here. thanks

7. Jun 10, 2006

### neutrino

Correct me if I'm wrong, Hootenanny, but isn't that the expression for the block which has fallen a distance of two metres from the initial position. Infact if you expand the terms, you get 196 = 31.25.

8. Jun 10, 2006

### Hootenanny

Staff Emeritus
My bad. I apologise the OP had it correct first time round. . The only excuse I can offer is that it was rather late last night when I posted . Thanks for pointing that out neutrino