# Physics work problem

1. Nov 16, 2008

### reaperkid

1. The problem statement, all variables and given/known data

A 0.179 kg ball is thrown straight up from 1.94 m above the ground. Its initial vertical speed is 10.00 m/s. A short time later, it hits the ground. Calculate the total work done by the force of gravity during that time.

m = .179 kg
vi = 10m/s (upward)
g = 9.8 m/s2
d = -1.94 m (since the displacement is downward)

2. Relevant equations

W = Fd
F = ma

3. The attempt at a solution

This seemed like a simple problem, I'm not sure why it's giving me so much trouble. Basically, I just need to find the force of gravity which I thought was just 9.8 * .179 but I'm clearly missing something.

I tried... W = -1.94 * (9.8 * .179)

Thanks

2. Nov 16, 2008

### asleight

$$x_f^2=x_i^2+2ad$$, $$W=Fd\rightarrow W=mg(x_f^2-x_i^2)/2a$$.

3. Nov 16, 2008

### reaperkid

That doesn't incorporate the initial velocity though does it?

Wouldn't

xi = 1.94 m ?
xf = 0 m ?

4. Nov 16, 2008

### borgwal

Ignore asleight's nonsense.

You did almost the correct thing: you chose the displacement as negative (fine, that's just a convention), but then you should take the acceleration due to gravity as negative, too! Both are downward.

And even without thinking about sign conventions: does gravity do positive or negative net work here?

5. Nov 16, 2008

### reaperkid

I seee, it's positive because it's in the direction of gravity.

Thank you very much sir! :)

6. Nov 16, 2008

### borgwal

You're welcome!