# Homework Help: Force and velocity

1. Jun 14, 2009

### brentwoodbc

I did

f=ma
(30+20)/2=2a
a=12.5m/s^2

vf^2=0+2•12.5•10
vf= 16 m/s

I remember doing a question like this on a test a while ago and I think the teacher said you dont know that the initial velocity is 0 or something like that?

anyways my way I dont get the right answer, the correct answer is 7.3m/s

How do I do this?

2. Jun 14, 2009

### rock.freak667

The area under the graph gives the total work done.

At 10m, the box will have two kinds of energy, one is kinetic, what is the other?

3. Jun 14, 2009

### brentwoodbc

ya there is ep and ek but you cant really find Ek at the start or at the end right?

unless you assume the starting velocity is 0 but what if its not?

4. Jun 14, 2009

### brentwoodbc

or do u use 250j total energy and the inititial Ep, I thinks thats right.

5. Jun 14, 2009

### rock.freak667

That's the nice thing about conservation of energy.

Ep+Ek= Total energy up to the point of the motion.

So at the beginning

mghinitial+1/2mv2initial = Total energy.

And at any other point

mgh + 1/2 mv2= Total energy at that point.

So if you want the energy at the end. You can get the total energy there using the area.

so mghend +1/2mv2end=Total energy at the end.

6. Jun 14, 2009

### brentwoodbc

thank you very much.