Solving Force and Velocity: Get Answer in 7.3m/s

[brentwoodbc]

I did

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

vf^2=vi^2+2ad
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 don't know that the initial velocity is 0 or something like that?

anyways my way I don't get the right answer, the correct answer is 7.3m/s

How do I do this?

 

[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?

 

[brentwoodbc]

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?

ya there is ep and ek but you can't really find Ek at the start or at the end right?

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

 

[brentwoodbc]

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

 

[rock.freak667]

ya there is ep and ek but you can't really find Ek at the start or at the end right?

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

That's the nice thing about conservation of energy.

E_p+E_k= Total energy up to the point of the motion.

So at the beginning

mgh_initial+1/2mv²_initial = 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 mgh_end +1/2mv²_end=Total energy at the end.

 

[brentwoodbc]

thank you very much.