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

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The discussion focuses on solving a physics problem involving force, velocity, and energy conservation. The initial calculations using the formula f=ma and kinematic equations lead to an incorrect final velocity of 16 m/s, while the correct answer is 7.3 m/s. Participants emphasize the importance of understanding initial conditions, specifically whether the initial velocity is zero, and the role of gravitational potential energy (Ep) and kinetic energy (Ek) in the total energy equation. They clarify that total energy is conserved throughout the motion, allowing for calculations at different points. The conversation concludes with a reinforcement of using energy conservation principles to find the total energy at various stages of the problem.
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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?
 
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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?
 
rock.freak667 said:
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?
 
or do u use 250j total energy and the inititial Ep, I thinks that's right.
 
brentwoodbc said:
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.

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.
 
thank you very much.
 
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