Work, potential and kinetic energy help

  • Thread starter PhizKid
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  • #1
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Homework Statement


A 30 kg block is slid down an inclined plane 5 m long with a height of 3 m from the floor. If the force of friction is 50 N, find the work done by the friction and the final velocity at the end of the plane.


Homework Equations


W = F*d
mgh = (1/2)mv^2

The Attempt at a Solution


50 N * 5 m * cos(180) = -250 J

Incorrect

30 kg * 9.8 m/s^2 * 3 m = (1/2) * 30 kg * v^2
882 = 15*v^2
58.8 = v^2
v = 7.6681 m/s

Incorrect

I don't know how else to approach this.
 

Answers and Replies

  • #2
Doc Al
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50 N * 5 m * cos(180) = -250 J

Incorrect
Nothing wrong with this. They probably wanted just the magnitude of that work.

30 kg * 9.8 m/s^2 * 3 m = (1/2) * 30 kg * v^2
882 = 15*v^2
58.8 = v^2
v = 7.6681 m/s
You forgot about the friction.
 
  • #3
476
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The answer says -150 J so I guess that's an error then.

How do I account for fricton in potential energy?
 
  • #4
Doc Al
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The answer says -150 J so I guess that's an error then.
Looks like they used the 3 m distance by mistake.
How do I account for fricton in potential energy?
Initial mechanical energy + work done by friction = Final mechanical energy
 
  • #5
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So then I get (30 kg * 9.8 m/s^2 * 3 m) + (-250 J) = (1/2) * 30 kg * v^2

I get 6.49 m/s which is also incorrect (solution is 7.3 m/s).
 
  • #6
Doc Al
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So then I get (30 kg * 9.8 m/s^2 * 3 m) + (-250 J) = (1/2) * 30 kg * v^2

I get 6.49 m/s which is also incorrect (solution is 7.3 m/s).
I agree with your answer. Even using their incorrect value for the work done by friction you won't get their answer.

What book is this from?
 
  • #7
476
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Something our professor wrote :-p

It's so intimidating because I feel like I'm not getting the concepts when it just turns out the solutions are incorrect...so I'm still not confident enough to say that my answer is correct and the answer key is not
 
  • #8
Doc Al
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It is unfortunate when the professor makes errors, but it happens.

Your textbook should have problems for you to work on. Often some of the answers are given.

You could also supplement your text with a problem book, such as a Schaum's Outline.
 

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