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Potential and Kinetic Energy Problem

  • Thread starter CeceBear
  • Start date
  • #1
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Homework Statement


You throw a softball (of mass 400g) straight up into the air. It reaches a maximum altitude of 15.2m and then returns to you.
What is the gravitational potential energy of the softball at its highest position? Assume the ball departed from and returned to ground level.
Assume no energy is lost by the softball while it is in the air.
What is the kinetic energy of the softball as soon as it leaves your hand?
What is the kinetic energy of the softball when it returns to your hand?
What is the speed of the ball?


Homework Equations


PE = mgh
KE = (1/2)mv^2


The Attempt at a Solution


I've already calculated the PE to be 59.584 J. I'm not sure how to calculate KE without a velocity.
 

Answers and Replies

  • #2
PhanthomJay
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You can try using the conservation of energy principle. Or use the kinematic motion equations. Have you tried either method?
 
  • #3
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You can try using the conservation of energy principle. Or use the kinematic motion equations. Have you tried either method?
But I only have distance. I don't have time, acceleration, or velocity for the kinematic equations. And according to the conservation of energy law, wouldn't KE just be -PE?
 
  • #4
63
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I think you are forgetting one more equation
total energy: KE+PE
so you can try setting up like this using conservation of energy
Initial Total Energy=Final Total Energy

as for kinematic equations, you do have acceleration. You can figure out velocity by finding time it takes to reach the top or fall to the ground using kinematics
funny I just had a question about Conservation of energy as well >_>...starting a new thread..

wouldn't KE just be -PE?
KE would equal to PE (not -PE) but this depends on when you are talking about. (it would be true if use KE when PE=0 and PE when KE=0)
 
Last edited:
  • #5
11
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Ok, I think I've got it now. Thank you very much!
 
  • #6
Using the known distance, gravity, and the fact that its velocity is zero at its highest point you can find its final velocity
 

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