Graph of Kinetic Energy v. Displacement of a Falling Object


by brendan3eb
Tags: displacement, energy, falling, graph, kinetic, object
brendan3eb
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#1
Feb13-08, 10:00 AM
P: 54
1. The problem statement, all variables and given/known data
Draw and label a graph with energy for the y axis and displacement for the x axis of the kinetic energy and potential energy of an object in free-fall that started with a horizontal speed of 30 m/s from a height of 80 m.


2. Relevant equations
U=mgh
k=1/2mv^2


3. The attempt at a solution
I drew a straight line with negative slope for potential energy because m and g are constants in U=mgh, so it should be a linear graph, as for kinetic energy, I am sitting on the fence on whether or not to draw a straight line or a curve The v^2 makes me want to draw a curve, however, if the the potential energy is a straight line and K = E - U, then shouldn't k be directly proportional to displacement as well?
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Doc Al
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#2
Feb13-08, 10:12 AM
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Quote Quote by brendan3eb View Post
The v^2 makes me want to draw a curve, however, if the the potential energy is a straight line and K = E - U, then shouldn't k be directly proportional to displacement as well?
Good thinking!

Why not use a bit of kinematics to double check? What's the relationship between v (or v^2) and displacement for a falling object?
brendan3eb
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#3
Feb13-08, 11:28 AM
P: 54
ahh..

v^2=v0^2+2a(x-x0)
but since initial velocity in the y direction is 0
v^2=-2g(x-x0)

and since -2g is a constant, v^2 is directly proportional to displacement, and thus kinetic energy is directly proportional to displacement!

Thanks Doc Al!


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