Phys151_PPLNine.F12[1]

# When the suitcase was raised from the floor to a

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When the suitcase was raised from the floor to a height of 1 m, its gravitational potential energy above the floor is 200 Joules. Note that I have to give a reference level to make the answer meaningful which is made obvious by this question: What if I had raised the suitcase from a hole in the floor?

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It’s Gone Kinetic The suitcase’s gravitational potential energy is larger with respect to the bottom of the hole! However, there is no contradiction .We are always “talking relative”. Now when I let the suitcase drop it loses height with respect to the floor, its GPE or “stored work” drops as well. Back at the floor, its GPE is 0. Yet that’s when the suitcase does its work on my foot! The suitcase’s ability to do work has not disappeared. So what happened to the stored work during the fall? The answer is that it now has been transformed to another kind of stored work which resides in the motion of the suitcase; this new stored work is called kinetic energy .
Kinetic Energy It’s the form of energy associated with the state of motion of the object. Disappearing GPE reappears as increasing KE. Take note of this conservation rule ”. Can kinetic energy be defined on its own? Yes. If an object of mass m has a speed/velocity of v then its kinetic energy is 1/2 m v 2 . Power of the work-energy concept: the payoff ! I drop the suitcase from 1 m. What is the speed just as it reaches the floor? Treating it as a freefalling object, I use d = 1/2.a.t 2 = 5.t 2 to find the time of fall. Then I plug this into v = a.t = 10.t to find v. The result is v = 10.(1/5) 1/2 = (20) 1/2 m/s.

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Work-Energy Connection We could have found the answer a little easier using the GPE-KE connection. The GPE at 1 m is 20.10.1 = 200 J. Since this disappears into KE and the KE of the suitcase just before it hits the floor is 1/2.20.v 2 , we must have 1/2.20.v 2 = 200 So v = (20) 1/2 m/s.
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