Concept for potential energy
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Concept for potential energy

[From: ] [author: ] [Date: 11-07-05] [Hit: ]
What if it mashed it down more?-C) Assumes a braking force F from the marshmallow stops the motion in d = .01 m.That is, the marshmallow is doing work on the object to stop it.And that work must sap all the kinetic energy PE = KE = 16J the object has when impacting the marshmallow.......
I'm not understanding this problem in the book and why the answer is what it is.

Concider an 8-Newton object that is 2 meters above the floor.
A) What is ti's PE with reference to the floor. (16J)
B) How much work can it do when it falls? (16J)
C) Suppose the object falls onto a marshmallow on the floor and mashes the marshmallow down 1cm. What force is exerted by the object on the marshmallow?
(W=F*d)
(16J=F*.01m)
(F=1600N)

What is happening here, why is there so many newtons. What if it mashed it down more?

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C) Assumes a braking force F from the marshmallow stops the motion in d = .01 m. That is, the marshmallow is doing work on the object to stop it. And that work must sap all the kinetic energy PE = KE = 16J the object has when impacting the marshmallow. That gives KE - WE = 0 so there is no energy left and the object stops. So we have PE = KE = 16J = F*.01 = WE and F = 1600 N is the braking force the marshmallow puts on the object.

If it smooshed (a technical term) even more, there would be less f < F braking force needed to sap that 16 J of energy. For example, if the entire mallow were smooshed, and d = .05 m; the braking force would be f = 16/.05 = 320 Newtons. This is in fact the concept behind air bags. Instead of coming to a halt in d = .01 m against the windshield, we halt in an air bag over d = 1 m; so the impact force is f' = 16/1 = 16 Newtons instead of 1600 N.

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(A) PE with reference to the floor = mgh (mg = weight = 8 N, h = 2 m)
= 8 x 2 = 16 J
(B) When it falls, work done = Force x displacement
= mg x h = 16 J
(C) F' d' = Fd = mg x h
Or
F' x 0.01 = 8 x 2 = 16
Or
F' = 16/0.01 = 1600 N
If it mashes down more, F' will become smaller.

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Please check your spelling and grammar before posting. I cannot understand what you are trying to ask.
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