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Normal Force Work Problem 
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#1
Apr1710, 03:23 PM

P: 11

1. The problem statement, all variables and given/known data
Can the normal force on an object ever do work on the object? Explain your answer. The answer in the textbook is: The normal force can do work on an object. For example, when you jump, you push down on the ground and the normal force pushes up on you and accelerates you up, giving you kinetic energy. 3. The attempt at a solution My initial answer before reading the textbook was, no, it can't do work. I understand that when an object is moving on a surface, the normal force is perpendicular to the distance traveled and so it is zero. With respect to the answer in the textbook, work is the energy transferred to an object when a force acting on it causes it to move a certain distance. Since, the normal force is a contact force, it only occurs when in contact with a surface. Wouldn't it would be zero as soon as the object loses contact or jumps. Wouldn't this mean that the normal force doesn't do work on the object..., or am i missing something? Thanks 


#2
Apr1710, 03:51 PM

HW Helper
P: 6,202

When jumping up, the normal force is in the same direction as the displacement. So it does work.
For an object moving horizontally on a surface, the normal force is perpendicular to the direction of the displacement, so that the work done by it is zero. 


#3
Apr1710, 04:00 PM

P: 3,014

If the constraining surface or line is moving itself, then the normal force can do work.



#4
Apr1710, 04:02 PM

P: 11

Normal Force Work Problem



#5
Apr1710, 04:10 PM

P: 88

In the act of jumping: From you start to jump till the moment your feet loose contact, your center of mass has moved some distance. This, I guess, can be seen as work done by the normal force.



#6
Apr1710, 04:11 PM

HW Helper
P: 6,202




#7
Apr1710, 04:15 PM

Mentor
P: 41,471




#8
Apr1710, 04:20 PM

Mentor
P: 41,471

Edit: Let me clarify. F_{net}*ΔX_{cm} = 1/2MV_{cm}^{2} (the kinetic energy of the center of mass), nonetheless the normal force itself did no work on the jumper. 


#9
Apr1710, 04:22 PM

P: 11




#10
Feb411, 05:50 PM

P: 35

The original question was "can the normal force do work?" for which the answer should be "yes," though I agree that their explanation is sloppy.
Consider the situation where the surface itself moves over some distance like the seat of a Ferris wheel, or a horse on a merrygoround. The normal force is responsible for the displacement and does positive work on the way up and negative work on the way down. 


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