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Just a clarification question based on an example I read about.

Can normal force do work on an object?

The answer is yes, with an example being a person jumping. The normal force causes work to be done.

However, I'm wondering, is that actually normal force, or is that the reaction force from applying force to the ground? There is a difference between the reaction force and normal force. I'm not sure if technically that example is correct. If it isn't, can someone provide a different one where normal force does do work?

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    $\begingroup$ It sounds like a question of semantics. When you jump you exert a force on the ground. The reaction force is the force the ground exerts on you. Both are normal to the ground. The reaction force does work on you. A magnetic field exerts a force on an electron that is normal to the electron's velocity. That can do no work on the electron. $\endgroup$
    – mmesser314
    Commented Nov 24, 2014 at 4:53
  • $\begingroup$ The reaction force is the normal force. If you think they are different then you'll probably see Newton's third law is being violated. $\endgroup$
    – DLV
    Commented Nov 24, 2014 at 5:14
  • $\begingroup$ It is an interesting question. Work is being done by one part of the body on another. You accelerate the upper part of the body and give it enough momentum to drag along the rest. Pretty complicated. And when you leave contact with the Earth, I wonder if it is as complicated as the rocket equation? You have inspired me to draw some pictures. $\endgroup$
    – C. Towne Springer
    Commented Nov 24, 2014 at 5:31
  • $\begingroup$ @David I was taught that the normal force is not the reaction force. That's a misconception. However, you're right that Newton's third law would seem to be violated. I'm not sure. $\endgroup$
    – Michael Yaworski
    Commented Nov 24, 2014 at 5:34
  • $\begingroup$ @David I don't know how it's taught in other countries, but the "normal force" here should be the normal component of the reaction force. In general, they are different. $\endgroup$
    – TZDZ
    Commented Nov 24, 2014 at 6:07

2 Answers 2

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** I'm wondering, is that actually normal force, or is that the reaction force from applying force to the ground?** There is a difference between the reaction force and normal force.

No, there is not a difference you can determine between "the reaction force" and this normal force because one is describing a relationship (reaction) and the other a reality (normal force).

It seems to me there is a fundamental misunderstanding of Newton's 3rd Law, aka, action-reaction forces. The normal force on the persons feet is caused by the interaction of the structural boundaries (the intermolecular bonds, etc) of the feet with the structural boundaries of the ground. Likewise, the normal force on the ground from the feet is caused by exactly the same interaction. You cannot say that one occurs in "reaction" to the other.

What Newton's 3rd Law says is that forces do not occur singularly. They are interactions, and as such, if you observed the result of a force or you conceptually determine there is one force on an object, there must, by symmetry, be another force due to the same interaction. It's not a cause and effect relationship (" which force is the reaction to the action?"), it's a there-must-be-another-force-somewhere relationship. Newton's 3rd law really is a statement about conservation of momentum. This is from Newton (translated from the Latin by Drake, I believe):

If a body impinges upon another, and by its force changes the motion of the other, that body also (because of the equality of the mutual pressure) will undergo an equal change, in its own motion, toward the contrary part. Teh changes made by these actions are equal, not in the velocities but in the motions of the bodies; that is to say, if the bodies are not hindered by any other impediments. For, as the motions are euqally changed, the changes of the velocities made toward contrary parts are reciprocally proportional to the bodies.

In this writing, Newton's motion is our momentum and his body is our concept of mass.

Nowhere does Newton say that an action causes a reaction. He says that forces come in pairs:

If you press a stone with your finger, the finger is also pressed by the stone.

The forces come from a mutual interaction; reaction is an unfortunate word.

Forces have root causes and those causes are not other forces. They are interactions: mass with mass, charge with charge, quarks with quarks.

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  • $\begingroup$ Okay, so action-reaction is a bad choice of words. They happen simultaneously in a mutual interaction. They act in both directions between the two objects. Am I correct? But still, jumping is a result of the force you apply on the ground. Whether or not it's called a "reaction force", it is still a force that is a result of the interaction between you and the ground. Correct? And that is unrelated to the normal force? Or is the force you applied to the ground affecting the normal force because you are affecting the structural boundaries by applying more force? $\endgroup$
    – Michael Yaworski
    Commented Mar 4, 2015 at 20:31
  • $\begingroup$ "Normal force" is the name we give to the force the ground exerts on the person if we are analyzing all the forces on the person. It would be fair to call the force on the ground a normal force also. It's called the normal force simply because it is perpendicular to contact surfaces, and that is an averaging of all the contact points.The contact forces ARE the interaction forces IS the normal forces. The one on the person by the ground is equal and opposite to the one on the ground by the person, and the pair of them is the N3Law pair. $\endgroup$
    – Bill N
    Commented Mar 4, 2015 at 22:34
  • $\begingroup$ Your explanation of the normal force makes sense to me. The confusion I had was because my high school physics teacher told me that the normal force was not a reaction force due to gravity. That is true, but I interpreted it incorrectly. Thanks. $\endgroup$
    – Michael Yaworski
    Commented Mar 5, 2015 at 0:23
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I would see the normal forces as a requirement to fulfil a constraint: staying on the surface of the earth. By defenition, no work is performed by constraint (reaction, normal) forces due to action-reaction.

It could be easier to look at a spring 'jumping' off a rigid table. First you compress it with your finger, which stores energy in the spring. The constraint forces make sure the spring does not enter the table, so there is no movement and therefore no work performed. When you release the spring, it will accelerate by pushing onto the table (the other end is free, so it moves). Due to inertia the spring will take off.

Now replace spring by muscles and body, and table by earth.

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  • $\begingroup$ I like the comparison, but I'm not sure that it's valid. Elastic potential energy causing acceleration is different than action-reaction force. $\endgroup$
    – Michael Yaworski
    Commented Nov 25, 2014 at 4:01
  • $\begingroup$ The action-reaction force is used to keep the spring from moving, your finger moves and therefore builds up potential, not the table. I think putting energy in a spring is comparable to tensioning your muscles. $\endgroup$
    – Rhino
    Commented Nov 25, 2014 at 6:46
  • $\begingroup$ So you're saying the action-reaction is considered the force making the person accelerate upward? Not normal? $\endgroup$
    – Michael Yaworski
    Commented Nov 25, 2014 at 14:00
  • $\begingroup$ Action-reaction == normal == reaction == constraint force. They all make sure there is not interpenetration between table and spring, or person and earth. $\endgroup$
    – Rhino
    Commented Nov 25, 2014 at 20:28

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