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I’ve been considering the applications of Newton’s 3rd Law and most of the examples I have seen shown to me I completely understand. For example, I understand that if an individual was to push against a wall with a given force the the reaction force from the wall would act on the person in the opposite direction with the same magnitude. However, when considering some other examples I have stumped myself. The situation that confuses me involves a person jumping onto an egg. If someone was to jump in the air and land on an egg then the force from the person (their weight) would act upon the egg and thus the egg would end up cracking as the maximum weight an egg could support is less than the applied weight force from the jumping person. However, if the egg can only handle a force much less than the person then how would it be possible for the egg to apply a reaction force back to the persons foot given the third law?

Qmechanic
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Brian Lamont
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4 Answers4

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then the force from the person (their weight) would act upon the egg.

That's not true. You can push on the egg, but that doesn't mean that you're actually applying that much force.

Push a wall, you can probably generate 100N or more against the wall. Now push a feather. You will not be able to apply that much force.

However, if the egg can only handle a force much less than the person then how would it be possible for the egg to apply a reaction force back to the persons foot given the third law ?

It cannot. Or rather, it can only apply that same force that it can before breaking. That is why the person does not decelerate (much) upon reaching the egg. Instead, they keep most of their velocity until they reach the floor.

The floor does not yield and supplies much larger forces to decelerate the person.

BowlOfRed
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  • Thank you very much BowlOfRed, it makes a lot of sense now. Thanks again for taking the time to answer my question ! – Brian Lamont Sep 17 '21 at 01:07
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You're kind of overthinking this. Newton's 3rd law does not say there have to be two forces for every interaction, it's stating a fact about the nature of what a force is. It is essentially saying there is no such thing as a "one sided" force -----> All forces are like this . <----->

In your example, what happens is the person accelerates down, then they contact the eggshell which very slightly decreases their acceleration until a force of (say) 1 lb builds up, then the egg shatters, the slightly slowed down person keeps falling until a combination of the egg pieces and the floor are able to resist the full load and stop him.

RC_23
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The person is not doing a force equal to his weight. As you well suspected, it can only make a force to the egg of similar magnitude than the egg does to the person. This should not be surprising, as the person falls he also makes force into the air (and the air does a force on him), but this force is so tiny that we usually ignore it for any practical purposes.

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Newton's third law applies to valid action-reaction pairs. If there is more than one force operating on either object, then it is not a valid action-reaction system.

Think about the forces that are acting in this system. The earth is pulling the person down. But on the egg's surface there be a normal force operating on the person's foot, and this same normal force must also resist a second force, gravity, and clearly it cannot which is why it breaks. So there are two forces here acting on the egg, and so this cannot be a a valid third law action-reaction pair.

Remember that Newton's third law is essentially a statement of the conservation of momentum, and momentum is always conserved.

joseph h
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