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Physics/Newtons third law

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Question
Hi Steve,

I imagine this is a very simple question but I just can't wrap my head around the understanding. For every action there is an equal and opposite reaction...I'll use the example that baffles me.

If I am standing on a concrete paver. Then the paver is opposing my weight with an equal amount of force. But if I were to step off of it then does this opposing force just cease to exist? How does an object such as the ground we walk on alter the force that it exerts on things?

I'm not sure if I have made it clear, but any help would be greatly appreciated.

Thanks

Answer
Hello Mat,

Let me start with a different example that I think you will be able to see how it relates to your example. Picture a steel table, perhaps a work bench. There is no support beneath the center of the table's top surface. But the steel is strong. If you climb up on the table and stand in the middle, the table will hold you. It provides a force equal and opposite your weight. You can picture how it does that as a spring in action. The table top reluctantly flexes slightly. If you were carrying a mug of coffee, it would have to flex slightly more so that it provided a force equal and opposite the weight of you and your coffee. When you get off the table, it returns to its normal shape. (Or perhaps if it were not as strong as I assumed originally, it would not return all the way to normal and would have a permanent dip in the center.)

The paver in your example is quite rigid. Concrete is not completely immune to flexing, but you might not be able to visualize that your weight makes it flex at all. But it is sitting on soil. With you standing on it, you and the paver exert more pressure on the soil than before you got on.

The pressure on the soil = (the paver's weight + your weight) / the contact area.

Newton's 3rd applies to the interface between the soil and the paver as well as between you and the paver. The soil exerts a pressure upward against the paver such that if you multiply the pressure by the contact area, the result is an upward force that is equal and opposite the weight of you and the paver.

If the paver had been recently set in its place, in the process the soil may get slightly compressed. When you step off the paver, then it is only the paver sitting on the compressed soil. If the soil has any resilience, it may exert a force on the paver that actually accelerates the paver up briefly. And then things would be in equilibrium with the force on the soil being only the paver's weight. (That compression of the soil may persist until the soil freezes and thaws, or until a beetle burrows through the area.)

Edit: So the force the table exerts on you, or the ground exerts on the paver, or the force the paver exerts on you is a response to the force you are exerting on those things. When you get off, there is no force on them that Newton would require them to respond to.

I hope this helps,
Steve

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Steve Johnson

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I would be delighted to help with questions up through the first year of college Physics. Particularly Electricity, Electronics and Newtonian Mechanics (motion, acceleration etc.). I decline questions on relativity and Atomic Physics. I also could discuss the Space Shuttle and space flight in general.

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I have a BS in Physics and an MS in Electrical Engineering. I am retired now. My professional career was in Electrical Engineering with considerable time spent working with accelerometers, gyroscopes and flight dynamics (Physics related topics) while working on the Space Shuttle. I gave formal classroom lessons to technical co-workers periodically over a several year period.

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BS Physics, North Dakota State University
MS Electrical Engineering, North Dakota State University

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