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CGP revision guide or physics teacher: which is correct?

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(edited 13 years ago)
Reply 1
Avatar for Lifesharker
Lifesharker
jfgwills5
newton's 3rd law of motion states each force experiences an opposite reaction force, i was just starting AS and learning this, CGP revision guide said that an apple sitting on a table was NOT an example, as there are two forces, the reaction force and weight
whereas my physics teacher said this WAS an example of newton's 3rd law of motion,
i personally think it's an example of newton's 1st law but i don't want to negate my teahcher as i obviously have respect for his profound knowledge in physics.


Your teacher is wrong as is the above poster.

The Newtons third law opposite is the gravitational pull of the apple on the earth, not the reaction force of the table, which has its own Newton third law partner in the reaction force of the apple on the table.

Think of it this way; if the apple was simply falling, it still has the gravitational force on it but not the reaction force of the table. However it still has a Newtons third law equal and opposite force in the gravitational pull of the apple on the earth.

The Newtons third law pair for the gravitational pull of the earth on the apple is the gravitational pull of the apple on the earth.

The Newtons third law pair for the reaction force of the table on the apple is the reaction force of the apple on the table. In this case, both pairs have the same magnitude.
(edited 13 years ago)
I think that Newton's third law says that the reaction force experienced by the apple from the table is equal in magnitude and opposite in direction to the force the table experiences from the apple.

This particular apple is in equilibrium; so its weight exactly cancels out the reaction force. And so the weight of the apple is exactly equal to the force the apple exerts on the table. I agree with your revision guide - this is not always so.

Your teacher isn't 'wrong' - just has simplified the explanation.
Your teacher was trying not to confuse you whereas CGP seem perfectly happy to do so.

Strictly, CGP are correct, the reaction does not directly oppose the weight of the apple. The weight of the apple produces a force on the surface of the table which the reaction force opposes. As all three forces have the same magnitude, CGP are being rather pedantic but I hope you can see that a force on an apple (the weight) cannot be opposed by a force on a table (the reaction) as they are two different objects.
(edited 13 years ago)

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