newton's third law!

hi

if everything exerts an equal force on everything else, how does anything accelerate (or even move, i guess)? like, how isn't everything immediately 'cancelled out'??

ty

Reply 1

KeyFingot

Original post by genuinelydense

hi

if everything exerts an equal force on everything else, how does anything accelerate (or even move, i guess)? like, how isn't everything immediately 'cancelled out'??

ty

Because the opposite force is not necessarily acting on the same object. A car "pushes" on the Earth and the Earth pushes on the car, which is why it can accelerate.

Reply 2

genuinelydense

Original post by KeyFingot

Because the opposite force is not necessarily acting on the same object. A car "pushes" on the Earth and the Earth pushes on the car, which is why it can accelerate.

but surely air particles and all that stuff push on the car, stopping the car from accelerating??

Reply 3

KeyFingot

Original post by genuinelydense

but surely air particles and all that stuff push on the car, stopping the car from accelerating??

You mean air resistance?

Reply 4

genuinelydense

Original post by KeyFingot

You mean air resistance?

i'm talking about _bodies_ (as in "When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction to that of the first body."). i guess i don't understand this law at all, because it seems to me that one molecule of...anything..which happens to get in the way of the car's motion would stop it from accelerating.

(edited 10 years ago)

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Reply 5

Stonebridge

Original post by genuinelydense

hi

if everything exerts an equal force on everything else, how does anything accelerate (or even move, i guess)? like, how isn't everything immediately 'cancelled out'??

ty

Because the forces in Newton's 3rd Law are acting on different objects, not the same object.
If they were acting on the same object they would cancel, but the Law says A applies a force to B and B applies an equal and opposite force on A.
So A just has the one force and B just has the one force. A can accelerate and so can B.
The Law does not say both forces act on A or both forces act on B.

If someone stands behind you and pushes you forwards with a force of 12N and I stand in front and push you back with a force of 5N, you have a resultant force of 7N pushing you forwards, so you accelerate forwards. Why would you not do so because of Newton's 3rd Law? All the 3rd Law says is that you also push me with a force of 5N. That force determines whether I accelerate, not you.

(edited 10 years ago)

Reply 6

krisshP

Original post by genuinelydense

hi

if everything exerts an equal force on everything else, how does anything accelerate (or even move, i guess)? like, how isn't everything immediately 'cancelled out'??

ty

Just as above posters have said, the forces in a Newton's third law pair act on different objects. Think of a bird in air. The bird exerts a downward force on the air and the air therefore exerts an upward force on the bird by Newton's third law. This is why the bird would accelerate upwards, assuming this force>the bird's weight and no other vertical forces on it. Note the objects the 2 forces act on. One force is exerted by bird on air and the other is exerted by the air on the bird.