# Applying Newton's 3rd Law

Use the third law of motion to explain why:
a) A person with ordinary shoes is able to walk on a sidewalk.
b) A rocket accelerates in the vacuum of outer space.

The 3rd law- for every action force, there is a reaction force equal in magnitude but opposite in direction.

I read over the textbook like 300 times..tell me where to start and i can go from there :|

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Char. Limit
Gold Member
Well, for starters, when you walk, what direction would you say that you are exerting a force (because you are) on the sidewalk?

Well, for starters, when you walk, what direction would you say that you are exerting a force (because you are) on the sidewalk?
umm..would you be exerting it on the sidewalk?..cause you're moving, the sidewalk isn't so, backwards..the opposite direction you're moving in? :S

Pengwuino
Gold Member
umm..would you be exerting it on the sidewalk?..cause you're moving, the sidewalk isn't so, backwards..the opposite direction you're moving in? :S
The sidewalk does move. However, since its attached to basically the earth, the force you exert on the sidewalk is basically the force you exert on the Earth which, obviously, is minuscule so you don't see the sidewalk (aka earth) move.

Char. Limit
Gold Member
umm..would you be exerting it on the sidewalk?..cause you're moving, the sidewalk isn't so, backwards..the opposite direction you're moving in? :S
Excellent. Now, apply Newton's Third Law of Motion to the equation. You are exertng a force on the sidewalk. What else must be going on?

Excellent. Now, apply Newton's Third Law of Motion to the equation. You are exertng a force on the sidewalk. What else must be going on?
penguin said the sidewalk is moving D:

the sidewalk must be exerting an equal force on you..in the opposite direction, and then there's gravity and the normal force..

Char. Limit
Gold Member
Ignore gravity and the normal force. They cancel each other out.

So, you have a net force on the sidewalk, and the sidewalk has an equal net force on you. Now, which one of you is more easily moved? You, or the Earth (via the sidewalk)?

Ignore gravity and the normal force. They cancel each other out.

So, you have a net force on the sidewalk, and the sidewalk has an equal net force on you. Now, which one of you is more easily moved? You, or the Earth (via the sidewalk)?

you, because u don't notice the earth moving

Char. Limit
Gold Member
you, because u don't notice the earth moving
And that's why you can move on a sidewalk.

And that's why you can move on a sidewalk.
thank youu :D
i got the second one already

Char. Limit
Gold Member
Nice.

And there you go... Application of Newton's Laws... I don't know what to say here...

Have a great day and a terrific Easter (it's still Easter here!)