Can anyone explain more for me Newton's 3rd Law of Motion?

[zeo2396]

As I learn about Newton's Third Law:For every force applied to object A by another object B, there is an equal and opposite force(reaction) applied to object B by object A.

I think if we had two equal and opposite forces exerts on a thing so it will not move!

Also, if there is an astronaut in outer space, what type of 2 forces can exerts on him? How can I recognize what force(action) exerts an equal and opposite on the second force (reaction) according to Newton'3rd Law?

Please help me to resolve these confuse!
Thank you!

 

[PhanthomJay]

As I learn about Newton's Third Law:For every force applied to object A by another object B, there is an equal and opposite force(reaction) applied to object B by object A.

yes, that's right

I think if we had two equal and opposite forces exerts on a thing so it will not move!

and this is wrong. Note from your first statement, the equal and opposite force acts on a different object, not the same object.

Also, if there is an astronaut in outer space, what type of 2 forces can exerts on him? How can I recognize what force(action) exerts an equal and opposite on the second force (reaction) according to Newton'3rd Law?

Please help me to resolve these confuse!
Thank you!

You are confusing Newton 3 with his other 2 laws. The first 2 laws state that a net unbalanced force acting on an object will accelerate that object in the direction of the net force, if there is one. The third law states what you said earlier, and applies to different objects. If a man is standing on a table, he doesn't move, because the weight force acting down on him by the earth, is balanced by the normal force of the table acting up on him. That's 2 forces (the gravity force of the Earth and the normal force of the table) acting on one object. Newton 3 says in this situation that since the table exerts a normal force on the man, the man exerts an equal and opposite normal force on the table; and since the Earth is exerting a force on the man, the man exerts an equal and opposite force on the earth. Each of these force pairs act on different objects.

Now suppose the table breaks and the man falls. Now there is just one force acting on the man, the Earth force of gravity, so the man must accelerate down, per Newton 2. Newton 3 says in this case, that the eatrth is exerting a force on the man, and the man is exerting an equal and opposite force on the earth. Of course, the man is still moving, due to Newton 2.

 

[zeo2396]

yes, that's rightand this is wrong. Note from your first statement, the equal and opposite force acts on a different object, not the same object.You are confusing Newton 3 with his other 2 laws. The first 2 laws state that a net unbalanced force acting on an object will accelerate that object in the direction of the net force, if there is one. The third law states what you said earlier, and applies to different objects. If a man is standing on a table, he doesn't move, because the weight force acting down on him by the earth, is balanced by the normal force of the table acting up on him. That's 2 forces (the gravity force of the Earth and the normal force of the table) acting on one object. Newton 3 says in this situation that since the table exerts a normal force on the man, the man exerts an equal and opposite normal force on the table; and since the Earth is exerting a force on the man, the man exerts an equal and opposite force on the earth. Each of these force pairs act on different objects.

Now suppose the table breaks and the man falls. Now there is just one force acting on the man, the Earth force of gravity, so the man must accelerate down, per Newton 2. Newton 3 says in this case, that the eatrth is exerting a force on the man, and the man is exerting an equal and opposite force on the earth. Of course, the man is still moving, due to Newton 2.

But if the astronaut is floating in the space, do you know what forces can exert on him and the other opposite force (reaction) (it's not gravity right?)

So, back to your example, if there is a man is standing on a table, he exerts a force on a table and a table also exerts an opposite equal force on him, so can you show me how to identify what force is action and what force is reaction (the man's force exerts or the table's force exerts).

I'm not sure I am right or wrong, but if I throw a boy to my friend, and he steps back a little bit, is it right that the ball's force exerts on my friend is action, and the force my friend exerts on a ball is reaction! I still have some confuse to identify action and reaction force!
Thanks.

 

[PhanthomJay]

But if the astronaut is floating in the space, do you know what forces can exert on him and the other opposite force (reaction) (it's not gravity right?)

Forget this action-reaction terminology. If an astronaut is floating in space, ther is just one force acting on him...and that would be his weight (the force of the Earth on him, the gravity force). The net force acting on him is the gravity force, since it is the only force acting on him. It causes him to fall, per Newton 2, but since he is moving tangentially around the Earth at tremendous speed, he is moving in a circle as he is falling, so he stays in orbit. Now also, since the Earth is exerting a force on him, he is exerting a force on the earth. Per Newton 3. A floating astronaut is a bad example to discuss Newton 3. A better example would be if one astronaut A pushed on another astronaut B ...If Astonaut A pushes on B, B moves and accelerates, per Newton 2, and since A is pushing on B, then B is pushing on A with an equal and opp force, per Newton 3, so A moves and accelerates also, the other way, per Newton 2.

So, back to your example, if there is a man is standing on a table, he exerts a force on a table and a table also exerts an opposite equal force on him, so can you show me how to identify what force is action and what force is reaction (the man's force exerts or the table's force exerts).

I guess that's why i don't like the action-reaction terminology. Ignore it.

I'm not sure I am right or wrong, but if I throw a boy to my friend, and he steps back a little bit, is it right that the ball's force exerts on my friend is action, and the force my friend exerts on a ball is reaction! I still have some confuse to identify action and reaction force!
Thanks.

Right...who or what is acting and who or what is reacting...and who cares?