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Sorry if this question's been asked. I looked through some of the questions but didn't find a similar enough question.

My question on Newton's Third Law: It's my understanding that if you were in outer space and you threw a tennis ball then both you and the tennis ball would move but in opposite directions. Because you exert a force on the ball and it exerts an equal force on you. So how come when you push a wall on earth you get pushed back but not when you throw a ball. Can't you throw a ball with the same force you pushed the wall- if that's the case how come you don't move back? It's putting a force on you equal to the wall's force, right?

I do appreciate all the answers I've got but I guess I'm still confused. I understand that the ground pushes me in opposition to the wall's direction of force when the wall pushes my body, therefore my feet, on the ground away from the wall. But how come your arm and consequently your torso moves back when pushing a wall but not when throwing a ball.

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    $\begingroup$ the friction on your feet, opposite to the force of the ball, keeps you from moving backwards $\endgroup$
    – user83548
    Jun 5 '16 at 1:46
  • $\begingroup$ I get that friction keeps your feet planted to the ground when you push a wall, but your torso still moves because your arm is being pushed back. I guess what I'm not understanding is that your torso or arm don't seem to be getting pushed back when you throw something. Can't you throw a ball with the same force that you push the wall, therefore the ball would push back with the same force the wall did? Do we just not notice it as humans? $\endgroup$
    – Jaull
    Jun 5 '16 at 2:14
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    $\begingroup$ Your entire body is pushed back, if you push a ball hard enough without making your body more or less rigid you will fall back $\endgroup$
    – user83548
    Jun 5 '16 at 2:51
  • $\begingroup$ The force exerted on the ball is small in comparison to other forces involved: you not only accelerate the ball, but also your hand and arm while throwing, and after the ball leaves your hand you exert force in the opposite direction to slow them down again. The effect is more obvious when you consider a large mass, say a block of concrete. If you put your feet together, and throw (push) that block with both hands as far as you can in front of you, without moving your feet, you'll probably lose your balance and fall backwards. $\endgroup$
    – Previous
    Jun 5 '16 at 3:12
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When you throw a ball, you push it and it pushes you back. It just weighs slightly less than a wall (or the entire earth) so you don't need to push as much to move it.

You push a ball with enough force to move by acceleration $a$. This force is $F= m a$. This force is also exerted by the ground as a reaction to keep you in place. If you try to throw a ball on ice you will notice you will slide backwards or fall down.

Now if you try to "throw a wall" you need a force $F = m a$ that exceeds both the traction on your feet and what you can physically provide so nothing happens, or you slip and fall down.

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The third law does not say that you have to move backwards. All it says is that the force that you exert on the ball (or the wall) is the same as the force that the ball (or the wall) exerts on you, but in the opposite direction. In the case where you are throwing a ball, the force that the ball exerts back on you is not the only horizontal force that is acting on you. There is also a frictional force that the ground exerts on you to help hold you in place. This is the same kind of frictional force that is there when you push on a wall.

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  • $\begingroup$ I get that friction keeps your feet planted to the ground when you push a wall, but your torso still moves because your arm is being pushed back. I guess what I'm not understanding is that your torso or arm don't seem to be getting pushed back when you throw something. Can't you throw a ball with the same force that you push the wall, therefore the ball would push back with the same force the wall did? Do we just not notice it as humans? $\endgroup$
    – Jaull
    Jun 5 '16 at 2:16
  • $\begingroup$ @Jaull I think the issue is simply that you've never thrown a ball as hard as you've pushed a wall. It is very easy to exert a large force on a wall, because it sits there and takes it; it's very hard to exert a large force on a ball, since when you exert just a small force, it immediately begins travelling away from your hand. $\endgroup$
    – Jahan Claes
    Jun 5 '16 at 2:37
  • $\begingroup$ @Jaull If you hold the ball still (say, by pushing it against a wall) you will feel the effect. $\endgroup$
    – Jahan Claes
    Jun 5 '16 at 3:19
  • $\begingroup$ @Jaull If you fire a gun, you get a "kick back." Here you have exerted enough force to push your hand back. $\endgroup$
    – Chet Miller
    Jun 5 '16 at 12:22
  • $\begingroup$ @Jaull When you throw a ball, your muscles are also giving your arm forward momentum. The ball just slows this down a little. Consider the cases where (a) you make a throwing motion with your arm, but without the ball present and (b) you make a throwing motion with your arm, but it is also accelerating the ball. The speed you can achieve with your arm is faster in (a) than in (b). Force doesn't produce a change in displacement, it produces a change in velocity. $\endgroup$
    – Chet Miller
    Jun 5 '16 at 12:34

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