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As Newton's Laws states an object should be in rest or in constant velocity if no external force is applied. A man inside a stopped car cannot push the car as he is not giving any external force.But a man inside a bubble ball can make it move. What is the theory behind this? https://www.holleyweb.com/images/human_sized_hamster_ball_free_walking.jpg

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    $\begingroup$ Will the bubble ball be in touch with the ground? Thats an external force. $\endgroup$ – Mikael Fremling Aug 4 '17 at 15:18
  • $\begingroup$ Here an external force is applied: friction! $\endgroup$ – Spirine Aug 4 '17 at 15:18
  • $\begingroup$ There are all kinds of external forces here! i.stack.imgur.com/gMGYX.jpg $\endgroup$ – uhoh Aug 4 '17 at 16:21
  • $\begingroup$ Surely a man inside a stopped vehicle can push the vehicle. A bicycle, for example. $\endgroup$ – safesphere Aug 4 '17 at 23:12
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The simple answer is that there is an external force, that of friction towards the ground. The man and the ball can thus change their velocity by using the friction provided by the ground, much in the same way we can walk by using the frictions on the ground.

An important follow up question, that the OP touches upon, is how the ball is actually made to move. Here again friction plays an important role by applying different amount of force in different direction a net movement can be achieved in any particular direction.

As an example, if the man decides to jump forward inside the ball, that will make the ball move forward. The reason is that when jumping forward the friction on the ground stops the ball from moving backwards, but when he lands, he transfers his momentum to the ball, which begins to roll.

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  • $\begingroup$ Wouldn't the man push off against a side of the ball when he jumps to another side? He imparts momentum to the ball when he jumps, which is cancelled by the momentum of his landing. The ball would wobble, or roll a short way in one direction and then a short way in the opposite direction. The ball moves, but it doesn't go anywhere, I think, unless the man leans over and falls without jumping. $\endgroup$ – Ernie Aug 4 '17 at 16:39
  • $\begingroup$ @Ernie You are thinking of a symmetric jump scenario, where momentum is transfered from left to right, but most of the time that is not what going to happen. Consider for example a jump that is almost vertical and only directed a little bit forward. The friction prevent the ball from moving back during the beginning of the jump, but then helps the ball to roll forward after the landing, due to torque. The net movement happens due to the asymmetric torque applied before and after the jump. $\endgroup$ – Mikael Fremling Aug 8 '17 at 8:37
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By moving his weight around inside the bubble ball, and utilizing friction between his body and the ball, coupled with friction between the ball and the ground, the man can cause gravitational force to topple the ball and move it over the ground.

If the center of mass of the ball moves to one side of the ball, gravity will topple the ball toward its center of mass. Because the ball is in a uniform gravitational field, the ball's center of gravity is also its center of mass. By changing the ball's center of mass, the man changes its center of gravity. When the center of gravity changes, gravity moves the ball.

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