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Newton's first law states that a body will remain at rest or in motion until an external force is applied on it. Now consider a dog sleeping in a room and after some time it suddenly woke up and started running so its state of rest changed to that of motion but no external force is applied on it. So this opposes the Newton's first law of motion. Does it means that a person cannot move by himself? And if this is beacuse of friction force then what if a person moves his hand in air then which external acts on his hand.

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I don't have enough reputation point to comment. Hence this answer ...

The sleeping dog is at rest. When it wakes up, it pushes the ground and the ground in return pushes it back. The dog moves by the reaction push given by the floor. This is as simple as that.

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    $\begingroup$ This shouldn't be a comment anyway; it was right to post it as an answer. $\endgroup$ – David Z May 20 '18 at 4:57
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    $\begingroup$ My humble request to you : Please don't downvote the question. It is not a homework question and the OP has some interest to learn, I think. $\endgroup$ – The Monk May 20 '18 at 5:01
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    $\begingroup$ Neither of those things are reasons not to downvote a question. $\endgroup$ – David Z May 20 '18 at 5:08
  • $\begingroup$ @DarkKnight it would not be wise to randomly accuse someone of downvoting the question. $\endgroup$ – QuIcKmAtHs May 20 '18 at 5:12
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    $\begingroup$ @QuIcKmAtHs it would not be wise to randomly accuse someone of randomly accusing someone of downvoting the question $\endgroup$ – pentane May 20 '18 at 6:24
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The external forces making the dog move is its paws making contact with the ground and moving so that it tries to make the floor move, but it doesn't. This results in friction, making the dog move.

The dog exerts forces on the floor and the floor exerts those same forces on the dog. This is how we walk and drive cars. Without friction, common movement as we know it would not be possible

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Well Newton’s third law states that for every force acting on a body there is a reaction force. Hence when the dog exerts a force on the ground, the ground also exerts a force which gives a dog acceleration, allowing it to move.

Another way to explain this is through energy. All living things possess chemical potential energy from what we eat, which can be converted to kinetic energy for movement. This is an internal energy, so it does not violate Newton’s first law at all!

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protected by Qmechanic May 20 '18 at 10:01

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