Newton's Three Laws of Motion

Newton's Three Laws of Motion are extremely important, and are the basis for much of physics.
First law of motion: A body continues in a state of uniform motion unless acted on by a net external force.
The converse is also true, so that if a body is in a state of uniform motion, there is no net external force. If a body is at rest, with no motion or acceleration - then there is no external force.
A body moving in a circle may be moving at a constant speed, but motion in a circle implies a change in the direction of motion, so the velocity is not constant and there is a force acting on the body.
Second Law of motion: The second law is summed up by the equation  
\[\mathbf{F}=m \mathbf{a}\]
. A bbody of mass  
  acted on by an external force  
  experiences an acceleration given by  
, in the direction of the force  
A body can be momentarily at rest, and still experience an acceleration. Newton's second law says nothing of velocity, only force and acceleration.
Third law of motion: To every action there is a reaction.
Every body involved in a collision experiences an impulse. Two bodies involved in a collision experience equal and opposite impulses. The net impulse in any collision is zero. Because of this, total momentum is conserved. This gives rise to its own principle - the principle of conservation of momentum. One final implication of the third law is that any such action and reaction forces must be equal in nature, both gravitational, both electrical etc.

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