Newton’s Laws
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Newton’s First Law of Motion. An object acted on by no external force has a constant velocity and zero acceleration.
- Forces are not required to sustain motion.
- Forces can perturb an already existing motion.
- This law is valid within an inertial frame of reference.
- Any frame of reference that moves relative to an inertial frame of reference with constant velocity is also inertial.
- Newton’s First Law means that no inertial observer is privileged over another — between two inertial observers, there is no way to say which observer is in motion or not. There is no absolute standard rest.
- The inertial characteristics of an object is characterized by its (inertial) Mass - the tendency of an object to change its motion
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Newton’s Second Law of Motion. If a net external force acts upon an object, the object accelerates in the direction of the net external force. This follows
- Another way to write this is with momentum
- Only the forces acting on an object determine its motion.
is not a force but a net force.
- Another way to write this is with momentum
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Newton’s Third Law of Motion. If
exerts a force on (an action), then also exerts an equal and opposite force n (a reaction). - Action reaction does not imply causal relationships.
- This is applicable regardless of how
and are moving. - The two forces in the equation are for two different objects. It does not imply any form of motion, it only relates action-reaction pairs.