Newton’s laws and periodic motion

 Objects at rest tend to stay at rest, while objects in motion tend to stay in motion, unless acted on by an outside force  Also called the Law of Inertia

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 A car traveling 12 mph crashes into a wall. The driver is injured because of Newton’s 1 st law  Seat belts and air bags seek to counteract Newton’s 1 st law  It takes energy to get out of a chair because of Newton’s 1 st law

 The tendency to resist change

 If object are in equilibrium, usually there is no motion.  Objects traveling at constant speed also exhibit equilibrium, though. Why?

 Objects travel in straight line unless acted on by an outside force  The force required to move an object depends on the object’s mass and the acceleration  Associate the 2 nd law with F = ma (or F = mg)

 For every action force there is an equal and opposite reaction force  Think of pushes and pulls

 Describes motion where an object moves, and then returns to it’s starting point

 Apparent weight – the force exerted by a scale the object Example : an elevator accelerating up or down would change your apparent weight how? Weightlessness is when apparent weight is 0 You still have mass, there is no contact force pushing on the scale, though.

 Static friction force – the force exerted by 1 object on another when there is no movement between the two objects  A book resting on the table

 Kinetic friction force - the force exerted by one object on another when there is motion between the two  Example: a car’s tires and the road while the car is moving  The force between a hockey puck and the ice as it slides across the rink

 Terminal velocity – when air resistance equals the pull of gravity  The object is falling, but is not accelerating any more

 Air drag (resistance) – the force exerted on objects moving thru the air