8.3 Newton’s Laws of 8.3 Newton’s Laws of MotionMotion

First Law of MotionFirst Law of Motion

An object at rest An object at rest remains at rest and remains at rest and an object in motion an object in motion keeps moving – keeps moving – unless acted on by a unless acted on by a forceforce

So the First Law is all about So the First Law is all about INERTIAINERTIA

Resistance to Resistance to motionmotionthe more mass in an the more mass in an object, the harderobject, the harderit is to get it moving,it is to get it moving,OR to stop it when it’s OR to stop it when it’s already moving.already moving.

Inertia = tendency of an object Inertia = tendency of an object to resist changes in its velocity.to resist changes in its velocity.

Inertia = tendency of an object Inertia = tendency of an object to resist accelerations.to resist accelerations.

You and the car are traveling at same speed. When the You and the car are traveling at same speed. When the car stops (is stopped by the wall), you keep going at the car stops (is stopped by the wall), you keep going at the

same speed – until YOU are acted on by the steering same speed – until YOU are acted on by the steering wheel. This is why you should wear a seat belt!wheel. This is why you should wear a seat belt!

Newton’s Second LawNewton’s Second Law

F = maF = maForces cause Forces cause accelerationacceleration

The unit for force is NEWTONThe unit for force is NEWTON

Newton gave credit to those who Newton gave credit to those who came before him...came before him...

Free Fall and WeightFree Fall and Weight

As crazy as he may As crazy as he may look, our friend does look, our friend does not wish to continue not wish to continue gravitational gravitational acceleration until he acceleration until he hits the ground. He hits the ground. He will soon make an will soon make an effort to slow his effort to slow his descent by pulling the descent by pulling the ripcord on his ripcord on his parachute. We hope.parachute. We hope.

Free FallFree Fall

All falling objects All falling objects (disregarding the effects (disregarding the effects of air resistance) fall at of air resistance) fall at the same rate.the same rate.

All falling objects All falling objects accelerate at the same accelerate at the same rate. (9.8 m/srate. (9.8 m/s22))

Why?Why?

INERTIA!INERTIA!

Sure, gravity is pulling downward.Sure, gravity is pulling downward.But the object is resisting.But the object is resisting.The more mass, the greater the pull The more mass, the greater the pull downward.downward.BUT, the more mass, the greater the BUT, the more mass, the greater the resistance.resistance.So everything ends up coming down at the So everything ends up coming down at the same rate. same rate.

WEIGHTWEIGHT

Measure of the force Measure of the force of gravity.of gravity.Weight = mass X Weight = mass X free-fall accelerationfree-fall accelerationweight = mass X 9.8 weight = mass X 9.8 m/sm/s22

w = mgw = mgunit is NEWTON unit is NEWTON

Terminal VelocityTerminal Velocity

Remember, air has mass.Remember, air has mass.So when you are falling, there are real So when you are falling, there are real molecules underneath you, that you can’t see. molecules underneath you, that you can’t see. They help to hold you up. Unfortunately, gravity They help to hold you up. Unfortunately, gravity wins. But there is a limit to how much you can wins. But there is a limit to how much you can speed up (accelerate) –because of the air speed up (accelerate) –because of the air underneath you as you fall. When the pushing underneath you as you fall. When the pushing up by the air, and the pulling down by gravity are up by the air, and the pulling down by gravity are equal, acceleration (speeding up) stops, and you equal, acceleration (speeding up) stops, and you keep falling, but you don’t fall any keep falling, but you don’t fall any faster.faster.

Sky divers Sky divers reach terminal reach terminal velocity at velocity at about 320 about 320 km /h (200 km /h (200 mi/h)mi/h)

Newton’s Third LawNewton’s Third Law

For every For every action, there action, there is an equal is an equal and opposite and opposite reaction.reaction.

Action – Reaction Pairs – foot Action – Reaction Pairs – foot presses on ice, you move in presses on ice, you move in

opposite direction.opposite direction.

Swimmer pushes against the wall, Swimmer pushes against the wall, and the wall pushes back, so she and the wall pushes back, so she

moves away from the wall!moves away from the wall!