8.3 Newton’s Laws of Motion

First Law of Motion An object at rest remains at rest and an object in motion keeps moving – unless acted on by a force

So the First Law is all about INERTIA Resistance to motion the more mass in an object, the harder it is to get it moving, OR to stop it when it’s already moving.

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

You and the car are traveling at same speed. When the car stops (is stopped by the wall), you keep going at the same speed – until YOU are acted on by the steering wheel. This is why you should wear a seat belt!

Newton’s Second Law F = ma Forces cause acceleration

The unit for force is NEWTON

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

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

Free Fall All falling objects (disregarding the effects of air resistance) fall at the same rate. All falling objects accelerate at the same rate. (9.8 m/s 2 ) Why?

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

WEIGHT Measure of the force of gravity. Weight = mass X free-fall acceleration weight = mass X 9.8 m/s 2 w = mg unit is NEWTON

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

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

Newton’s Third Law For every action, there is an equal and opposite reaction.

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

Swimmer pushes against the wall, and the wall pushes back, so she moves away from the wall!