Falling Objects and Gravity

Air Resistance

• When an object falls, gravity pulls it down.• Air resistance works opposite of gravity and

opposes the motion of objects that move through the air.

• Air resistance causes different objects to fall with different accelerations at different speeds.– Amount depends on the speed, size, and shape of

the object.

What if air resistance didn’t exist?

This shoes a feather and an apple falling in avacuum.

There is no air resistance in a vacuum so the feather and apple fall with the same acceleration.

Outside of the vacuum, the apple falls muchfaster due to its mass and shape.

Terminal Velocity

• As an object falls faster, air resistance increases.• Eventually the upward air resistance will equal

the downward force of gravity causing the net force on the object to be zero.

• Terminal velocity – highest speed a falling object will reach

• Terminal velocity depends on size, shape, and mass of the falling object.

Terminal Velocity

The force of air resistance on an openparachute balances the force of gravity.

The parachute opens and the terminalvelocity of the sky diver becomes smallenough that he/she can land safely.

Gravity• Gravity – an attractive force between two

objects– Depends on the masses of the objects and the

distance between them

You

Your book is closeenough to exert aforce you can feel but too small.

Jupiter is largeenough to exert a noticeableforce but too far away.

The Earth is large enough andclose enough to exert and noticeableforce.

The Law of Universal Gravitation

• Newton’s equation enables the force of gravity to be calculated between any two objects if their masses and the distance between them is known.

G = universal gravitation constantm1 = mass of the 1st objectm2 = mass of the 2nd objectr = distance between the two masses

What Does the Law of Universal Gravitation Mean?

• The gravitational force between two masses decreases rapidly as the distance between the masses increases.

• No matter how far apart two objects are, the gravitational force never completely goes to zero!

The Earth’s Gravitational Acceleration

• Close to the Earth’s surface, the acceleration of an object in free fall is 9.8 m/s2.

• This acceleration constant is sometimes labeled g.

• By Newton’s 2nd law of motion, the force of Earth’s gravity on a falling object is

F = mg

Example

• What is the gravitational force on a sky diver with a mass of 600 kg?

Weight

• Even if you aren’t falling, the force of Earth’s gravity is still pulling you downward.

• Weight – the gravitational force exerted on an object

• Weight can be calculated by the following equation:

W = mg

Weight and Mass

• Weight and mass ARE NOT THE SAME THING!

• Weight is a measure of force.• Mass is the measure of the amount of matter

in an object.

• Weight changes as gravity changes; mass does not.