Gravity

F 34-40

Why would air make a difference?

• If you drop a ball and a feather from the same height at the same time, the ball would hit the ground first– This means the ball has fallen at a greater

rate• It is important to realize that when the ball and

feather are falling they both pass through air• air offers resistance to the motion of objects

• Since the ball has more mass, the air resistance is greater on the feather than on the ball– This causes the ball to fall faster than the

feather• Scientists have learned that if you take

away air resistance, objects of different weights will fall at the same rate (like on the moon)

What makes object fall at the same rate?

• Aristotle said that heavy things fell faster than lighter things– This was believed to be true until about 400

years ago• In the early 1600’s Galileo challenged

Aristotle when he said that things fall at the same rate– He was ignoring air resistance

• To test his theory Galileo carried out experiments and concluded that :– object accelerate steadily as they fall – an objects weight (mass) does not affect how

fast it accelerates when falling• We know today that Galileo was right

because we know about gravity– The attraction between the mass of Earth and

the mass of an object

• Objects with a large mass are pulled on by gravity with more force but they also have more inertia (resistance to change in motion)

• Because of this objects with greater mass will fall with the same acceleration as less massive objects

What is the acceleration of falling objects?

• There is a legend that Newton cam up with the idea of gravity from being hit on the head while sitting under an apple tree

• What is for sure is that Newton came up with the idea that there is an invisible force pulling objects to the ground

• This is the same force that keeps the Moon in the Earth’s orbit

• In the case of a falling apple the unbalanced force acting upon the apple is gravity– We give the force of gravity on any object a

name weight • It is the weight of the apple that makes it accelerate

to the ground

• A combination of the Moon’s inertia and the force of gravity between the Earth and the Moon that keep it in orbit

How can gravity be universal?

• Newton said that as mass increases the force of gravity does also

• Newton’s Law of Universal Gravitation– The force of gravity between two objects increases

with the mass of the object and it decreases with the distance between them squared

• It is universal because it applies to all objects not just moons, planets and stars

• Look at F 40 for times when added weight may be helpful