Gravity Ch5.4 8th

Gravity Chapter 5 Section 4

Objectives: Describe gravity and its effect on matter Explain the law of universal gravitation Describe the difference between mass and

weight

Gravity: The force of attraction between objects

that is due to their masses The force of gravity can change the motion

of an object by changing its speed, direction, or both

The Effects of Gravity on Matter: All matter has mass Gravity is a result of mass All objects experience an attraction toward all

other objects The gravitational force pulls objects toward each

other

Can you feel it? The mass of most objects is too small to cause

a force large enough to move objects toward each other Earth is massive enough to feel the pull!

The Size of Earth’s Gravitational Force Earth has a huge mass Gravitational force is large Force must be applied to overcome this

gravitational force Everything is pulled toward the center of the

Earth

Newton and the Study of Gravity 1665 – Sir Isaac Newton:

The Law of Universal Gravitation Describes the relationships between

gravitational force, mass, and distance Applies to all objects in the universe

The Law of Universal Gravitation: All objects in the universe attract each

other through gravitational force The size of the force depends on the

masses of the objects and the distance between the objects Formula

Understanding the Law in 2 Parts: Part 1: Gravitational Force Increases as

Mass

Increases

Imagine an elephant and a cat… Gravitational force is small between objects

that have small masses Gravitational force is large when the mass of one or both objects is large Figure 3 page 136 in text.

Law of Universal Gravitation: Part 2: Gravitational Force Decreases as

Distance

Increases

You the sun and the Earth… Planets stay in orbit because they have large masses Gravitational force is strong when the distance

between 2 objects is small If the distance increases, the gravitational force pulling them together decreases rapidly (Figure 5, pg. 137 in text)

Weight as a Measure of Gravitational Force Weight is a measure of gravitational force on an object Differences between weight and mass: Weight changes when the gravitational force changes Mass is the amount of matter in an object (does not

change) The moon’s gravitational force is about 1/6 the Earth’s

gravitational force

Units of Weight & Mass: SI unit of force = newton (N) Gravity is a force, weight is a measure of

gravity, weight is also measured in newtons SI unit of mass = kilograms (kg) On Earth, a 100g object weighs about 1 N.

Quick Quiz: What is the difference between mass and

weight? What must you know to calculate the

gravitational force between 2 objects? Where would you weigh the most, on a

boat, on the space shuttle, or on the moon?