# Gravity and Motion

Gravity and Motion

State Standard
2.D.1.d: Recognize that the pull of gravity causes the pattern of motion of celestial objects

Objectives
1.2.1 Identify what determines the strength of the force of gravity between two objects.
1.2.2 Describe two factors that keep the moon and Earth in orbit.

Big ideas
1. Earth revolves around the sun in the same way the moon orbits Earth.
2. Gravity attracts all objects toward each other.

Enduring Understanding
1. The strength of the force of gravity between two objects depends on the masses of the objects and the distance between them.
2. Inertia and gravity keep Earth in orbit around the sun and the moon in orbit around Earth
3. The law of universal gravitation states that every object in the universe attracts every other object.

Essential Questions
1. How are gravity and weight related?
2. Why doesn’t Earth simply fall into the sun?
3. What determines the strength of the force of gravity between two objects?
4. What two factors combine to keep the moon and earth in orbit?

OVERVIEW
Earth revolves around the sun in a nearly circular orbit. The moon orbits Earth in the same way, But what keeps Earth and the moon in orbit? Why don't they just fly off into space?
The first person to answer these questions was the English scientist Isaac Newton. Late in his life, Newton told a story of how watching an apple fall from a tree in 1666 had made him think about the moon’s orbit. Newton realized that there must be force acting between Earth and the moon that kept the moon in orbit. A force is push or a pull. Most everyday forces require objects to be in contact. Newton realized that the force that holds the moon in orbit is different in that it acts over long distances between objects that re not in contact.

# Gravity and introduction

Discovery Activity
Can you remove the bottom penny?

Materials – 25 pennies, thin plastic ruler or thin spatula
Procedure:
1. Place 25 or so pennies in a stack on a table.
2. Write down your prediction of what will happen if you attempt to knock the bottom penny out of the stack.
3. Quickly slide a ruler along the surface of the table and strike the bottom penny. Observe what happens to the stack of pennies.
4. Repeat step 3 several times, knocking more pennies from the bottom of the stack

Think in over: Developing Hypothesis
Explain what happened to the stack of pennies as the bottom penny was knocked out of the stack.

GRAVITY
Newton hypothesized that the force that pulls an apple to the ground also pulls the moon toward Earth, keeping it in orbit. This force, called gravity, attracts all objects toward each other. In Newton’s day, most scientists thought that forces on Earth were different from that elsewhere in the universe. Although Newton did not discover gravity, he was the first person to realize that gravity occurs anywhere. Newton’s law of universal gravitation states that every object in the universe attracts every other object.
The force of gravity is measured in units called newtons, named after Isaac Newton. The strength of the force of gravity between two objects depends on two factors: the masses of the objects and the distance between them.

Gravity, Mass, and Weight
According to the law of universal gravitation, all of the objects around you, including Earth and even this book, are pulling on you, just as you are pulling on them. Why don’t you notice a pull between you and the book? Because the strength of gravity depends in part on the masses of each of the objects. Mass is the amount if matter in an object.
Because Earth is so massive, it exerts a much greater force on you than this book does. Similarly, Earth exerts a gravitational force on the moon, large enough to keep the moon in orbit. The moon also exerts a gravitational force on Earth.
The force of gravity on an object is known as it weight. Unlike mass, which doesn’t change, an object’s weight can change depending on its location. For example, on the moon you would weigh about one sixth of your weight on Earth. This is because the moon is much less massive than Earth, so the pull of the moon’s gravity on you would be far less than that of Earth’s gravity.

 The strength of the force of gravity between two objects depends on their masses and the distance between them.

To see the difference between mass and weight and its relative connection to gravity, watch the video.

International Space Station

Here's a cool game that Galileo Galilei himself experimented to test gravity. Try it yourself.

Galileo drops the ball

Here's another simulation activity.

http://www.messenger-education.org/Interactives/ANIMATIONS/grav_assist/gravity_assist.html

Gravity and Distance
The strength of gravity is affected by the distance between two objects as well as their masses. The force of gravity decreases rapidly as distance increases. For example, if the distance between two objects were doubled, the force of gravity between them would decrease to one fourth of its original value.

INERTIA AND ORBITAL MOTION
If the sun and Earth are constantly pulling on one another because of gravity, why doesn’t Earth fall into the sun? Similarly, why doesn’t the moon crash into Earth? The fact that such collisions have not occurred shows that there must be another factor at work. That factor is called inertia.
The tendency of an object to resist a change in motion is inertia. You feel the effects of inertia every day. When you are riding in a car and it stops suddenly, you keep moving forward. If you didn’t have a seat belt on, your inertia would cause you to bump into the car’s windshield or the seat in front of you. The more mass an object has, the greater its inertia. An object with greater inertia is more difficult to star or stop.
Isaac Newton stated his ideas about inertia as a scientific law. Newton’s first law of motion says that an object at rest will stay at rest and an object in motion will stay in motion with a constant speed and direction unless acted on by a force.

Orbital Motion
Why do Earth and the moon remain in their orbits? Newton concluded that two factors – inertia and gravitycombine to keep Earth in orbit around the sun and the moon in orbit around Earth.

 A combination of gravity and inertia keeps the moon in orbit around Earth. If there were no gravity, inertia would cause the moon to travel in a straight line. What would happen to the moon if it were not moving in orbit?