Gravity and the Universe

Gravity, Inertia & Orbits / Study Guide

The Earth's gravity is the reason materials, people, plants, and animals are held to its surface. Gravity is a force based on an object's mass that causes it to attract, or pull, other objects toward it.

All objects have some gravitational force. This is known as the law of universal gravitation, which states that every object in the universe attracts every other object. However, gravity's strength is determined by the object's mass, or the amount of matter in that object. Therefore objects with a greater mass have a stronger force of gravity.

The distance between two objects also impacts the gravitational force between them. Objects that are closer together will exert more gravitational force on one another, while objects that are farther apart will exert less. Objects such as stars, planets, and moons are very large, making their gravitational force more noticeable than the gravitational force of a smaller object, like a computer.

Weight is the force on an object due to gravity. While mass stays the same regardless of location, weight can increase and decrease depending on the force of gravity. An object in a location with weaker gravitational force would have a lower weight than the same object in a location with stronger gravitational force.

Multiple Select

Which of the following objects have gravitational force? Select all that do.

• moons

• comets

• stars

• planets

• asteroids

Multiple Choice

Which two objects would have the greatest gravitational force between them.

Multiple Choice

Which two objects would have the least gravitational force between them.

Multiple Choice

The mass of planets can vary. This also impacts their gravity. Use the information about Venus and Mars shown at left (click to enlarge) to complete this sentence.

An object would have the same ______ on both planets

• mass

• weight

Multiple Choice

The mass of planets can vary. This also impacts their gravity. Use the information about Venus and Mars shown at left (click to enlarge) to complete this sentence.

An object's weight on _______ would be less than the same objects weight on _______.

• Mars; Venus

• Venus; Mars

Multiple Choice

_______ is determined by the gravitational force on an object.

• mass

• weight

A person on the Earth who jumps into the air is quickly brought back down to the Earth's surface due to gravity. With this example in mind, how is it that larger objects like the Moon are not brought crashing down to the Earth's surface in the same way? The answer lies in a principle called inertia.

Inertia is the tendency of an object to resist a change in motion. For example, an object on a car is moving forward at the same rate as the car. If the car comes to a sudden stop, the object will continue to move forward unless it is secured. This is inertia at work.

This is inertia at work. The object in the car resists a change to its motion. An object's inertia also acts to keep it moving in a straight line at the same speed. Without the interference of another force (the seatbelt), an object would continue moving the same direction and speed.

This idea led scientist Isaac Newton to develop Newton's first law of motion, which states that an object at rest will stay rest and an object at motion will stay in motion with a constant speed and direction unless acted on by a force. Newton's first law of motion helps explain why objects revolving around other objects in space maintain their orbits without crashing into each other's surface.

Gravity and inertia combine to keep planets in orbit around the Sun, moons in orbit around planets, and other objects in orbit around each other in the solar system. As gravity pulls a smaller object in space twards a larger object, the smaller object's inertia acts against gravity to keep its direction and speed from changing. This interaction curves the smaller object's path, or orbit, causing it to revolve around the larger object in a mostly circular path.

Multiple Choice

Jeremiah is traveling in a car that suddenly comes to a stop. Jeremiah's seat belt helps stop his ________.

• gravity

• mass

• inertia

• forward

Multiple Choice

Without the seat belt, Jeremiah's body would continue moving ______ until acted byu _______.

• backward; gravity

• backward; another force

• forward; mass

• forward; another force

Multiple Choice

Samantha is observing two objects. Object A is at rest, while Object B is in motion. If no forces act on the objects, Newton's first law predicts Object A will ______, while Object B will _______.

• start moving; slow unti it comes to a rest

• remain at rest; remain in motion but change speed and direction

• start moving; remain in motion at a constant speed and direction

• remain at rest; remain in motion at a constant speed and direction

Multiple Choice

Gravity is _______.

• the force based on an object's weight that causes it to repel other objects

• the force based on an object's weight that causes it to attract other objects

• the force based on an object's mass that causes it to repel other objects

• the force of attraction between two objects due to their masses and distance from one another

Multiple Choice

What is the definition of mass?

• a measure of the distance across an object

• a measure of the amount of gravity in an object

• a measure of the amount of matter in an object

• a measure of the amount of space an object takes up

Multiple Choice

What is weight?

• a measure of the amount of space an object takes up

• a measure of the force of inertia on an object

• a measure of the force on an object due to gravity

• a measure of the amount of matter in a given amount of space

Multiple Choice

Inertia is _______.

• the tendency of an object to change direction

• the tendency of an object to cause a change in motion

• the tendency of an object to resist a change in motion

• the tendency of an object to increase in speed

Multiple Choice

What is the law of universal gravitation?

• a scientific law that states that every object in the universe attracts every other object

• a scientific law that states that every object in the universe repeals every other object

• a scientific law that states that every object in the universe only attracts objects that are larger than itself

• a scientific law that states that every object in the universe only attracts objects that are smaller than itself

Multiple Choice

What is Newton's first law of motion?

• a scientific law that states that all objects attract one another with the same force and strength

• a scientific law that states that an object at rest will stay at rest and an object in motion will stay in motion with a variable speed and direction unless acted on by a force

• a scientific law that states 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

• a scientific law that states that an object at rest and an object in motion cannot be acted on by any forces other than gravity

Multiple Choice

The Earth's gravity does not attract the Sun.

• True

• False

Multiple Choice

The Sun's gravity does not attract the moon.

• True

• False

Multiple Choice

Only the Moon's gravity keeps it in orbit around the Earth.

• True

• False

Multiple Choice

Which word from this unit of study goes with Label 1 on the diagram at left (click to enlarge).

• inertia

• gravity

• orbit

Multiple Choice

Which word from this unit of study goes with Label 2 on the diagram at left (click to enlarge).

• inertia

• gravity

• orbit

Multiple Choice

Which word from this unit of study goes with Label 3 on the diagram at left (click to enlarge).

• inertia

• gravity

• orbit

Multiple Select

Jamal has drawn a diagram to represent how gravity works between two objects. What is wrong with his diagram? Check all that are true.

• The force of gravity attracts two objects together, but it does not repel them.

• The gravity of objects that are closer together is weaker than the gravity of objects that are farther apart.

• Objects with a lower weight have a higher gravitational force.

• An object's mass, not its weight, determines the strength of its gravity.

Multiple Select

Which factors influence an object's orbit around another object in space? Check all that are true.

• inertia

• gravity

• electric charge

• tides