Universal Gravitation

Universal Gravitation

High School

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Learning Objectives

• Recall that gravity is an attractive force between all objects with mass.

• Identify the factors that influence the force of gravity, namely mass and distance.

• Distinguish between the concepts of mass and weight.

• Explain how gravity and inertia work together to keep objects in orbit.

Key Vocabulary

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Gravity

A universal force that attracts all objects with mass toward one another.

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Mass

The total amount of matter that is contained within an object and remains constant regardless of location.

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Weight

A measure of the force of gravity acting on an object's mass, which can change with location.

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Inertia

The natural tendency of any object to resist a change in its current state of motion.

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Universal Gravitation

Newton's law stating that every object in the universe attracts every other object with a measurable force.

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The Universal Force of Gravity

• A force is simply a push or a pull that acts on an object.

• Gravity is a fundamental force that attracts any two objects with mass.

• The strength of this force depends on the mass of each object.

• It also depends on the distance separating the two objects from each other.

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Solved Example 1
Two students with masses of 70 kg and 80 kg are standing 1 meter apart. What is the gravitational force between them? (G = 6.67 x 10^-11 N·m²/kg²)

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Step 1: Analyze and Sketch the Problem

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Solved Example 1
Two students with masses of 70 kg and 80 kg are standing 1 meter apart. What is the gravitational force between them? (G = 6.67 x 10^-11 N·m²/kg²)

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Step 2: Solve for the Unknown

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Solved Example 1
Two students with masses of 70 kg and 80 kg are standing 1 meter apart. What is the gravitational force between them? (G = 6.67 x 10^-11 N·m²/kg²)

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Step 3: Evaluate the Answer

Multiple Choice

According to Sir Isaac Newton's foundational observations, what two factors determine the force of attraction between any two objects?

• Their masses and the distance between them.

• Their temperature and their chemical composition.

• Their density and their shape.

• Their speed and their volume.

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Law of Universal Gravitation

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Multiple Choice

According to the Law of Universal Gravitation, if the distance between two objects doubles, what happens to the gravitational force?

• It is halved.

• It quadruples.

• It doubles.

• It is reduced to one-fourth of its original value.

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Distinguishing Mass from Weight

• Mass is the amount of matter in an object and is always constant.

• Weight is the force of gravity acting on an object’s mass.

• An object's weight can change depending on its location in the universe.

• You would weigh less on the Moon, but your mass remains the same.

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Solved Example 3
An 80 kg person stands on Earth. Given Earth's mass is 5.97x10²⁴ kg and its radius is 6.37x10⁶ m, what is the person's weight (the force of gravity acting on them)?

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Step 1: Analyze and Sketch the Problem

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Solved Example 3
An 80 kg person stands on Earth. Given Earth's mass is 5.97x10²⁴ kg and its radius is 6.37x10⁶ m, what is the person's weight (the force of gravity acting on them)?

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Step 2: Solve for the Unknown

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Solved Example 3
An 80 kg person stands on Earth. Given Earth's mass is 5.97x10²⁴ kg and its radius is 6.37x10⁶ m, what is the person's weight (the force of gravity acting on them)?

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Step 3: Evaluate the Answer

Multiple Choice

If an astronaut travels from Earth to the Moon, which of their physical properties would decrease?

• Their weight

• Neither their mass nor weight

• Both their mass and weight

• Their mass

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Inertia and Orbital Motion

• An object with more mass has greater inertia, resisting changes in motion.

• Newton's first law states an object in motion stays in constant motion.

• Inertia and gravity work together to maintain stable orbits for celestial bodies.

• The Moon's inertia pushes it forward while Earth's gravity pulls it inward.

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Multiple Choice

What two principles combine to keep the Moon in orbit around the Earth?

• Force and acceleration

• Speed and velocity

• Mass and weight

• Gravity and inertia

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Common Misconceptions About Gravity

Misconception	Correction
Gravity only exists on planets.	Gravity is a universal force between any two objects with mass.
Mass and weight are the same.	Mass is an object's matter. Weight is the force of gravity on it.
There is no gravity in space.	Astronauts in orbit feel weightless because they are in constant freefall.

Multiple Choice

How would the gravitational force between two celestial bodies change if the mass of one body was tripled while the distance between them remained the same?

• The force would be reduced to one-third.

• The force would triple.

• The force would be halved.

• The force would remain the same.

Multiple Choice

According to Newton's First Law of Motion, what would happen to a planet orbiting the Sun if the Sun's gravity suddenly disappeared?

• It would fly toward the center of the galaxy.

• It would continue moving in a straight line into space.

• It would stop moving and remain stationary.

• It would begin orbiting the nearest, most massive planet.

Multiple Choice

Planet X has twice the mass of Earth but the same radius. How would your weight on Planet X compare to your weight on Earth?

• It would be half.

• It would be double.

• It would be four times as much.

• It would be the same.

Multiple Choice

Imagine two asteroids of equal mass in space. If the distance between them is reduced by half, what is the resulting effect on the gravitational force between them?

• The force becomes four times stronger.

• The force becomes one-quarter as strong.

• The force becomes half as strong.

• The force becomes two times stronger.

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Summary

Poll

On a scale of 1-4, how confident are you about the concepts covered in today's review?

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