Gravitational Forces

Gravitational Forces

Middle School

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Learning Objectives

• Define gravity and its relationship with the mass and distance between two objects.

• Differentiate between the concepts of mass and the measurement of weight.

• Describe gravity's role in creating orbits and causing tides in the solar system.

• Explain how gravity and momentum result in the elliptical orbits of planets.

Key Vocabulary

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Gravity

A force of attraction that exists between any two or more objects that have mass.

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Mass

The specific measure of the amount of matter contained within any physical body or object.

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Weight

The measure of the force of gravity that is exerted on an object's mass.

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Law of Universal Grav.

The attractive force between two objects decreases as the distance between them increases.

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Force at a Distance

A force that can affect an object without ever physically coming into contact with it.

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Orbit

The curved path that an object follows as it revolves around a much more massive object.

Key Vocabulary

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Elliptical Orbit

The oval-shaped path a celestial object follows in its orbit around another body in space.

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Tides

The regular rise and fall of the ocean's water level caused by the Moon's gravitational pull.

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What Is Gravity?

• Gravity is a force of attraction between any two objects that have mass.

• The more mass an object has, the stronger its gravitational pull becomes.

• The force of gravity gets weaker as the distance between objects increases.

• This is why planets far from the Sun feel a weaker pull.

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Multiple Choice

What is gravity?

• A force of attraction between any two objects that have mass.

• A force that pushes two objects apart.

• A type of energy that objects with mass contain.

• A force that only affects planets and stars.

Multiple Choice

What is the relationship between an object's mass and its gravitational pull?

• The gravitational pull between them becomes stronger.

• The gravitational pull between them becomes weaker.

• The gravitational pull is not affected by mass.

• The gravitational pull only changes with distance.

Multiple Choice

If two planets have the same mass, which one will experience a stronger gravitational pull from the Sun?

• The planet that is closer to the Sun.

• The planet that is farther from the Sun.

• The planet that has less mass.

• The planet that is spinning faster.

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Mass vs. Weight

Mass

• Mass is the total amount of matter that an object contains.

• An object’s mass remains constant and does not change based on its location.

• For instance, an astronaut has the exact same mass on Earth as on the Moon.

Weight

• Weight measures the force of gravity that is pulling on an object’s mass.

• An object’s weight can change depending on the gravitational force at its location.

• An astronaut weighs much less on the Moon because the Moon’s gravity is weaker.

Multiple Choice

Which of the following best defines an object's mass?

• The total amount of matter an object contains

• The force of gravity pulling on an object

• The size of an object in space

• The speed at which an object is moving

Multiple Choice

What is the primary difference between an object's mass and its weight?

• Weight is affected by gravity, while mass is not.

• Mass changes depending on location, while weight is constant.

• Mass and weight are two different words for the same thing.

• Weight is measured in grams, while mass is measured in pounds.

Multiple Choice

Imagine a robot is sent from Earth to Jupiter, where the force of gravity is much stronger. What will happen to the robot's mass and weight?

• Its mass will stay the same, but its weight will increase.

• Its weight will stay the same, but its mass will increase.

• Both its mass and its weight will increase.

• Both its mass and its weight will stay the same.

Forces That Act at a Distance

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Gravity

• A force that acts at a distance influences objects without physical contact.

• Gravity is a primary example of this type of invisible pulling force.

• Earth pulls a falling apple toward the ground without any direct contact.

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Magnetic Force

• This force is created by an object’s invisible but powerful magnetic field.

• This field can affect certain other objects without ever touching them directly.

• A magnet can easily pick up a paperclip from a short distance.

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Electrical Force

• This force exists between any two objects that have an electrical charge.

• Objects with opposite charges attract, while objects with like charges will repel.

• A charged balloon attracts hair, showing this force acting at a distance.

Multiple Choice

What is the defining characteristic of a force that acts at a distance?

• It can influence an object without making physical contact.

• It requires two objects to be physically touching.

• It is a force that always pushes objects apart.

• It only works on objects that are very large.

Multiple Choice

Based on the information, what do gravity, magnetic force, and electrical force have in common?

• They are all forces that can affect objects without touching them.

• They are all forces that only pull objects closer together.

• They are all created by an object's electrical charge.

• They are all forces that only affect metal objects.

Multiple Choice

If two balloons are observed pushing away from each other without any physical contact, what is the most likely explanation?

• A gravitational force pulling the two objects together.

• A magnetic force affecting two objects made of plastic.

• An electrical force between two objects with opposite charges.

• An electrical force between two objects with like charges.

Gravity in the Solar System

• The Sun's powerful gravity holds planets, asteroids, and comets in orbit.

• An orbit is the path an object takes around a larger object.

• Moons and satellites are held in orbit around planets.

Multiple Choice

What is an orbit?

• The path an object takes around a larger object.

• A collection of stars and planets held together.

• The force that pulls objects towards each other.

• A natural satellite that revolves around a planet.

Multiple Choice

What is the relationship between the Sun and the planets' orbits?

• The Sun's gravity keeps the planets in their paths.

• The planets are trying to move closer to the Sun for warmth.

• The moons of the planets push them forward.

• The planets all have the same size and speed.

Multiple Choice

What is the best explanation for a moon revolving around a distant planet?

• The planet's gravity is holding the moon in orbit.

• The Sun's gravity is stronger than the planet's gravity.

• The moon is a captured asteroid from the asteroid belt.

• The moon generates its own light and heat.

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Elliptical Orbits and Momentum

• An orbit is a balance between an object's forward momentum and gravity.

• Without this balance, an object would fly away or fall into the Sun.

• Planetary orbits are elliptical, so the planet's distance to the Sun changes.

• Planets move faster when closer to the Sun and slower when farther away.

Multiple Choice

What is required for a planet to maintain a stable orbit around the Sun?

• A balance between its forward momentum and the pull of gravity

• The speed at which the planet is spinning on its axis

• The distance between the planet and other planets

• The amount of light the planet reflects from the Sun

Multiple Choice

How does a planet's speed change as it travels in its elliptical orbit?

• The planet moves faster when it is closer to the Sun

• The planet's speed stays the same throughout its orbit

• The planet moves slower when it is closer to the Sun

• The planet's speed is determined by its size, not its distance

Multiple Choice

What would most likely happen if a planet's forward momentum suddenly increased, upsetting its balance with gravity?

• The planet would begin to move farther away from the Sun

• The planet would fall directly into the Sun

• The planet's orbit would become a perfect circle

• The planet's speed would immediately decrease

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Common Misconceptions about Gravity

Misconception	Correction
Mass and weight are the same thing.	Mass is matter in an object. Weight is the force of gravity on it.
There is no gravity in space.	Gravity is everywhere in space. It keeps planets and satellites in orbit.
Planets orbit the Sun in perfect circles.	Planetary orbits are actually elliptical, or oval-shaped.
Gravity is a force that only pulls objects down.	Gravity attracts any two objects with mass, pulling them toward each other.

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Summary

• Gravity is a non-contact force of attraction that depends on mass and distance.

• An object's mass is constant, but its weight changes with gravity's force.

• The Sun's gravity holds planets in orbit, which is a balance with momentum.

• The Moon's gravity is the primary force that causes the Earth's tides.

Poll

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

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