Force of Gravity

Force of Gravity

Middle School

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Learning Objectives

• Define gravity as a force that depends on mass and distance between objects.

• Differentiate between mass as a measure of matter and weight as gravity's force.

• Describe how gravity controls the orbital motions of planets, moons, and galaxies.

• Explain how gravity holds together the solar system and the Milky Way galaxy.

Key Vocabulary

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Gravity

An attractive force that exists between any two objects with mass, pulling them toward each other.

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Mass

The measure of the amount of matter or substance that makes up a physical object.

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Weight

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

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Net Force

The overall force acting on an object when all individual forces are combined together.

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Orbit

The curved path that an object, like a planet, takes as it revolves around another.

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Gravitational Field

The invisible region of influence surrounding a massive object where its gravitational force can be felt.

Key Vocabulary

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Solar System

The Solar System includes the Sun and all of the objects that orbit around it.

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Galaxy

A galaxy is a vast collection of stars, gas, and dust held together by gravity.

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Attractive Force

An attractive force is a force that is responsible for pulling different objects toward each other.

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Centripetal Force

Centripetal force is a force that acts on an object moving in a circular path.

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Microgravity

Microgravity is the condition of apparent weightlessness that is experienced by all objects in free fall.

Net Force and Apparent Weight

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Elevator at Rest

• ​Gravity's downward pull is balanced by the floor pushing up.

• ​​The two forces are equal, so the net force is zero.

• ​You feel your normal weight because the forces are balanced.

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Accelerating Downward

• ​The floor pushes up with less force than gravity pulls down.

• ​​The net force is unbalanced and points in the downward direction.

• ​This downward net force causes you to feel significantly lighter.

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Accelerating Upward

• ​The floor pushes up with more force than gravity pulls down.

• ​​The net force is unbalanced and points in the upward direction.

• ​This upward net force causes you to feel significantly heavier.

Multiple Choice

What happens when the downward pull of gravity on a person in an elevator is perfectly balanced by the upward push of the floor?

• The net force is zero, and you feel your normal weight.

• The net force is upward, and you feel heavier.

• The net force is downward, and you feel lighter.

• The net force is zero, and you feel weightless.

Multiple Choice

What causes a person to feel lighter when an elevator is accelerating downward?

• The floor pushes up with less force than gravity pulls down.

• The floor pushes up with more force than gravity pulls down.

• The forces of gravity and the floor become zero.

• The elevator is no longer affected by gravity.

Multiple Choice

Which statement provides the best explanation for why a person feels heavier when an elevator accelerates upward?

• The upward push from the floor is greater than the downward pull of gravity, creating an upward net force.

• The downward pull of gravity is greater than the upward push from the floor, creating a downward net force.

• The forces are balanced, creating a net force of zero, which makes you feel heavier.

• The elevator's speed becomes constant, which increases the force of gravity.

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Mass vs. Weight

Mass

• Mass is the amount of matter, or 'stuff', in an object.

• An object's mass is constant and does not change based on its location.

• Your mass is the same whether you are on Earth or the Moon.

Weight

• Weight measures the force of gravity pulling on an object's mass.

• Because it depends on gravity, an object's weight can change.

• You would weigh less on the Moon because its gravity is weaker.

Multiple Choice

Which statement best defines an object's mass?

• The amount of matter, or 'stuff', the object contains.

• The force of gravity pulling on the object.

• The size or volume of the object.

• The location of the object in space.

Multiple Choice

What is the main reason an object's weight can change, but its mass cannot?

• Weight depends on gravity, which can change depending on location, while mass is constant.

• Mass is measured in kilograms, while weight is measured in newtons.

• An object's matter changes with its location, but gravity is always the same.

• Weight is a property of large objects, while mass is a property of small objects.

Multiple Choice

If an astronaut travels from Earth to the Moon, where gravity is weaker, what would happen to her mass and weight?

• Her mass would be the same, but her weight would be less.

• Her mass would be less, but her weight would be the same.

• Both her mass and her weight would be less.

• Both her mass and her weight would be the same.

The Force of Gravity

• Gravity is an attractive force between any two masses.

• The force of gravity increases with the mass of the objects.

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

Multiple Choice

What is gravity?

• A force that pushes two masses apart

• An attractive force between any two masses

• A force that only affects planets

• A force that only exists on Earth

Multiple Choice

What happens to the force of gravity between two objects if their mass increases?

• The force of gravity decreases

• The force of gravity increases

• The force of gravity stays the same

• The force of gravity disappears

Multiple Choice

If the distance between the Earth and the Moon were twice as large, how would the gravitational pull between them change?

• It would be stronger than before

• It would be weaker than before

• It would stay the same

• It would reverse direction

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Gravity and the Solar System

• The solar system includes the sun, planets, their moons, and asteroids.

• All objects are held in orbit by the sun’s immense gravitational pull.

• Gravity pulled a disk of dust and gas together to form the solar system.

• This is why everything orbits the sun, the most massive object.

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Multiple Choice

What is the primary force that holds planets, moons, and asteroids in orbit within the solar system?

• Gravity

• Magnetism

• Friction

• Light

Multiple Choice

What is the relationship between the sun's mass and its role in the solar system?

• The sun is the most massive object in the solar system.

• The sun is the only object that produces light.

• The planets are all much larger than the sun.

• The asteroids are located in the center of the solar system.

Multiple Choice

Based on the information provided, what is the best explanation for how the solar system was formed?

• The force of gravity pulled the dust and gas together to form the sun and planets.

• The spinning motion of the cloud caused it to get hotter and form a star.

• The planets formed first and then captured the sun with their gravity.

• The asteroids collided with each other to create the larger planets.

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Gravity's Role in Galaxies

• Earth and our solar system are located in the vast Milky Way galaxy.

• A galaxy is a massive collection of stars, gas, and dust.

• The force of gravitational attraction is what holds the entire galaxy together.

• This gravity makes our Sun and other stars orbit the galactic center.

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Multiple Choice

What holds the massive collection of stars, gas, and dust in a galaxy together?

• The light from all the stars

• The force of gravitational attraction

• The rotation of the planets

• The heat generated by the galactic center

Multiple Choice

What is the primary effect of gravity on the Sun and other stars within the Milky Way galaxy?

• It causes stars to become hotter and brighter.

• It causes stars to orbit around the galactic center.

• It pushes other galaxies farther away.

• It helps create new dust and gas clouds.

Multiple Choice

If the force of gravity within a galaxy were to suddenly disappear, what would be the most likely outcome?

• The stars would all collapse into the galactic center.

• The stars, gas, and dust would begin to drift apart.

• The stars would stop shining and go dark.

• The galaxy would attract other galaxies more strongly.

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What is an Orbit?

• An orbit is the path an object takes around another object in space.

• It is a balance between moving in a straight line and gravity’s inward pull.

• Gravity acts as a centripetal force that creates the object's curved path.

Multiple Choice

What is an orbit?

• The path an object takes around another object in space.

• The force that pulls two objects towards each other.

• An object's tendency to move in a straight line.

• The speed at which an object travels through space.

Multiple Choice

What is the function of gravity in maintaining an orbit?

• It pushes the object away, causing it to speed up.

• It makes the object travel in a straight line through space.

• It pulls the object toward the center, preventing it from flying off in a straight line.

• It has no effect on the path of an orbiting object.

Multiple Choice

What would most likely happen to a planet if the Sun's gravity suddenly disappeared?

• The object would immediately stop moving.

• The object would continue to travel in its circular path.

• The object would travel in a straight line away from the object it was orbiting.

• The object would fall directly into the object it was orbiting.

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Gravity in Space

• Gravity is present in space, affecting astronauts on the International Space Station (ISS).

• They experience microgravity, a feeling of weightlessness, due to constant free fall.

• The ISS stays in orbit by falling toward Earth and moving sideways rapidly.

• Every object with mass has a gravitational field that weakens over distance.

Multiple Choice

What characteristic of an object creates a gravitational field?

• Its mass

• Its speed

• Its distance from Earth

• Its temperature

Multiple Choice

Why do astronauts in orbit feel weightless?

• Because there is no gravity in space.

• Because they are in a state of constant free fall.

• Because their spacesuits protect them from gravity.

• Because they are moving faster than the speed of light.

Multiple Choice

What is the best explanation for how the International Space Station remains in a stable orbit?

• It has escaped Earth's gravitational pull completely.

• It is moving fast enough to continuously miss Earth as it falls.

• Its engines are always pushing it away from the planet.

• The Sun's gravity balances Earth's pull.

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Common Misconceptions about Gravity

Misconception	Correction
Feeling 'weightless' in space means there is no gravity.	Weightlessness is the feeling of being in continuous free fall around the Earth.
An object's mass and weight are the same thing.	Mass is matter in an object; weight is gravity's force on that mass.
Gravity only exists between large objects like planets.	Gravitational force exists between any two masses, but is only noticeable for large objects.

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Summary

• Gravity is an attractive force that depends on mass and distance.

• Mass is constant, while weight is the measure of gravitational force.

• Gravity holds the solar system together, causing smaller objects to orbit larger ones.

• An orbit is a stable path balancing an object's motion and gravity's pull.

Poll

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

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