Mass

Mass

Middle School

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Learning Objectives

• Define mass and weight, and learn the key differences between them.

• Explain why mass is constant, but weight changes depending on the location.

• Describe how to use a triple beam balance to measure an object's mass.

Key Vocabulary

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Mass

Mass is the amount of matter in an object and is measured in grams or kilograms.

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Weight

Weight measures the force of gravity on an object and changes based on the location’s gravitational pull.

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Scalar Quantity

A quantity, like mass, that has only a magnitude or size but does not have a direction.

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Vector Quantity

A quantity, such as weight, that possesses both magnitude and a specific direction associated with it.

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Triple Beam Balance

A lab instrument used to find an object's mass by balancing it against known masses.

Mass vs. Weight

• An object's mass is the amount of matter in it and is constant.

• Weight is the force of gravity on an object, so it can change.

• An object has the same mass everywhere, but its weight can change.

Multiple Choice

An astronaut travels from Earth to the Moon. Which statement correctly describes what happens to her mass and weight?

• Her mass stays the same, but her weight decreases.

• Both her mass and weight stay the same.

• Both her mass and weight decrease.

• Her mass decreases, but her weight stays the same.

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Measuring Mass with a Triple Beam Balance

• A triple beam balance is a tool used to accurately measure mass.

• ​You slide riders on three beams until the pointer balances at zero.

• The beams measure mass in 100s, 10s, and single grams with decimals.

• Add all three readings to find the object's total mass (e.g. 225.4 g).

• A triple beam balance measures mass (which stays the same everywhere), unlike a spring balance, which measures weight and changes with gravity.

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Multiple Choice

To find the total mass of an object on a triple beam balance, what must you do?

• Add the readings from all three beams together.

• Use only the reading from the largest beam.

• Average the readings of the three beams.

• Subtract the smallest reading from the largest reading.

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Common Misconceptions About Mass and Weight

Misconception	Correction
Mass and weight are the same thing.	Mass is matter in an object. Weight is the force of gravity on it.
An object's mass changes when it goes to another planet.	An object's mass is constant, but its weight changes with gravity.
Weight is measured in kilograms.	Kilograms (kg) measure mass. Weight is measured in Newtons (N).

Multiple Choice

Why is an object's weight different on the Moon compared to the Earth, while its mass remains the same?

• The object loses matter when it travels to the Moon.

• The Moon is smaller and has a weaker gravitational pull.

• The scales used on the Moon work differently.

• The definition of mass changes in space.

Multiple Choice

If a triple beam balance shows a reading of 300g on the middle beam, 70g on the back beam, and 2.5g on the front beam, what is the total mass of the object?

• 300.725 g

• 372.5 g

• 232.5 g

• 370.25 g

Multiple Choice

An object has a mass of 50 kg. If it is moved to a planet with twice the gravitational force of Earth, what can you predict about its mass and weight?

• Its mass will be 50 kg, and its weight will be halved.

• Its mass will be 50 kg, and its weight will double.

• Its mass will be 100 kg, and its weight will stay the same.

• Its mass will be 100 kg, and its weight will double.

Multiple Choice

You have a spring balance and a triple beam balance. How could you determine which is which by taking measurements on Earth and then on the Moon?

• The triple beam balance would give the same reading in both locations, while the spring balance reading would decrease on the Moon.

• The spring balance would give the same reading in both locations, while the triple beam balance reading would decrease on the Moon.

• Both devices would show lower readings on the Moon.

• Both devices would show the same readings in both locations.

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Summary

• An object's mass is a constant measure of the matter inside it.

• An object's weight is a force that changes depending on the pull of gravity.

• Mass is a scalar quantity, while weight is a vector quantity.

• A triple beam balance is used to measure the mass of an object.

Poll

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

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• 4