Physics·General Physics · NSSCO 2.5
Mass & weight
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In everyday life we use 'mass' and 'weight' as if they mean the same thing — but in physics they are two very different ideas. Mass is the amount of matter in an object, measured in kilograms with a balance, and it never changes. Weight is the pull of gravity on that mass, measured in newtons with a spring balance, and it changes with location. This lesson links them with one simple formula, W = m × g, and shows why you would weigh less on the Moon but keep exactly the same mass.
What you'll learn in this lesson
By the end you should be able to (NSSCO Physics 2.5):
- Define the mass of a body as a measure of the matter in it, depending on the number of particles and the mass of those atoms
- Define inertia as the property of mass which resists a change in motion (Newton's first law)
- Recall that mass is measured in kilograms with a balance and is the same everywhere
- Recall that weight is the gravitational force on a mass, measured in newtons with a spring balance
- Recall and use the equation weight = mass × gravitational field strength, and know g ≈ 10 N/kg near the Earth's surface
- Determine the mass of an object from its weight using a balance or a spring
- Define gravitational field strength g = W ÷ m and state that it equals the free-fall acceleration
- Explain that weight changes with location (e.g. on the Moon) while mass does not, and describe falling with and without air resistance including terminal velocity
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