Gravity – JSS2 Intermediate Science Lesson Note (Third Term)

Gravity is the force of attraction that pulls all objects toward each other.

Behavioural objectives

1. Define gravity, mass and weight.
2. State four effects of gravity.
3. State the units and measuring instruments for mass and weight.
4. Explain why mass stays the same but weight changes from place to place.
5. Calculate weight using W = m × g.

What is gravity?

Gravity is a force of attraction that pulls every object with mass toward every other object.

• Mass: the amount of matter (the “stuff”) in an object.
• Small objects: a pen or a chair also pulls other things, but the pull is too weak to feel.
• Large objects: we notice gravity only near something massive, such as the Earth.
• Ball thrown up: it slows down, stops, then falls back because the Earth pulls it down.
• People in Australia: they do not fall off the Earth, because gravity pulls everyone toward the centre of the Earth.

What is the Earth’s pull?

The Earth’s pull is the gravity of the Earth, which pulls every object toward the centre of the Earth.

• Strong pull: the Earth has a very large mass, so its gravity is strong.
• Meaning of “down”: down always means toward the ground, wherever you stand on the planet.
• Falling objects: a dropped mango, a raindrop and a stone all fall because of this pull.

What are the effects of gravity?

Gravity keeps things on the ground, holds the air around the Earth, keeps the Moon in orbit, causes tides and gives objects weight.

• It keeps us on the ground: without gravity we would float. Gravity brings your foot back to the floor with every step.
• It holds the atmosphere: gravity pulls on the air, so the oxygen we breathe does not escape into outer space.
• It keeps the Moon in orbit: the Earth pulls on the Moon, so the Moon travels in a circle around the Earth instead of flying away.
• It keeps the planets in orbit: the Sun’s gravity keeps the Earth and other planets moving around it.
• It creates tides: the Moon’s gravity pulls on the oceans, so the water level rises (high tide) and falls (low tide).
• It gives things weight: without gravity, an object would have no weight.

What is mass?

Mass is the amount of matter an object contains.

• Unit: kilogram (kg).
• Instrument: beam balance.
• Does it change? No. A mass of 50 kg on Earth is still 50 kg on the Moon or in space.

What is weight?

Weight is the force of gravity pulling on an object.

• Unit: newton (N).
• Instrument: spring balance.
• Does it change? Yes. It depends on the strength of gravity where you are.
• On the Moon: the Moon is smaller than the Earth, so its gravity is weaker. Your weight there is about one-sixth of your weight on Earth.
• Your mass on the Moon: it stays the same, because your body still contains the same amount of matter.

What is the difference between mass and weight?

Mass is the amount of matter in an object, while weight is the pull of gravity on that object. In daily life people use the two words as if they mean the same thing, but in science they are different.

How do we calculate weight?

Weight is calculated by multiplying mass by the strength of gravity: W = m × g.

• W: weight, in newtons (N).
• m: mass, in kilograms (kg).
• g: strength of gravity. On Earth we use about 10 m/s², which means 10 N for every 1 kg.

Worked example 1: finding weight

A student has a mass of 40 kg. What is the weight on Earth?

1. Write the formula: W = m × g.
2. Write the values: m = 40 kg and g = 10 m/s².
3. Substitute: W = 40 × 10.
4. Answer: W = 400 N.

Worked example 2: finding mass

A box weighs 250 N on Earth. What is its mass?

1. Rearrange the formula: m = W ÷ g.
2. Substitute: m = 250 ÷ 10.
3. Answer: m = 25 kg.

Evaluation

1. Why do objects fall toward the ground when dropped?
2. If you travel to Mars, where gravity is weaker than on Earth, will your mass change? Give a reason.
3. What is the S.I. unit for weight?
4. A bag of rice has a mass of 10 kg. Calculate its weight in newtons (g = 10 m/s²).
5. State two effects of gravity.

Assignment

Copy the table of differences between mass and weight. Then calculate the weight on Earth of an object of mass 65 kg (g = 10 m/s²).