2.2Chapter 2: Forces

Gravity — Alhussan Physics - Stage 7 Revision Notes & Topical Questions

5 min read 7 parts Lesson 2 of 7 2 quizzes

📚 Alhussan Physics - Stage 7 Hub / Topical Revision

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Lesson summary

2.2 Gravity — Gravitational Field Strength, Mass (kg) and Weight (N) — Alhussan International Academy — Physics (Stages 7–9) notes: Describe how the force of gravity arises and its factors, and distinguish between mass and weight using gravitational field strength.Force of gravit…

Always attractive
Gravitational field strength (g)
Different planets
Fields in physics
Rearranging
On the Moon

Part 1 of 5The force of gravity

Force of gravity

Force of gravity a force of attraction between all objects that have mass.

It pulls you down towards the centre of the Earth, and forces come in pairs, so you also pull the Earth with a force of gravity.

Always attractive

Always attractive the force of gravity is an attractive force, and it always acts towards the centre of the planet.

Moon

Moon there is no atmosphere (air) on the Moon,
- so astronauts wear spacesuits to breathe,
- but there is gravity there, about one-sixth as strong as on Earth.

Factors

Factors the size of the force of gravity between two objects depends on the mass of the two objects and how far apart they are.
A more massive planet exerts a bigger force on objects of the same mass, so Jupiter has stronger gravity than Earth.

Part 2 of 5Gravitational field strength

1Gravitational field

Gravitational field a region where a mass experiences a force, and there is one around all objects.

2Gravitational field strength (g)

Gravitational field strength (g) the force per kilogram, the weight of each kilogram of mass.

On Earth g is 10 N/kg (more exactly 9.8 N/kg).

3Different planets

Different planets when people say gravity is different on different planets, they mean the gravitational field strength is different.

4Distance

Distance gravitational field strength decreases as you move away from a planet.

Place	g (N/kg)	Weight of 1 kg (N)
Earth	10	10
Moon	1.6	1.6
Mars	3.7	3.7
Jupiter	25	25

Fields in physics
- the idea of a field is very important and is linked to forces, and there are other fields such as magnetic fields.

Part 3 of 5Mass and weight

Mass

Mass the amount of matter in an object, measured in kilograms (kg).

Mass is the same everywhere, measured with a balance.
An object with a small mass accelerates faster than one with a big mass when the same force is applied.

Weight

Weight the force of the Earth's gravity on an object, measured in newtons (N) with a force meter.

Weight is a force and it depends on where the object is.

Astronaut

Astronaut an astronaut's weight changes on another planet but the mass stays the same.

Confusion

Confusion scales often show "weight" in kilograms,
which is scientifically wrong,
because a reading in kilograms or grams is a mass and a weight should be in newtons.

Part 4 of 5Calculating weight

Formula

Formula weight (N) = mass (kg) × gravitational field strength (N/kg), W = m × g.

Rearranging

Rearranging m = W ÷ g and g = W ÷ m.

On the Moon

On the Moon weight is about one-sixth of the Earth value, so a person can jump higher.

Check

Check weights on Earth are about ten times the mass in kilograms.

Procedure

Procedure write the formula, substitute with units and give the answer with the correct unit.

Part 5 of 5Gravity beyond Earth

Space exploration

Space exploration a rover with a mass of 900 kg has a weight of 9000 N on Earth but only 3330 N on Mars (g = 3.7 N/kg),
- which affects the design of landing systems.

Weightlessness

Weightlessness astronauts in orbit feel weightless because they and the spacecraft are in continuous free fall, not because gravity has vanished.
At the height of the International Space Station gravity is still about 90% of its strength at the surface.

Free fall

Free fall near Earth, free-falling objects speed up by about 10 m/s every second when air resistance is small.

Bathroom scales

Bathroom scales they measure the normal force, so the reading would be different on another planet.

Key terms

Term	Meaning
Mass	the amount of matter in an object, in kg.
Weight	the force of gravity on an object, in N.
Gravitational field strength	the force per kilogram due to gravity, in N/kg.
Force meter	an instrument for measuring force.
Gravitational field	a region where a mass experiences a force.
Free fall	falling with only the force of gravity acting.