PHYSICS · Stage 7 · Chapter 2: Forces
Physics 2.2 Gravity — Gravitational Field Strength, Mass (kg) and Weight (N)
5 min read 5 parts Lesson 2 of 7
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
1Force of gravity1 point
= 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.
2Always attractive
— the force of gravity is an attractive force, and it always acts towards the centre of the planet.
3Moon
— 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.
4Factors1 point
— 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
- Gravitational field = a region where a mass experiences a force, and there is one around all objects.
- 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).
- Different planets — when people say gravity is different on different planets, they mean the gravitational field strength is different.
- 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
1Mass2 points
= 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.
2Weight1 point
= 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.
3Astronaut
— an astronaut's weight changes on another planet but the mass stays the same.
4Confusion
— 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
- Formulaweight (N) = mass (kg) × gravitational field strength (N/kg), W = m × g.
- Rearrangingm = W ÷ g and g = W ÷ m.
- On the Moonweight is about one-sixth of the Earth value, so a person can jump higher.
- Checkweights on Earth are about ten times the mass in kilograms.
- Procedurewrite the formula, substitute with units and give the answer with the correct unit.
Part 5 of 5Gravity beyond Earth
- 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 — 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 — near Earth, free-falling objects speed up by about 10 m/s every second when air resistance is small.
- Bathroom scales — they measure the normal force, so the reading would be different on another planet.
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