Weight
Contents
Key Stage 2
Meaning
Weight is how heavy or light something is.
About Weight
- Objects have weight because of the gravity of the Earth.
- Weight can be measured using a Newton Meter or a Measuring Scale.
- Weight is measured in Newtons but can also be measured in stone, pounds and ounces.
- Weight depends on the amount of mass an object has.
Golf Ball | Beach Ball | Bowling Ball |
The Golf Ball is not made of much material so it is lightest. | The Beach Ball might be the biggest but it is not the heaviest. | The Bowling Ball is made of the most material, so it is the heaviest. |
Key Stage 3
Meaning
Weight is the the force on an object that is in a gravitational field.
About Weight
- Weight is a force so it is measured in Newtons.
- Weight is a non-contact force because an object does not need to be touching the ground to be attracted to the Earth.
- Weight always acts downwards.
- All objects on Earth have weight because the Earth has a gravitational field.
- Different planets have a different strength gravitational field, so the same object might weigh a different amount on different planets.
- On Earth the gravitational field strength is roughly 10N/kg
Examples
Weight makes a tennis ball fall to the ground. | Weight holds the bowling ball to the ground. | If the weight of a boat were bigger than the upthrust the boat would sink. |
The weight of the car acts from its centre of mass. | The submarine has the same weight under the water as it would on land, but in the water this is balanced by the upthrust. | Planes can be very heavy but their weight is balanced by the lift. |
Equation
- Weight = Mass x Gravitational Field Strength
\(W = m \times g \)
Where:
- W = Weight
- m = Mass
- g = Gravitational Field Strength
Example Calculations
Calculate the weight of a 25kg object on Earth. | A 550kg space probe is sent into deep space beyond the gravitational field of any other object. Calculate the weight of the space probe. | The Moon has a gravitational field strength of 1.6N/kg. Calculate the weight of a 85kg astronaut on The Moon. |
m = 25kg g_{Earth} = 10N/kg \(W = m \times g \) \(W = 25 \times 10 \) \(W = 250N \) |
m = 550kg g_{Deep Space} = 0N/kg \(W = m \times g \) \(W = 550 \times 0 \) \(W = 0N \) |
m = 85kg g_{The Moon} = 1.6N/kg \(W = m \times g \) \(W = 85 \times 1.6 \) \(W = 136N \) |
Key Stage 4
Meaning
Weight is the force acting on an object in a gravitational field.
About Weight
The magnitude of the weight depends on:
- The mass of the object - The more massive the object the greater the force of weight.
- The gravitational field strength - The greater the gravitational field strength the greater the force of weight.
- Weight always acts towards the centre of mass of a moon, planet or star.
- The weight of an object acts from its centre of mass (or centre of gravity) of an object.
- On Earth the gravitational field strength is 9.8N/kg correct to two significant figures.
Examples
Weight makes a tennis ball fall to the ground. | Weight holds the bowling ball to the ground. | If the weight of a boat were bigger than the upthrust the boat would sink. |
The weight of the car acts from its centre of mass. | The submarine has the same weight under the water as it would on land, but in the water this is balanced by the upthrust. | Planes can be very heavy but their weight is balanced by the lift. |
Equation
Weight = Mass x Gravitational Field Strength
\(W = m \times g \)
Where
\(W\) = The weight of the object.
\(W = m \times g \) = The mass of the object.
\(W = m \times g \) = The gravitational field strength of the gravitational field the object is in.
Calculating Weight from Mass and Gravitational Field Strength
Calculate the weight of a 25kg object on Earth correct to two significant figures | A 550kg space probe is sent into deep space beyond the gravitational field of any other object. Calculate the weight of the space probe correct to two significant figures. | The Moon has a gravitational field strength of 1.6N/kg. Calculate the weight of a 85kg astronaut on The Moon correct to two significant figures. |
1. State the known quantities in SI Units.m = 25kg
g_{Earth} = 9.8N/kg |
1. State the known quantities in SI Units.m = 550kg
g_{Deep Space} = 0N/kg |
1. State the known quantities in SI Units.m = 550kg
m = 85kg g_{The Moon} = 1.6N/kg |
2. Substitute the numbers into the equation and solve.
\(W = m \times g \) \(W = 25 \times 9.8 \) \(W = 245N \) \(W \approx 250N \) |
2. Substitute the numbers into the equation and solve.
\(W = m \times g \) \(W = 550 \times 0 \) \(W = 0.0N \) |
2. Substitute the numbers into the equation and solve.
\(W = m \times g \) \(W = 85 \times 1.6 \) \(W = 136N \) \(W \approx 140N \) |
References
AQA
- Weight, page 120, GCSE Physics, Hodder, AQA
- Weight, page 204, GCSE Combined Science; The Revision Guide, CGP, AQA
- Weight, page 52, GCSE Physics; The Revision Guide, CGP, AQA
- Weight, pages 129, 130, GCSE Combined Science Trilogy; Physics, CGP, AQA
- Weight, pages 149, 150, GCSE Physics; The Complete 9-1 Course for AQA, CGP, AQA
- Weight, pages 152-3, GCSE Physics; Student Book, Collins, AQA
- Weight, pages 210, GCSE Combined Science Trilogy 2, Hodder, AQA
- Weight, pages 4-5, 10-11, 146-147, GCSE Physics; Third Edition, Oxford University Press, AQA
Edexcel
- Weight, page 150, GCSE Combined Science; The Revision Guide, CGP, Edexcel
- Weight, pages 17, 102, GCSE Physics; The Revision Guide, CGP, Edexcel
- Weight, pages 2, 16-17, GCSE Physics, Pearson Edexcel
- Weight, pages 37, 38, 333, GCSE Physics, CGP, Edexcel