Explain weight as the gravitational force on an object, calculate it using W = mg, and describe its action at the centre of mass and measurement with a newtonmeter.
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GCSE Physics Revision
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GCSE Physics revision
Gravity
Forces and their interactions
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AQA student objectives
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Apply the specified scientific knowledge of gravity to a relevant example.
Analyse an unfamiliar example of gravity using the specified scientific ideas.
Revision summary
Key knowledge
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Gravitational Fields
- A gravitational field is the region of influence surrounding an object where its gravitational force can be felt.
- The strength of a gravitational field is called gravitational field strength, represented by the letter g.
- Earth's gravitational field strength is approximately 9.8 N/kg (sometimes rounded to 10 N/kg), while the Moon's is only 1.6 N/kg due to its smaller mass.
- The Sun's gravitational field strength is around 274 N/kg because of its enormous mass, which is why it strongly influences the Earth despite the large distance between them.
Defining Weight
- Weight is defined as the force exerted on an object due to gravity, and it is measured in Newtons (N).
- Weight depends on both the mass of the object and the gravitational field strength acting upon it.
- Unlike mass, weight is not a fixed property — it changes depending on which planet or star the object is near.
Calculating Weight
- The formula for weight is W = mg, where W is weight (N), m is mass (kg), and g is gravitational field strength (N/kg).
- For example, a person with a mass of 60 kg on Earth has a weight of 60 × 9.8 = 588 N.
- The same person on the Moon would have a weight of only 60 × 1.6 = 96 N, because the Moon's gravitational field strength is much weaker.
Mass vs. Weight: Key Differences
- Mass is a property of an object that measures the amount of matter it contains, and it remains the same everywhere in the universe.
- Weight is the force acting on an object due to gravity, so it varies depending on the gravitational field strength at that location.
- In everyday language, 'weight' is often used to mean mass (e.g. 'I weigh 60 kg'), but in physics these are two distinct concepts with different units.
- What is Gravitational Potential Energy?
- Gravitational potential energy (GPE) is the energy stored in an object when it is lifted against the force of gravity.
- When work is done to lift an object, energy is transferred into the object's gravitational potential energy store.
- Gravitational potential energy is a form of energy and is therefore measured in Joules (J).
Calculating Gravitational Potential Energy
- The formula for gravitational potential energy is E_p = mgh, where m is mass (kg), g is gravitational field strength (N/kg), and h is height (m).
- It is important to ensure all values are in the correct units before substituting into the formula — for example, converting grams to kilograms by dividing by 1000.
- For example, an apple of mass 100 g (0.1 kg) thrown 3 m into the air has a GPE of 0.1 × 9.8 × 3 = 2.94 J.
Common Mistakes & Exam Tips
- Always convert mass into kilograms (kg) before using the GPE or weight formulae, as grams will give an incorrect answer.
- Remember that g = 9.8 N/kg on Earth, but this value changes on other planets and moons — always check which value to use.
- Do not confuse weight (a force measured in Newtons) with mass (measured in kilograms), as mixing these up is a common error in exams.