Weight, Mass & Gravity (OCR GCSE Physics A (Gateway Science Suite)): Revision Notes
5.1.4 Weight, Mass & Gravity
Weight
- The weight of an object is the force acting on it due to gravity (N).
- The weight of an object does depend on where the object is.
- The weight of an object is directly proportional to the mass of the object.
- If we double the mass, then the weight also doubles.
Mass
- The mass of an object tells us how much matter the object has in it (kg).
- Mass is a scalar quantity as it has no direction.
- The mass of an object does not depend on where the object is.
Gravity
- The gravitational field strength (g) on the surface of the Earth is 9.8N/kg.
- This is a measure of the force of gravity in a particular location.
- Objects on the surface of the Earth experience a force of 9.8N for every kg of their mass.
Weight and Gravity
The Link Between Weight and Gravity
- Weight is the force on an object due to gravity. Without gravity, objects would just have mass. With gravity, objects have a force pulling them down due to their mass; this is the weight of an object.
- The gravitational field is responsible for the force of gravity close to Earth. Around the earth, there is a gravitational field which pulls objects down. Anything with mass experiences this gravitational pull, including objects (e.g. a chair) and living things (e.g. humans).
Calculating Weight
Where:
- Weight () is in newtons (N)
- Mass () is in kilogrammes (kg)
- Gravitational field strength ( in newtons per kilogramme, N/kg.
Example Qestion: Question: A car has a mass of 1600kg. Calculate the weight of the car on Earth, assuming the gravitational field strength is 9.8 N/kg.
1. Write the equation.
2. Put in the numbers.
Weight and mass
Weight and mass are directly proportional. Weight and mass are closely linked, and are said to be directly proportional. This means that they do the 'same thing'. If the mass halves, then the weight will halve. If the mass triples, then the weight will triple.
The difference between weight and mass
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Weight is a force. Weight is measured in Newton's and changes according to the force of gravity. This means that your weight on the Earth is different to your weight on the Moon.
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Mass is just a value. Mass is the 'amount' of object that you have. Your mass on Earth would be the same as your mass on the Moon.
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Weight is determined by the action of gravity, whereas mass is not. This means that wherever you are in the Solar System, your mass will be the same, but your weight will not. Therefore mass resists changes in motion.
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The centre of mass will be at the centre of a symmetrical object. The centre of mass is the point at which the object will balance. It is often found in the centre of a symmetrical object.
Measuring Weight with a Newton meter
A Newton metre is a tool used to measure the weight of an object. It works like a calibrated spring balance:
- When you attach an object to the end of the Newton metre, the weight of the object pulls on the spring, causing it to stretch.
- The amount the spring extends depends on the force of gravity acting on the object (its weight).
- You can then read the weight directly from the scale on the Newton metre, usually in Newtons (N).
Finding the Centre of Mass of an Irregular-Shaped Object
Method:
- Gather your equipment. You will need a clamp, a plumb line, a pin and a lamina.
- Attach the object. Attach the irregularly shaped lamina to a pin.
- Allow the card to swing. Hang up the card and let it turn until the centre of mass is vertically under the pin.
- Attach a plumb line. Repeat using a plumb line. Mark the position of the thread.
- Repeat. Mark the position of the thread at least two more times, changing the position of the pin.
- Determine the centre of mass. The point at which the lines cross is the centre of mass.