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Cambridge IGCSE0625

Mass and weight

Physics 0625 Chapter Notes

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Mass and weight
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In this chapter

  1. 1Mass and weight are different quantities
  2. 2Gravitational field strength
  3. 3Weight is the effect of a gravitational field

1. Mass and weight are different quantities

Mass is a measure of the quantity of matter in an object at rest relative to the observer.

Weight is a gravitational force on an object that has mass. It is a force, so its unit is the newton.

Weights, and masses, may be compared using a balance.

Examiner insight

“Many incorrectly believed that weight is the gravitational force per unit mass rather than the gravitational force on an object”

Worked example 12 marks

A student holds a rubber ball above a hard floor.

(a) State what is meant by the mass of the ball.

(b)(i) The student releases the ball. The ball accelerates downwards due to a force W. State the name of force W.

  1. 1

    Mass is a measure of the quantity of matter in the ball. [1]

  2. 2

    Weight. Also credited: gravitational force, or force due to the Earth's gravitational field. [1]

2. Gravitational field strength

Gravitational field strength g is the force per unit mass. It is equivalent to the acceleration of free fall.

Rearranged, the same equation gives weight from mass, and mass from weight.

g = W/m

W = mg

Examiner insight

“Some weaker candidates could not recall the correct equation to calculate weight and instead divided the mass by the gravitational field strength.” “If candidates multiplied the mass of the bear by 10, they needed to state explicitly that they were using the value g = 10 N / kg in their calculation.”

Worked example 13 marks

The weight of a block of ice is 650 N. Calculate the mass of the block of ice.

  1. 1

    mass = weight ÷ gravitational field strength [1]

  2. 2

    = 650 ÷ 9.8 [1]

  3. 3

    = 66 kg [1]

3. Weight is the effect of a gravitational field

Weight is the effect of a gravitational field on a mass. An object always has mass, but it has weight only when a gravitational field acts on it.

Where the gravitational field strength is different, the weight is different and the mass is the same.

Examiner insight

“A large number of candidates had the misconception that gravitational field strength depends on atmospheric pressure”

Worked example 11 mark

The gravitational field strength at the surface of the Earth varies slightly from place to place. At the South Pole it is 9.86 N / kg and in London it is 9.81 N / kg. An explorer collects a rock at the South Pole and moves it to London. How does this affect the mass and the weight of the rock?

  1. 1

    No effect on the mass. The weight decreases. [1]

Recap

  • Mass: a measure of the quantity of matter in an object.
  • Weight: a gravitational force on an object that has mass, in newtons.
  • g = W ÷ m, the force per unit mass, equivalent to the acceleration of free fall.
  • A balance compares weights and masses.
  • Extended: a different gravitational field changes the weight, not the mass.

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. State what is meant by the mass of an object.1 mark
  2. A student writes ‘the weight of the bag is 5 kg’. State what is wrong with this statement.1 mark
  3. A box has a mass of 3.0 kg. Calculate its weight.2 marks
  4. A crate has a weight of 147 N. Calculate its mass.2 marks
  5. An apple has a mass of 400 g. Calculate its weight.3 marks
  6. At a certain place the gravitational field strength is 9.8 N / kg. State the acceleration of free fall at that place and give a reason.2 marks
  7. Two objects are placed one on each pan of a balance, and the balance is level. State what this shows about the weights of the objects and about their masses.2 marks
  8. On the Moon the gravitational field strength is 1.6 N / kg. An astronaut has a mass of 75 kg on the Earth. (a) State her mass on the Moon. (b) Calculate her weight on the Moon.3 marks
  9. A rock has a weight of 49 N on the Earth. On another planet the same rock has a weight of 18.5 N. Calculate the gravitational field strength on that planet.4 marks
  10. An astronaut of mass 60 kg is in a spacecraft far from any planet or star. A student says, ‘She is weightless, so she has no mass.’ (a) Explain why the student is wrong. (b) Calculate the astronaut's weight on the Earth.4 marks

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