Mass measuring instruments: mass is not weight

Two different things

At the shop we ask for the weight of flour. In physics, that word means something else. Mass is the quantity of matter in a body. Weight is the force pulling the body towards the Earth.

A spring balance measures weight: the pull stretches its spring (Fig. 1.8). Mass is found by comparing the object with known standard masses. This comparing is called weighing.

The laboratory physical balance (Fig. 1.9) works on the principle of levers. The object goes on the left pan and standard masses on the right, until the beam balances.

The most precise balance in your book is the digital electronic balance. It can measure mass of the order of 0.1 mg.

Mass is how much matter; weight is how hard the Earth pulls. A physical balance compares masses; a spring balance measures the pull.

Key terms

Mass

How much matter a body has; SI unit kilogram.

Weight

The force by which the Earth attracts a body.

Weighing

Finding mass by comparing with known standard masses.

Physical balance

The laboratory balance that compares an object with standard masses.

Short questions with model answers

1. Q1. The digital electronic balance reads to 0.1 mg. Write this in kilograms.

   • 0.1 mg = 0.1 × 10⁻³ g = 1 × 10⁻⁴ g
   • 1 × 10⁻⁴ g = 1 × 10⁻⁴ × 10⁻³ kg = 1 × 10⁻⁷ kg

   1 × 10⁻⁷ kg: a ten-millionth of a kilogram.

2. Q2. Standard masses of 50 g, 20 g, 5 g and 2 g balance a stone. What is its mass?

   • Pointer on zero, so the pans balance.
   • 50 g + 20 g + 5 g + 2 g = 77 g

   77 g.

Common mistakes

• ✗ Writing that weight is the quantity of matter in a body.

  ✓ That is mass. Weight is the force by which the Earth attracts the body.

• ✗ Saying a spring balance measures mass.

  ✓ A spring balance measures weight; a physical balance finds mass.

• ✗ Putting the object on a pan before levelling the base and beam.

  ✓ Steps 1 and 2 come first; a tilted balance gives a wrong comparison.

MCQs

1. 1. A physical balance works on the principle of:

   1. (a) Springs
   2. (b) Levers
   3. (c) Parallax
   4. (d) Screws
   Show answer

   (b) Your book, p. 16.

2. 2. On a physical balance, the object to be weighed goes on:

   1. (a) The right pan
   2. (b) Either pan
   3. (c) The left pan
   4. (d) The beam
   Show answer

   (c) Step 3: object on the left pan; standard masses on the right.

3. 3. A spring balance measures:

   1. (a) Weight
   2. (b) Mass
   3. (c) Volume
   4. (d) Density
   Show answer

   (a) Fig. 1.8: the pull of the Earth stretches the spring.

4. 4. Weight is:

   1. (a) The quantity of matter in a body
   2. (b) The space a body occupies
   3. (c) The total of standard masses
   4. (d) The force by which the Earth attracts a body
   Show answer

   (d) The first and third describe mass; the second is volume.

5. 5. The most precise balance in your book measures mass of the order of:

   1. (a) 1 g
   2. (b) 0.1 mg
   3. (c) 1 kg
   4. (d) 0.1 g
   Show answer

   (b) The digital electronic balance: 0.1 mg (p. 16).

Quick revision

• Mass is the quantity of matter; weight is the Earth's pull on it.
• Spring balance → weight. Physical balance, by weighing against standard masses → mass.
• Level first, object left, masses right, pointer on zero, then add the masses.