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P1.3P1. Motion, Forces and Energy

Mass and weight — Combined Science 0653 - Physics Revision Notes & Topical Questions

5 min read 9 parts Lesson 4 of 15 1 quiz

📚 Combined Science 0653 - Physics Hub / Topical Revision
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Lesson summary

P1.3 Mass and weight — COMBINED SCIENCE notes: Distinguish mass from weight, define gravitational field strength and use W = mg.Mass = a measure of the quantity of matter in an object.Properties — mass is a scalar, measured in kilograms (kg), and is the same everywhere in the Uni…

  • Unit conversion
  • Effects of weight
  • Gravitational field strength (g)
  • Link to free fall
  • What the symbols mean
  • Proportionality

Part 1 of 7Mass

Mass
Mass a measure of the quantity of matter in an object.
Properties
  • Properties mass is a scalar, measured in kilograms (kg), and is the same everywhere in the Universe.
Inertia
  • Inertia the greater the mass, the harder it is to speed the object up, slow it down or change its direction.
Unit conversion
  • Unit conversion divide grams by 1000 to get kilograms, because 1000 g = 1 kg.

Part 2 of 7Weight

Weight
Weight the gravitational force acting on an object that has mass.
Properties
  • Properties weight is a force, so it is a vector measured in newtons (N) and acts downwards towards the centre of the planet.
Effects of weight
  • Effects of weight objects stay on the ground, objects fall when released, and satellites stay in orbit.

Part 3 of 7Gravitational field strength

Gravitational field strength
Gravitational field
Gravitational field a region in which a mass experiences a gravitational force.
Gravitational field strength (g)
Gravitational field strength (g) the force per unit mass acting on an object in a gravitational field.
  • g = W ÷ m, in N/kg.
  • Near the Earth's surface g ≈ 9.8 N/kg.
Link to free fall
  • Link to free fall g in N/kg is numerically equal to the acceleration of free fall in m/s², so 1 N/kg = 1 m/s².

Part 4 of 7Weight equation

Weight
  • W = m × g, with W in N, m in kg and g in N/kg.
    What the symbols mean: W = weight (N) · m = mass (kg) · g = gravitational field strength (9.81 N kg−1 on Earth)
Mass
  • Mass m = W ÷ g.
Proportionality
  • Proportionality weight is directly proportional to mass at a given place, so doubling the mass doubles the weight.
Units
  • Units convert grams to kilograms before using the equation.

Part 5 of 7Mass and weight in different places

QuantityOn EarthOn the Moon
MassUnchangedUnchanged
Gravitational field strengthAbout 9.8 N/kgAbout 1.6 N/kg
WeightLargerAbout one-sixth of the Earth value

Part 6 of 7Comparing masses and weights

Balance
  • Balance compares the weights of two objects, and because g is the same for both, it also compares their masses.
  • Balances can be digital or analogue, and read in kg or g.
Newton meter
  • Newton meter a spring and hook that reads the weight of a hanging object in newtons.
1
Zero a newton meter and hang it vertically.
2
Hang masses of known value from it one after another.
3
Read each weight at eye level.
4
Plot weight (y) against mass (x), which gives a straight line through the origin.
5
Find the gradient, which equals g.

Part 7 of 7More from the specification

1Mass and weight
  • Mass and weight mass is the amount of matter in kilograms, weight is the gravitational force on it, W = mg
2Gravitational field strength
  • Gravitational field strength g is about 10 N/kg near the Earth's surface and differs on other bodies
3Density
  • Density density = mass/volume, measured in kg per m3 or g per cm3
4Measuring density
  • Measuring density find the mass of a liquid with a balance and volume with a measuring cylinder,
  • and repeat for an average; for an irregular solid use water displacement
5Floating and upthrust
  • Floating and upthrust an object floats if its density is less than the fluid, because upthrust equals its weight
6Mass and weight
  • Mass and weight mass is the amount of matter in kilograms, weight is the gravitational force on it, W = mg
7Gravitational field strength
  • Gravitational field strength g is about 10 N/kg near the Earth's surface and differs on other bodies
8Density
  • Density density = mass/volume, measured in kg per m3 or g per cm3
9Measuring density
  • Measuring density find the mass of a liquid with a balance and volume with a measuring cylinder,
    • and repeat for an average; for an irregular solid use water displacement
10Floating and upthrust
  • Floating and upthrust an object floats if its density is less than the fluid, because upthrust equals its weight
11Apparatus
  • The equipment used to carry out an experiment.
12Variable
  • Anything that might change during an experiment.
13Independent variable
  • The variable that is changed on purpose.
14Dependent variable
  • The variable that is measured.
15Density
  • Mass per unit volume of a substance.
16Volume
  • The amount of space that something takes up.
17Liquid
  • A state of matter with a fixed volume that takes the shape of its container.
18Upthrust
  • The upward force exerted by a fluid on an object in it.

Key terms

TermMeaning
MassThe quantity of matter in an object, measured in kg.
WeightThe gravitational force acting on an object with mass, measured in N.
Gravitational fieldA region in which a mass experiences a gravitational force.
Gravitational field strengthThe force per unit mass in a gravitational field, g = W ÷ m.
NewtonThe SI unit of force.
InertiaThe resistance of an object to a change in its motion.

Avoid these mistakes

1
Saying mass changes on the Moon — mass is constant, only weight changes.
2
Measuring weight in kilograms — weight is a force in newtons and mass is in kilograms.
3
Using grams in W = mg — convert to kilograms first.
4
Thinking a balance and a newton meter read the same quantity — a balance compares masses and a newton meter reads weight.

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