National 5PhysicsSQARevision NotesDynamicsNewton’s LawsMass & Weight

Mass & Weight (SQA National 5 Physics): Revision Note

Exam code: X857 75

Katie M

Written by: Katie M

Reviewed by: Caroline Carroll

Updated on

Mass & weight

  • Weight and mass are different in physics

  • Mass is a measure of how much matter there is in an object

    • Mass has magnitude but not direction

    • Therefore, mass is a scalar quantity

  • Weight is a force 

    • Forces have magnitude and direction

    • Therefore, weight is a vector quantity

Weight and gravitational field strength

  • Weight is the effect of a gravitational field on a mass

  • Weight is defined as:

The force experienced by an object with mass when placed in a gravitational field

  • Planets have strong gravitational fields

    • Hence, they attract nearby masses with a strong gravitational force

  • Because of weight:

    • Objects stay firmly on the ground

    • Objects will always fall to the ground

    • Satellites are kept in orbit

5-1-4-weight-force

Some of the phenomena associated with gravitational attraction and the weight force

Weight, mass and gravitational field strength

  • Weight, mass and gravitational field strength are related using the equation:

W space equals space m g

  • Where:

    • W = weight, measured in newtons (N)

    • m = mass, measured in kilograms (kg)

    • g = gravitational field strength, measured in newtons per kilogram (N kg-1)

  • The gravitational field strength on Earth is 9.8 N kg-1

  • g is also used to describe the acceleration of an object in freefall in a gravitational field

    • The acceleration of freefall on Earth is 9.8 m s-2

    • These quantities are two ways of describing the thing 

  • The mass of an object and the weight acting on it are directly proportional

    • If one doubles, the other also doubles

    • If one is halved, the other is also halved

Worked Example

A weightlifter applies an upwards force of 784 N to a barbell to hold it still.

(a) Describe how the upward force exerted by the weightlifter on the barbell compares to the weight of the barbell.

(b) Calculate the mass of the barbell.

Answer:

(a)

  • The upward force and weight are balanced, meaning they are equal in size and opposite in direction

(b)

Step 1: List the known values

  • Gravitational field strength, g = 9.8 N kg-1

  • Weight of barbell, W = 784 N

Step 2: State the equation linking weight and mass and rearrange for mass

W space equals space m g

  • Divide both sides by g:

m space equals space W over g

Step 3: Substitute the values of weight and gravitational field strength into the equation

m space equals space fraction numerator 784 over denominator 9.8 end fraction

Step 4: Calculate the mass of the barbell

m space equals space 80 space kg

Examiner Tips and Tricks

It is a common misconception that mass and weight are the same, but they are in fact very different

  • Since weight is a force - it is a vector quantity

  • Since mass is an amount - it is a scalar quantity

You do not need to remember the value of g on Earth; it will be given to you in the exam.

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    Author
    Reviewer
    Katie M

    Author: Katie M

    Expertise: Physics Content Creator

    Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.

    Caroline Carroll

    Reviewer: Caroline Carroll

    Expertise: Physics & Chemistry Subject Lead

    Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about creating high-quality resources to help students achieve their full potential.