National 5PhysicsSQARevision NotesSpaceSpace ExplorationCalculating Weight across the Universe

Calculating Weight across the Universe (SQA National 5 Physics): Revision Note

Exam code: X857 75

Katie M

Written by: Katie M

Reviewed by: Caroline Carroll

Updated on

Calculating weight across the universe

  • 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)

  • Different planets have different gravitational field strengths

    • This depends on the mass of the planet

    • More massive planets have stronger gravitational fields

  • The gravitational field strengths at the surface of different objects in the solar system are shown in the table

Gravitational field strengths in the solar system

Object

Gravitational field strength on the surface (N kg-1)

Earth

9.8

Jupiter

23

Mars

3.7

Mercury

3.7

Moon

1.6

Neptune

11

Saturn

9.0

Sun

270

Uranus

8.7

Venus

8.9

Examiner Tips and Tricks

This table will be provided in the Data Sheet for your exam, so you will be expected to identify when to use these values, rather than being given them in the question.

Mass vs. weight

  • An object’s mass always remains the same, regardless of its location in the universe

  • The weight force acting on the object will vary depending on the gravitational field strength at the location in the universe

  • For example, the gravitational field strength on the Moon is 1.63 N kg-1, meaning an object’s weight will be about 6 times less than on Earth

Comparison of weight on Earth and on the Moon

Man standing on Earth and Moon, illustrating weight differences due to gravity. Earth: weight 687 N, Moon: weight 114 N, both with 70 kg mass.
On the Moon, a person's mass will stay the same but their weight will be much lower

Worked Example

A student estimates they would have a weight of 185 N on Mars.

Calculate the weight of the student on Earth.

Answer:

Step 1: List the known quantities

  • Weight on Mars, WM = 185 N

  • Gravitational field strength on Mars, gM = 3.7 N kg-1

  • Gravitational field strength on Earth, gE = 9.8 N kg-1

Step 2: Write out the equation relating mass and weight and rearrange for mass

W space equals space m g

  • Divide both sides by g:

m space equals space W over g

Step 3: Calculate the student’s mass

  • The student’s mass is the same at all locations in the universe

m space equals space W subscript M over g subscript M space equals space W subscript E over g subscript E

m space equals space fraction numerator 185 over denominator 3.7 end fraction space equals space 50 space kg

Step 4: Calculate the student’s weight on Earth

W subscript E space equals space m cross times g subscript E space equals space 50 cross times 9.8 space equals space 490 space straight N

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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.