National 5PhysicsSQARevision NotesDynamicsEnergyKinetic Energy

Kinetic Energy (SQA National 5 Physics): Revision Note

Exam code: X857 75

Katie M

Written by: Katie M

Reviewed by: Caroline Carroll

Updated on

Kinetic energy

  • Kinetic energy is defined as:

The energy an object has due to its movement

  • This means that all moving objects have kinetic energy due to their mass and speed

    • If an object speeds up, energy is transferred to it as kinetic energy

    • If an object slows down, its kinetic energy is transformed into a different type of energy

Kinetic energy of a moving object

A blue car of mass m moving to the right with speed v and kinetic energy equal to half times mass times speed squared
The kinetic energy of a moving car depends on its mass and speed

Calculating kinetic energy

  • The kinetic energy possessed by a moving object can be calculated using the relationship:

E subscript k space equals space 1 half m v squared

  • Where:

    • E subscript k = kinetic energy, measured in joules (J)

    • m = mass of the object, measured in kilograms (kg)

    • v = speed of the object, measured in metres per second (m s-1)

  • The kinetic energy equation demonstrates that if the mass of an object is doubled for a given speed, then its kinetic energy will double

    • This is because kinetic energy is directly proportional to mass

    • E subscript k space proportional to space m

  • If the speed of the object is doubled for a given mass, it will have four times the kinetic energy

    • This is because kinetic energy is directly proportional to velocity squared

    • E subscript k space proportional to space v squared

Worked Example

Calculate the kinetic energy stored in a vehicle of mass 1200 kg moving at a speed of 27 m s-1.

Answer:

Step 1: List the known quantities

  • Mass of the vehicle, m space equals space 1200 space kg 

  • Speed of the vehicle, v space equals space 27 space straight m divided by straight s

Step 2: Write down the equation for kinetic energy

E subscript k space equals space 1 half m v squared

Step 3: Calculate the kinetic energy

E subscript k space equals space 1 half space cross times space 1200 space cross times space open parentheses 27 close parentheses squared

E subscript k space equals space 437 space 400 space straight J

Step 4: Round the final answer to 2 significant figures

E subscript k space equals space 440 space 000 space straight J space open parentheses 2 space straight s. straight f. close parentheses

Examiner Tips and Tricks

When performing calculations using the kinetic energy equation, always double-check that you have squared the speed. Forgetting to do this is the most common mistake that students make.

You will most likely need to rearrange the kinetic energy equation in your exam. The kinetic energy equation is one of the more difficult rearrangements, so make sure you are comfortable doing it before your 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.