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Kinetic Energy (CIE AS Physics)

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Derivation of KE = 1/2mv2

  • Kinetic energy is energy an object has due to its motion (or velocity)
  • Resultant forces make objects accelerate as described by Newton's second law F = ma
  • Work is done by that force and energy is transferred to the object
  • Using this concept of work done and an equation of motion, the extra work done due to an object's speed can be derived

  • Consider a mass, m, at rest which accelerates to a velocity, v, over a displacement, s
  • The work done in accelerating the mass is:

W space equals space F s

  • Newton's Second Law states that:

F space equals space m a

  • The suvat equation for velocity is:

v to the power of 2 space end exponent space equals space u squared space plus space 2 a s

    • If the initial velocity is zero, u = 0
    • And displacement, s 
    • Then:

v squared space equals space 2 a s

    • Rearranging to make a the subject:

a space equals fraction numerator space v squared over denominator 2 s end fraction

  • Substituting this expression for a into Newton's Second Law gives:

F space equals space m a space equals fraction numerator space m v squared over denominator 2 s end fraction

  • Substituting this expression for F into the work done equation gives:

W space equals fraction numerator space m v squared over denominator 2 s end fraction cross times space s space equals space 1 half m v squared

  • The mass is now able to do extra work due to its speed
    • The amount of extra work is equal to 1 half m v squared
  • The mass now has: 

E subscript k space equals space 1 half m v squared

Kinetic Energy

  • The faster an object is moving, the greater its kinetic energy
  • When an object is falling, it is gaining kinetic energy since it is gaining speed
  • This energy transferred from the gravitational potential energy it is losing
  • An object will maintain this kinetic energy unless its speed changes

  • The amount of kinetic energy an object has is determined by its mass and its speed:

E subscript k space equals space 1 half m v squared

  • Where:
    • Ek = kinetic energy in joules (J)
    • m = mass in kilograms (kg)
    • v = velocity in metres per second (m s-1)

 

A car travelling forwards

Kinetic energy diagram, downloadable AS & A Level Physics revision notes

Kinetic energy is the energy an object has when it is moving, determined by its mass and its speed

Worked example

A body travelling with a speed of 12 m s-1 has kinetic energy 1650 J. The speed of the body is increased to 45 m s-1. Determine the body's new kinetic energy.

Answer:

Step 1: List the known quantities

  • Initial speed, vi = 12 m s-1
  • Initial kinetic energy, Ek i = 1650 J
  • Final speed, vf = 45 m s-1

Step 2: State the kinetic energy equation

E subscript k space equals space 1 half m v squared

Step 3: Determine the object's mass

  • The mass will not change
  • Therefore, mass can be calculated from the initial kinetic energy

m space equals fraction numerator space 2 E subscript k space i end subscript over denominator v subscript i squared end fraction

m space equals space fraction numerator 2 space cross times space 1650 over denominator 12 squared end fraction

m space equals space 23 space kg

Step 4: Substitute the known values to calculate the final kinetic energy

E subscript k space f end subscript space equals space 1 half m v subscript f squared

E subscript k space f end subscript space equals space 0.5 space cross times space 23 space cross times space 45 squared

E subscript k space f end subscript space equals space 23 space 000 space straight J

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Leander

Author: Leander

Expertise: Physics

Leander graduated with First-class honours in Science and Education from Sheffield Hallam University. She won the prestigious Lord Robert Winston Solomon Lipson Prize in recognition of her dedication to science and teaching excellence. After teaching and tutoring both science and maths students, Leander now brings this passion for helping young people reach their potential to her work at SME.