Kinetic Energy (CIE A Level Physics)

• 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:

• Newton's Second Law states that:

• The suvat equation for velocity is:

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

• • Rearranging to make a the subject:

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

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

• The mass is now able to do extra work due to its speed
  • The amount of extra work is equal to 
• The mass now has: 

• 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:

• Where:
  • E_k = 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 is the energy an object has when it is moving, determined by its mass and its speed