Power (CIE A Level Maths: Mechanics)

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Power

What is power?

  • Power is developed as an engine or machine does work
  • Power is the rate of doing work, usually by the driving force of an engine
    • It is the same as work done per second
    • The work done is converting fuel (a type of chemical energy) into a driving force
  • In your Mechanics A Level course, power will usually be in questions about moving vehicles
  • Power is a scalar quantity and can only take a positive value

How is power calculated?

  • Power is the rate of doing work and can be calculated by the formula

P space equals space fraction numerator W D over denominator t end fraction

Where WD is the work done by the driving force in Joules and is the time in seconds

  • If a driving force, F N, is acting on an object moving with velocity, v m s-1 then power can be calculated using the formula

begin mathsize 16px style P space equals F v end style

    • The velocity must be in the same direction as the force
    • For constant and the above formula can be derived by recalling the formula

v space equals space s over t

                      where v is speed or velocity and is distance or displacement.

3-2-1-m2-power-diagram-1

    • The derivation can also be shown for a constant force, F N and considering the rate of doing work over a very small interval of time
      • P space equals space F fraction numerator delta s over denominator delta t end fraction
      • As the time δt gets smaller and approaches  begin mathsize 16px style fraction numerator d s over denominator d t end fraction end stylethen
      • P space equals F fraction numerator d s over denominator d t end fraction equals F v 

  • The units for power are watts (W) or kilowatts (kW)
    • 1 watt = 1 joule per second
    • 1 kilowatt = 1000 watts

How is power used in calculating maximum speed?

  • As a vehicle gains speed the driving force can decrease to create the same amount of power
  • If the vehicle maintains maximum power its driving force will decrease as its speed increases and it will eventually reach a point where its resultant force is zero (the driving force will balance the resistance forces)
    • When the resultant force becomes zero, by Newton’s Second Law, acceleration will also become zero and the vehicle will be travelling at maximum speed
    • The maximum power output of a vehicle can be found when the vehicle is moving at its maximum speed and the acceleration is zero
    • If the maximum power output is known, then maximum speed can be found when the vehicle is travelling at its maximum power with zero acceleration and the resultant forces are balanced

Worked example

A car of mass 1300 kg, including the driver, moves forwards on a straight horizontal road. There is a constant resistive force of 900 N acting on the car. Its maximum possible speed is 40 m s-1  Calculate the maximum power that the engine of the car can produce.

3-2-1-m2-power-worked-example

Examiner Tip

  • Make sure you are using the correct force in your calculation, power is only generated by the driving force of an engine and so only this force should be used in the formula.
  • Always draw a diagram and add the forces. If the question involves an inclined slope remember to resolve the weight into components parallel and perpendicular to the slope first.
  • Remember to check the units carefully, power questions could be given in watts or kilowatts. It is also important to give your answer in the correct units, or if not specified, choose the most appropriate units for the question.

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Amber

Author: Amber

Expertise: Maths

Amber gained a first class degree in Mathematics & Meteorology from the University of Reading before training to become a teacher. She is passionate about teaching, having spent 8 years teaching GCSE and A Level Mathematics both in the UK and internationally. Amber loves creating bright and informative resources to help students reach their potential.