Edexcel IGCSE Physics 复习笔记 4.2.1 Work Done

• Whenever any work is done, energy gets transferred (mechanically) from one store to another
• The amount of energy transferred (in joules) is equal to the work done (also in joules)

Energy transferred (J) = Work done (J)

• If a force acts in the direction that an object is moving, then the object will gain energy (usually in the form of kinetic energy)
• If the force acts in the opposite direction to the movement then the object will lose energy (usually as heat)
• One example of work done is when an object is moved over a distance by a force applied in the direction of its displacement
  • It is said that the force does work on the object
  • If a force is applied to an object but doesn’t result in any movement, no work is done

   

Work is done when a force is used to move an object

Examples of Work

• Work is done on a ball when it is lifted to a height above the ground
  • The energy transfer is from kinetic energy to lift the ball from the ground to gravitational potential energy

   

The weight on the ball produced by the gravitational field does work on the ball over a distance equal to the height of the ball

• Work is done when a bird flies through the air
  • The bird must travel against air resistance and therefore some energy is wasted through this resistance

   

Air resistance (drag) does work against the bird as it flies through the air

• The amount of work that is done is related to the size of the force and the distance moved by the object in the direction of the force
• To calculate the amount of work done on an object by a force the following formula is used

W = f × d

• Where:
  • W = work done in Joules (J) or newton-metres (N m)
  • F = force in Newtons (N)
  • d = distance in metres (m)

   

• This equation can be rearranged with the help of a formula triangle:

Work, force, distance formula triangle

Step 1: List the known quantities

• • Distance, d = 23 m
  • Force, F = 500 N

   

Step 2: Write out the equation relating work, force and distance

W = F × d

Step 3: Calculate the work done on the car by the brakes

W = 500 × 23 = 11 500 J