Work Done (HL IB Physics)

• The work done by a force is equivalent to a transfer of energy
  • The units of work done are newton metres
  • 1 N m = 1 J
• The work done by a resultant force on a system is equal to the change in energy in that system

• Mechanical work is defined as

The transfer of energy when an external force causes an object to move over a certain distance

• If a constant force is applied in the line of an object's displacement (i.e. parallel to it), the work done can be calculated using the equation:

• Where:
  • W = work done (J)
  • F = constant force applied (N)
  • s = displacement (m)

• In the diagram below, the man’s pushing force on the block is doing work as it is transferring energy to the block

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

• When pushing a block, work is done against friction and energy is transferred from the man to the block
• The kinetic energy is transferred to other forms of energy such as heat and sound

• When plotting a graph of average force applied against displacement, the area under the graph is equal to the work done

• Sometimes the direction of motion of an object is not parallel to the direction of the force
• If the force is at an angle θ to the object's displacement, the work done is calculated by:

• Where θ is the angle, in degrees, between the direction of the force and the motion of the object
  • When θ is 0 (the force is in the direction of motion) then  and

• For horizontal motion, cos θ is used
• For vertical motion,  sin θ is used
  • Always consider the horizontal and vertical components of the force
  • The component needed is the one that is parallel to the displacement

When the force is at an angle, only the component of the force in the direction of motion is considered for the work done