EXAM GUIDELINES P1
Work, Energy and Power
(This section must be read in conjunction with the CAPS, p. 117–120.)
Work
- Define the work done on an object by a constant force F as:
where
(Work is done by a force on an object – the use of 'work is done against a force', e.g. work done against friction, should be
avoided.)
- Draw a force diagram and free-body diagrams.
- Calculate the net/total work done on an object.
- Distinguish between positive net/total work done and
negative net/total work done on
the system.
Work-energy theorem
- State the work-energy theorem:
The net/total work done on an object is equal to the change in the object's kinetic energy OR the work done on an object by a resultant/net
force is equal to the change in the object's kinetic energy.
In symbols:
- Apply the work-energy theorem to objects on horizontal, vertical and inclined planes
(for both frictionless and rough surfaces).
Conservation of energy with non-conservative forces present
- Define a conservative force as a force for which the work done in moving an object
between two points is independent of the path taken.
Examples are gravitational force, the elastic force in a spring and electrostatic forces (coulomb forces).
- Define a non-conservative force as a force for which the work done in moving an object
between two points depends on the path taken. Examples are frictional force, air
resistance, tension in a chord, etc.
- State the principle of conservation of mechanical energy: The total mechanical energy
(sum of gravitational potential energy and kinetic energy) in an isolated system remains
constant. (A system is isolated when the resultant/net external force acting on the
system is zero.)
- Solve conservation of energy problems using the equation:
- Use the relationship above to show that in the absence of non-conservative forces,
mechanical energy is conserved.
Power
- Define power as the rate at which work is done or energy is expended.
In symbols:
- Calculate the power involved when work is done.
- Perform calculations using:
when an object moves at a constant speed along a rough horizontal surface or a rough inclined plane.
- Calculate the power output for a pump lifting a mass (e.g. lifting water through a height
at constant speed).