3.3  Work, Energy and Power

 

3.3.1  Work and conservation of energy

 

Learners should be able to demonstrate and apply their knowledge and understanding of:

(a) work done by a force; the unit joule

I know it doesn't sound like it, but work done is an energy and thus measured in Joules.

 

​​​​​​

 

(b)       for work done by a force

If the force and the displacement are in the same direction then theta is zero, therefore cos θ = 1 and the equation becomes the familiar one of work done = force x distance moved.

 

​​​

 

(c) the principle of conservation of energy

this is still as true as it ever was!  Energy can still not be created or destroyed.

 

​​

 

(d) energy in different forms; transfer and conservation

Same old stuff that you have had every year since Upper Three.

 

​​

 

(e) transfer of energy is equal to work done.

This is important.  It comes up in a lot of questions.  If you think it is a suvat question but it just doesn't seem to work, then it might be this that you need.

 

​​

 

3.3.2 Kinetic and potential energies

 

Learners should be able to demonstrate and apply their knowledge and understanding of:

 

(a) kinetic energy of an object;    Learners will also be expected to recall this equation and derive it from first principles.

You derive this by starting with one of the equations of motion.  I won't tell you which one, wouldn't want to spoil the excitement.

 

​​​​

 

(b) gravitational potential energy of an object in a uniform gravitational field; Ep = mgh Learners will also be expected to recall this equation and derive it from first principles.

Derivation of this comes from the definition of work done.  Look at the second link for details.

 

​​​​

 

(c) the exchange between gravitational potential energy and kinetic energy.

This is all about equating the two equations for GPE and KE.  The important thing to notice is that the mass cancels and thus you don't need to be told it in the question.

 

​​

 

 

 

 

3.3.3  Power

 

(a) power; the unit watt; P = Wt

Again, nothing much new here.  Power is still how quickly energy is transferred.  Lots of calculation questions to be asked on this topic.  Make sure you try the questions below.

 

​​​​​​

 

(b) P = Fv   Learners will also be expected to derive this equation from first principles.

Start from power = work done/ time then substitute in work done = force x distance and speed = distance/time

 

(c) efficiency of a mechanical system;

 

Equation has not changed but the questions you will be asked about it have.  Make sure you get plenty of practice with them and understand what the equation is actually telling you.

 

​