Topic  3

What happens to movement energy when things speed up or slow down?

Force, Energy, and Work

Energy enables us to do jobs of work.  When work is done, energy is transferred.  We can say:

Energy transferred = work done

Both energy and work are measured in joules (J).  Many people call work energy, but the best way to think about it is that energy is the potential to do work.

Question 1  Is there likely to be more work got out than energy transferred?  ANSWER

We can relate work and force by the simple equation which you need to learn for the exam:

In Triangle form:

Note that the direction of the force and the direction of the movement have to be the same.  If they are at right angles, zero work is done.  To explain this, look at the picture of the crane:

The crane does work by lifting the load vertically.  If the load is hanging and swings from side to side, work is not done.

Worked Example

A common bear trap is to fail to convert a mass to a weight.

Question 2  A box is dragged 5 m across a floor with a force of 20 N.  How much work is done?  How much energy is transferred?  ANSWER

Work Done and Friction

If you look at the example above, you will see that the work done is NOT against the weight of the box.  The work done is in the direction of the force, horizontally.  This work is done against the force of friction.

Friction arises because even the smoothest of surfaces are rough if you look at them under a microscope.

Friction can be reduced by lubricating the surfaces with oil.  PTFE and ice have very low friction, so they are very slippery.

When work is done against friction, most of the energy is turned to heat.  You can see this very clearly when a plane lands on a runway:

You can see the smoke coming from the tyres.  The wheels are stationary just before touch-down, so they are dragged along the runway for several metres before they are turning at the landing speed.  The tyres got very hot and smoke.  Plane tyres are thicker than car tyres for this reason.  Even so they have to be periodically replaced.  You may wonder why the wheels are not spun to prevent this heating on landing.    The reason for this is that there would be extra weight for each motor.  Also if one motor failed, the aeroplane would be difficult to control on landing.

Question 3  What other kind of energy other than heat do you get when you do work against friction?  ANSWER

Elastic Potential Energy

You can store up energy by deforming a solid material.  This is called elastic potential energy.  The little boy in the picture below is winding the propeller of his toy plane.

Energy is stored in the rubber band as elastic potential energy.  When he releases the plane, the energy stored in the rubber band is released as kinetic energy in the propeller, which makes the plane fly.

Another example is the elastic energy stored in the wood of this archer's bow.

Clockwork motors in many toys use elastic potential energy.

Question 4  Where is the elastic energy stored in a clockwork motor?  ANSWER

Kinetic Energy

Movement energy is called kinetic energy (from the Greek "kinein" - to move).  All moving objects have kinetic energy:

• objects moving in a straight line;

• objects going round in a circle;

• objects vibrating.

All the devices above convert the elastic potential energy into kinetic energy.

Working out Kinetic Energy (HT only)

The equation for kinetic energy needs to be learned for the exam:

Worked Example

Question 5  A model racing car has a mass of 0.1 kg and kinetic energy of 5 J.  How fast is it travelling?  ANSWER

Now try the Topic Quiz

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