What can create an unbalanced force?

Unbalanced forces are generally due to a forward force (or thrust) and a frictional force working against each other.

Friction is a force that opposes motion. It is present whenever two surfaces rub over each other, such as when you rub your hands together, or when you apply the brakes on a bike or in a car. Friction also prevents an object from starting to move, such as a shoe placed on a ramp. When friction acts between two surfaces that are moving over each other, some kinetic energy is transformed into thermal (heat) energy.

Friction can sometimes be useful. For example, we rely on the soles of our shoes to not slip over the ground that we are walking on, and the friction between a car tyre and the road surface helps cars to speed up, slow down and turn corners. Sometimes though, friction can be a nuisance. For example, the friction between a wheel and the axle that it rotates on wastes energy, so we try to minimise the friction using bearings and lubricants.

A demonstration of friction

Energy transferred due to frictional forces

Energy is 'lost' or not used effectively when frictional forces occur. For example, when a bike applies its brakes and slows down the kinetic energy of the bike is transferred into heat energy in the brakes.

How much energy is lost depends on the frictional force ( F) and the distance over which the frictional force is acting ( d).

This is called the Work Done against Friction ( E_{w}).

Work Done = frictional force x distance

 E_{w}=F\times\,d

Example

Travelling between stops, the average frictional force on a bus was 8\cdot2\times10^{3}N over a distance of 500 m.

Calculate the work done by the bus engine to overcome this frictional force.

 E_{w}=Fd

E_{w}=8\cdot2\,\times10^{3}\,\times500

 E_{w}=4\cdot1\,\times10^{6}\,J