(a)(i) Laws of solid friction states that:
— friction opposes motion
— friction depends on the nature of e surfaces
— friction is independent of the area of the surfaces in contact
— frictional force F is proportional to the normal reaction R
— friction is dependent of relative velocity between the surfaces.

ii) Advantages of friction:
— it prevents man from ipping when walking
— it provides gripping effect of beIts in machines
— for grinding when using grinding ones
— used for the production of electric charges used in the brake systems of vehicles etc.

iii) Methods of reducing friction:
— by lubrication by streamlining
— by smoothing
— by using ball bearings or rollers.

b)


(c)(i) Efficiency = $\frac{\text{Work output}}{\text{work output}}$ x 100%
= $\frac{\text{load x distance by load}}{\text{effort x distance moved by effort}}$
= M.A x $\frac{1}{VR}\times \frac{100}{1}$

(ii) $e=\frac{\text{Work output}}{\text{Work input}}$ x 100
Work input = $\frac{\text{Work output}}{E}$ x 100
= $\frac{300\times 10\times 4\times 100}{80}$
= 15,000J or 15KJ