(a) State two;
(i) laws of solid friction;
(ii) advantages of friction;
(iii) methods of reducing friction.
(b) Draw and label a diagram of a pulley system with velocity ratio of 5.
(c)(i) Show that the efficiency L, force ratio M.A. and the velocity ratio V.R. of a machine are related by the equation (E = frac{M.A.}{V.R}) x 100%.
(ii) The efficiency of a machine is 80%. Calcuate the work done by a person using the machine to raise a load of 300 kg through a height of 4 m.[ g = 10 ms(^{-2}) ]
Explanation
(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