Physics Paper 2, Nov/Dec. 2010  
Questions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Main
General Comments

Question 11
Part II : Candidates are expected to answer any three form this part

(a) Define:
        (i) work;
        (ii) power
(b) Distinguish between static friction and dynamic friction
(c)Why do tyres have treads?
(d) A car of mass M starts from rest and moves with a constant acceleration, a. it attains a
        velocity, v after time t, and maintains this velocity until time t2. It is then brought to rest with
        uniform retardation, d at time t3.
                        (i)          Draw and label a velocity-time graph for the motion.
                        (ii)         Deduce an expression for the force on the car during the period of retardation.



Part (a) was satisfactorily attempted by most candidates however; some confused work with moment of a force. Part (b) was poorly attempted. Part (c) showed that lessons are probably presented in an abstract manner as performance was poor. In part (d), the graph shape was correctly drawn by many but majority quoted the formula F = ma instead of deducing it.

The expected answers are:

(a)(i) Work is the product ofa force (applied) and the distance moved in the direction of the force
The product of displacement and the component of the force in the direction of the displacement.
(ii) Power is the time rate at which energy is expended.
Power is the time rate of doing work.

NOTE: Mathematical definitions are acceptable provided symbols are correctly defined.

(b)          Any correct difference

Static Friction Dynamic Friction
It exists between surfaces of
stationary bodies
It exists between surfaces in relative

- For the same system the static frictional force is greater than dynamic frictional force

(c) The tyres have treads in them to provide better grip of roads due to increased friction thus preventing vehicles from skidding

(ii) Retardation d = slope (oflast segment of graph)

= v - 0
T3 - t2
= v
t3 - t2

F = md

= Mv/t3 - t2

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