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7. a. The motor of an automobile moving with a constant velocity is switched off. According to first law of motion it should continue its motion. But in practice it comes to rest. Why? (1) b. A bats man hits back a ball straight in the direction of bowler with out changing its initial speed of 12ms-1. If the mass of the ball is 0.15kg determine the impulse of the ball. (1½) c. A block of mass m is Sliding down an inclined plane of angle ?. If ? is the Coefficient of sliding friction between the block and inclined plane, what Is the acceleration of the body? (1½) d. Schematically represent the various forces acting on a car moving with a velocity v along a curved road which is banked at an angle ? with the horizontal (2)

a) First law: If there is no net force on an object, then its velocity is constant. The object is either at rest (if its velocity is equal to zero), or it moves with constant speed in a single direction.Here we apply an external force for for stopping the car.so it comes to rest.

b) m = 0.15 kg, u = 12 m/s, v = - 12 m/s

        Impulse = m (v-u) = 0.15(-12 – 12) = - 3.6 Ns

The negative sign indicates that the direction of the impulse is from batsman to the bowler.

c) The acceleration is produced by an unbalanced force.The sum of frictional force and the applied force gives the net force.If the mass of the block is m then,

                    net firce Fnet= mg sin? + ?mg cos?

                                 Fnet=ma

therefore a = g sin? + ?g cos?

                 = g( sin? + ?cos?)

d)the forces acting are ---
1) centripetal force acting towards the centre of circle
2) frictional force acting tangential to the wheels along the path
3) normal reaction of the path acting normally upward
4) weight of the car acting vertically downward
5) centrifugal reaction acting opposite to the centripetal force
and also 6) a centrifugal force which is not real in earth frame, but real in rotation frame.


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