Solved Examples


1.    A body of mass 400 g slides on a rough horizontal surface . If the frictional force is 3.0 N, find (a) the angle made by the contact force on the body with the vertical and (b) the magnitude of the force. Take g = 10 m/s2.
Sol.    Let the contact force on the block by the surface be F which makes an angle  with the vertical (figure)

2.    A heavy box of mass 20 kg is pulled on a horizontal force. If the coefficient of kinetic friction between the box and the horizontal surface is 0.25, find the force of frictionless exerted by the horizontal surface on the box.
Sol.    The situation is shown in figure (6.6). In the vertical direction there is no acceleration , so 

3.    A boy (30 kg) sitting on his horse whips it. The horse speeds up at an average acceleration of 2.0 m/s2.(a) If the boy does not slide back, what is the force of friction exerted by the hoerse on boy.(b) If the boy slides back during the acceleration , what can be said about the cofficientof static friction between the horse and the boy ? Take g = 10 m/s2.          
Sol.    (a) The horse acting on the boy are 
    (i) the weight Mg1
    (ii) the normal contact force  and
    (iii) the static friction fs.


4.    A wooden block is kept on

a polished wooden plank and the inclination of the plank is gradually incresed. It is found that starts sliping when the plank makes an angle of 18º with the horizontal . However, once  strated, the block can continue with uniform speed if the inclination is reduced to 15°. Find the coefficients of static and kinetic fraction between the block and the plank.


Questions for Short answer

1.    For most of the surfaces used in daily life, the friction cofficient is less than 1. Is it always necessary that the friction cofficient is less than 1?

2.    Why is it easier to push a heavy block from behind than to press it on the top and push ?

3.    What is the average friction force when a person has a usual 1 km walk ?

4.    Why is it difficult to walk on solid ice ?

5.    Can you acceleration a car on a frictionless horizontal road by putting more petrol in the engine?Can you stop a car going on a frictionless horizontal road by applying brakes?

6.    Spring   fitted doors close by themselves when relesed.You want to keep the door open for  a long time say for