Question 1Suppose your writing desk is tilted a little. A book kept on it starts sliding down. Show the direction of frictional force acting on it.
Frictional force will act upward i.e., direction opposite to that of sliding book.
Question 4Iqbal has to push a lighter box and Seema has to push a similar heavier box on the same floor. Who will have to apply a lager force and why?
We know that the force of friction is increased if the two surfaces are pressed harder. A heavier box will apply more pressure on the floor and hence Seema will experience more frictional force.
Question 5Explain why the sliding friction is less than the static friction.
The friction is due to the interlocking of irregularities in the two surfaces. When the object starts sliding the contact points on its surface, do not get enough time to lock into the contact points on the floor. So the sliding friction is slightly less than the static friction.
Question 6Give example to show that friction is both a friend and foe.
(i) Friction as a Friend
Frictional force is very useful in several situations. If there were no friction between the wheels and the road, no vehicle would be able to move. If there were no brakes in a bicycle there would be many accidents. It is due to the frictional force between the road and the wheel or between the wheel and the brake that vehicles can move and stop.
In the same way, when we walk, because of friction the sole of our footwear rubs against the hard surface below. If there were no friction we would slip. The reason for slipping on a banana peel is because the friction is low.
When two objects are rubbed against each other heat is produced. That is the reason you rub your hands together in winter. But for friction it would not be possible to light a matchstick.
(ii) Friction as a Foe
Machine parts that come in contact with one another wear out because of friction. Similarly the soles of our shoes wear out because of friction.
Question 7Explain why objects moving in fluids must have special shape.
When objects move through fluids, they have to overcome the friction acting on them. Efforts are therefore made to minimize the friction so objects are given special shapes. Idea of such shapes come from the body structure of birds and fishes which have to move about in fluids all the time. Such shapes are called streamlined. Giving such shapes to the ships, plane and cars is called streamlining.