Question 4Answer the following.
Question 5What is easier to move a box from rest, or to move it when it is already in motion?
When the box starts sliding, the contact points on its surface, do not get enough time to look into the contact points on the floor. So, the sliding friction is slightly less than the static friction and you find it easier to move the box already in motion than to get it started.
Question 6Does the smoothness of the sliding surface affect friction? Explain.
Friction is caused by the irregularities on the two surfaces in contact. Even those surfaces which appear very smooth have a large number of minute irregularities on them. Irregularities on the two surfaces lock into one another. When we attempt to move any one surface over other, we have to apply a force to overcome interlocking. On rough surface, there are a large number of irregularities. So the force of friction is greater if a rough surface is involved.
Question 7Why the friction between two surface increases when they are pressed harder. Give an activity to show the above effect.
Since the friction is due to the interlocking of irregularities in the two surfaces which slide with respect to each other, it is obvious that the force of friction will increase if the two surfaces are pressed harder. One can experience it by dragging a mat when no body is sitting on it and when a preson is sitting on it.
Question 8What would happen if there were no force of friction? Imagine and describe in your own words.
If there were no friction many problems could have been arise one cannot able to walk if there were no friction at all. One could not write with pen or pencil, is there were no friction. When one is writing with chalk on the blackboard, its rough surface rubs off some chalk particles. So, we can see the writing on the black board. If any object just started moving, it would never stop. Had there been no friction between the tyres of the automobiles and the road, they could not be started or stopped or turned to change the direction of motion. One could not fix a nail in the wall or tie a knot. Without friction no building could be constructed.
Question 10Describe some examples where heat is produced due to friction.
Friction can also produce heat. When we rub our hands together vigorously for few minutes we feel hot.
When we strike a match stick, against the rough surface, it catches fire. We also observe that the jar of a mixer becomes hot when it is run for a few minutes.
Question 11Describe some examples where we deliberately increase friction.
We deliberately increase friction by using brake pads in the brake system of bicycles and automobiles. When we are riding a bicycle, the brake pads do not touch the wheels. But when we press the brake lever, these pads arrest the motion of the rim due to friction. The wheel stops moving. Kabaddi players rub their hands with the soil for an easy grab of their opponents. Gymnasts apply some coarse substance on their hands to increase friction for better grip.
Question 12Describe some events when we reduce the friction.
We sprinkled fine powder on the carom board. When a few drops of oil are poured on the hinges of a door moves smoothly. A cycle or a motor mechanic uses grease between the moving parts of the cycle. In all the above cases, we want to reduce friction in order to increase efficiency. When oil, grease or graphite is applied between the moving part of a machine, its thin layer is formed there and moving surfaces do not directly rub against each other. Interlocking of irregularities is avoided to a great extent. Movement becomes smooth. The substances which reduce friction are called lubricants. In some machines it may not be advisable to use oil as lubricant. There is an air cushion between the moving parts is used to reduce friction.
Question 13Explain why it is convenient to pull the luggages fitted with rollers.
When one body rolls over the surface of another body, the resistance to its motion is called the rolling friction. Rolling reduces friction. It is always easier to roll than to slide a body over another. That is the reason it is convenient to pull the luggages fitted with rollers.
Question 14What is fluid friction?
Air is very light and thin. Yet it exerts frictional force on objects moving through it. Similarly, water and other liquids exert force of friction when objects move through them. In science, the common name of gases and liquids is fluids. So we can say that fluids exert force of friction on objects in motion through them.
The frictional force exerted by fluids is also called drag.
The friction force on an object in a fluid depends on its speed with respect to the fluid. The frictional force also depends on the shape of the object and the nature of the fluid.
Question 18Enlist any four methods of reducing friction.
(i) by applying a lubricant
(ii) by using ball bearing
(iii) by polishing surfaces
(iv) In the machines where oil cannot be used air cushion between the parts is used to reduce friction.
(v) by streamlining.