SHORT ANSWER TYPE QUESTIONS
Q. 1. What is the cause of friction?
Ans. Friction is caused by the irregularities on the two surfaces in contact. The irregularities on the two surfaces lock into one another. Due to the interlocking of two surfaces friction arises.
Q. 2. Push the book on a table. You observe that after some time it stops. Explain why?
Ans. When we push a book on a table then a force acts on the book which opposes its motion. This force is called force of friction. The friction opposes the movement due to which the book stops.
Q. 3. What is a spring balance?
Ans. Spring balance is a device used for measuring the force acting on an object. It contains a coiled spring. The spring gets stretched when a force is applied on it. Stretching of the spring is measured by a pointer moving on a graduated scale. The reading on the scale gives the magnitude of force.
Q. 4. How does the friction get affected by the nature of surface?
Ans. Nature of surface is the major factor which affects the friction. When we attempt to move any surface, we have to apply a force to overcome the interlocking of the surfaces. On rough surfaces, there are a larger number of irregularities. So the force of friction is greater if a rough surface is involved.
Q. 5. Take two bricks, one is wrapped in jute while other is wrapped in polythene. Which one will be easier to drag and why?
Ans. It is easier to drag the brick which is wrapped in polythene. Polythene has a smooth surface. It has less friction than the jute. Thus, it requires lesser force to drag it. Jute has rough surface so it needs more force to drag it.
Q. 6. Why is it easy to drag a mat from the floor, but it is difficult to drag the mat when somebody is sitting on it?
Ans. We know that friction is caused by the interlocking of the irregularities in the two surfaces. It is obvious that the force of friction will increase, if the two surfaces are pressed harder. So it is easy to drag a mat when nobody is sitting on it but when a person is sitting on it then it produces more friction and not easy to drag it.
Q. 7. What are static and sliding frictions?
Ans. The force required to overcome friction at the instant an object starts moving from rest is a measure of static friction. On the other hand, the force required to keep the object moving with the same speed is a measure of sliding friction.
Q. 8. Which is smaller, static friction or the sliding friction and why?
Ans. 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 smaller than the static friction.
Q. 9. What happens, if the floor we walk on is frictionless?
Ans. We would not be able to walk on the surface if there is no friction on the floor. It is the friction which helps us to stand or walk on the surface. The grooves of our feet or shoes are locked into the irregularities of the floor and make us stand.
Q . 10. Why is it not easy to move an object from its static position?
Ans. When an object is at rest, it has better hold of the surface on which it is placed. In the static position the irregularities of the surface are interlocked properly due to which more force is required to overcome the friction. So it is not easy to move an object from its static position.
Q . 11. Why can’t we write with a chalk if there were no friction?
Ans. We cannot write with a chalk, pen, pencil if there were no friction. If we are writing with a chalk on frictionless surface then no chalk particles stick to the surface. So we cannot write with a chalk if there were no friction. On the other hand when we are writing with a chalk on the blackboard, its rough surface rubs off some chalk particles which stick to the blackboard.
Q . 12. The soles of our shoes get worn out after a period of time. Explain why?
Ans. The soles of our shoes get worn out after a period of time due to friction. When we walk on the road, then the friction arises. Our shoes have to overcome friction to walk due to which our shoes get worn out slowly.
Q . 13. Our hands become warm when we rub them. Why?
Ans. When we rub our hands with each other the friction arises between two hands. The friction produces heat, this heat makes our hands warm. This is why our hands become warm when we rub them. We can feel this warmth by touching our hands on our face.
Q . 14. Mention three examples which show that friction produces heat.
Ans. Some examples which show that friction produces heat are following:
(i) Warming of our palms when we rub them.
(ii) Jar of a mixer becomes hot when it is run.
(iii) Warming of the parts of a machine when it is operated.
Q . 15. We observe that in some cases we want to increase friction. Explain why.
Ans. If we do not increase friction, vehicles will slip on the roads and they will never stop. We need to increase friction for better grip and to control the speed of the moving objects. It is not easy to walk and write without friction. We increase friction by making roads with concrete and by applying brakes in vehicles to stop them.
Q . 16. Why do we need to decrease friction and how it can be decreased?
Ans. Friction is harmful in some of the cases. It causes a lot of wear and tear in objects. It also causes damage to the machine parts. So we need to decrease the friction. It can be decreased by rolling, sliding and lubricating the surface in contact. We use ball bearings, wheels and lubricants like grease, oils and creams to decrease the friction.
Q . 17. How do wheels reduce friction?
Ans. We use wheels to reduce friction. The wheels reduce the resistance against motion. So it is easy to roll the body over one another than to slide a body. That is the reason that wheels reduce friction.
Q . 18. What are the factors on which frictional force depends in fluids?
Ans. There are following factors on which force of friction depends in fluids:
(i) The frictional force on an object in a fluid depends on its speed with respect to the fluid.
(ii) Frictional force also depends on the shape of the object and the nature of the fluid.
Q . 19. Why do we shape aeroplanes like that of a bird?
Ans. The objects are given special shapes to minimise friction. Aeroplane and birds both fly in the air and have to face friction exerted by air. They are so shaped that they do not have to lose energy while overcoming the frictional force exerted by air to fly. Therefore the aeroplanes are also shaped streamlined, so that they can overcome the frictional force of air.