A. Fill in the blanks.
1. Friction always opposes any relative motion between two surfaces that are in contact with each other.
2. Static friction is always greater than sliding friction.
3. Sportsmen use shoes, fitted with spikes, to increase friction between their shoes and the ground.
4. Sprinkling of talcum powder, on the carrom-board, helps to decrease friction.
5. Frictional force, on an object moving in a fluid, depends on its viscosity.
6. The shape of an aeroplane is streamlined to reduce, the effects of friction, due to air.
B. State True or false for the following statements.
1. When a body slides over smooth and wet surfaces, the amount of interlocking of irregularities, of the two surfaces in contact, increases.
Ans- False
2. If a car moves eastwards, the force of friction acts southwards.
Ans- False
3. Friction depends on how hard the two surfaces press against each other.
Ans- True
4. The sole of shoes and the tyres of the vehicle are 'treaded' to decrease the effects of friction.
Ans- False
5. Lubricants can be liquid, semi-liquids or solids.
Ans- True
6. Friction can be increased by using ball bearings between the moving parts of machines.
Ans- False
C. Tick the correct option.
1. Suppose your writing desk is tilted to a position where a book kept on it just starts sliding down. The figure, showing the correct direction of frictional force acting on it, is----
Ans- D
2. Two boys are applying oppositely directed, and equal in magnitude, forces on a box as shown in the figure.
In such a case, the force of friction, that would exist between the lower end of the box and the ground, would be called----
Ans- Static friction
3. Four students were asked to arrange the (frictional) force, due to rolling, static, and sliding, in increasing order. Their arrangements are listed below. The correct arrangement is----
Ans- Rolling, sliding, static.
4. The energy, 'used up' in overcoming friction, gets converted mainly into----
Ans- Heat energy
5. Rahul rolls a ball on a wooden surface. The ball covers a certain distance before coming to rest. To make the same ball cover a (much) longer distance, before coming to rest, Rahul should----
Ans- Sprinkle talcum powder on the wooden surface.
6. A block is sliding on a horizontal surface. The force of friction between the two can be increased by----
Ans- By putting a second identical block on top of the given block.
D. Answer the following question in brief.
1. Define the term (a) static friction (b) sliding friction.
Ans- a) Static friction- Force of friction acting between two surfaces in the address is called static friction.
b) Sliding friction- Force of friction acting between two surfaces when one surface slides on another surface is called sliding friction.
2. State the meaning of the term (a) rolling friction (b) fluid friction.
Ans- a) Rolling friction- The force of friction acting between two surfaces when one surface or both surfaces roll on each other is called rolling friction.
b) Fluid friction- The force of friction experienced by fluid or among the different layers of the fluid is called fluid friction.
3. State the likely causes of friction.
Ans- Interlocking between the two surfaces causes friction between them.
4. A box is resting on the floor. To move it, a (variable) force is applied as shown in the diagram below.
a) Give the (likely) value of the force of limiting friction.
Ans- 2F
b) Why does the box stay at rest in the cases corresponding to figure (a) and figure (b)?
Ans- Box stays at rest as the applied force is not being able to beat the frictional force between the block and the surface, which means that applied force in both cases is less than the static friction between the block and the surface.
(c) Mark the direction of frictional force, and mention the kind of frictional force that comes into play, in each case.
Ans- Direction of frictional force in all cases will be opposite of applied force.
i) Static friction
ii) Static friction
iii) Limiting friction
5. Name the kind of frictional friction, that comes into play, when a book, kept on a collection of cylindrical pencils, is moved by pushing.
Ans- Rolling friction
6. Why are objects given special shapes when they are moving through fluids? Write the names given to these special shapes.
Ans- Objects are given special shapes to reduce fluid friction.
The name given to these special shapes is streamlined shapes.
E. Answer the following questions.
1. Define the force of friction. List the factors affecting the force of friction. Explain with examples.
Ans- Friction is a contact force acting on two surfaces that oppose the relative motion between one another.
Factors affecting the force of friction----
i) Nature of the surfaces in contact.
ii) Force pressing or weight of the two surfaces acting together.
For example- When we rub our hand, friction is produced.
2. A ball is allowed to roll down an inclined wooden plane from a given height. At the foot of the inclined plane, it moves on a horizontal surface, differently 'covered', one by one, as shown in the following figures.
In which case, is the ball likely to move the longest distance? Give reason for your answer.
Ans- In the case of the plane glass sheet, the ball is likely to move the longest distance because the plane glass sheet has low friction.
3. Explain why sliding friction is less than static friction.
Ans- Frictions comes into play when irregularities present in the surfaces of two objects in contact get interlocked with each other. In sliding the time given for interlocking is very small. Hence, interlocking is not strong. Therefore, less force is required to overcome this interlocking. Because of this reason, sliding friction is less than static friction.
4. Give reason for the following.
(a) We tend to slip when we step on a banana peel.
Ans- The friction between the banana peel and the surface is very less so, when we step on a banana peel, no grip is provided by the surface and we slip down.
(b) It is easier to push a lighter box than a similar heavy box on the same floor.
Ans- It is easier to push a lighter box than a similar heavy box on the same floor because frictional force will be more in the case of a heavy box so, it is difficult to move a heavy box than a lightbox.
(c) The force, needed to start a cart, is (somewhat) greater than the force needed to keep it moving with a uniform speed.
Ans- The force, needed to start a cart, is (somewhat) greater than the force needed to keep it moving with a uniform speed because at first, the body remains in a state of rest. In order to brake its state of rest, a larger force is required. In the second case, the body acquires linear momentum. It allows it to gain a force there. So, it is easier to move the body once it is in motion.
(d) Sportsmen use shoes fitted with spikes.
Ans- Sportsmen use shoes fitted with spikes because they want grip and get grip from the friction created by the spikes.
(e) Ball bearing are used in machines.
Ans- Ball bearing are used in machines.
(f) Tyres of the vehicles needs to be changed regularly.
Ans- Tyres of the vehicles needs to be changed regularly because with passing time they get worn off and due to friction they become more smoother.
(g) Machine parts are frequently oiled or greased.
Ans- Machine parts are frequently oiled or greased to reduce friction and help in the smoother running of the machine.
(h) Sports cars, aeroplanes and boats are designed to have a streamlined shape.
Ans- Sports cars, aeroplanes and boats are designed to have a streamlined shape to reduce fluid friction.
(i) Metal chains are wrapped on tyres when they run on icy roads.
Ans- An ice is slippery and there is almost less friction, so tyres are wrapped with chains to create high friction. So, cars or trucks can move easily on them.
(j) Rollers are used for transporting luggage.
Ans- Rolling decreases friction and makes the movement of things easier. That's why luggage are fitted with rollers.
5. State and explain, with examples, how friction can help the cause motion in certain situations.
Ans- We can walk on the road or floor due to friction. Also, the vehicles run on the road due to friction between their tyres and the road. We slip and the vehicles skid in the absence of friction. Thus, friction can cause motion in these situations.
