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Class- 8 DAV Science Friction Question and Answer
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 shape.
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 |
| 4. The sole of shoes and the tyres of the vehicle are ‘treaded’ to decrease the effects of friction. | Ans- True |
| 5. Lubricants can be liquid, semi-liquids or solids. | Ans- False |
| 6. Friction can be increased by using ball bearings between the moving parts of machines. | Ans)True |
| 7. Fluid friction can be minimized by giving suitable shapes to the objects moving through the fluid. | Ans- True |
C. Tick the correct option
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.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.