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Q10. Explain why objects moving in fluids must have special shapes.

When an object is moving through a liquid there is an opposing force offered by the liquid against the motion of the object. To reduce this opposing force object must have special shapes. 

Q9. Give examples to show that friction is both a friend and a foe.

Q9. Friction acting as a friend: (a) It aids us to walk and cars to move on the roads. (b)Fire can be ignited using friction. Friction acting as a foe: (a) It causes the efficiency of engines to decrease and also damages machinery. (b) Friction causes soles of shoes and tires to wear off.

Q8. Explain why sliding friction is less than static friction.

Sliding friction is less than static friction as the frictional force between two surfaces depends on the interlocking between the irregularities on the surfaces. This interlocking in case of sliding decreases as the contact points does not get enough time to interlock properly thus reducing the frictional force.

Q7. Iqbal 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 larger force and why?

Q7. Seema would have to apply a larger force as she would have to overcome a greater frictional force. Frictional force between two surfaces exists due to the interlocking of the irregularities on the surface. The magnitue of this frictional force depends on how tight this interlocking is which in this case depends on the weight of the boxes which is more in case of seema.

Q)6. Explain why sportsmen use shoes with spikes.

Sportsmen use shoes with spikes to increase the grip between their feet and the ground by increasing the friction with the help of spikes. This aids them while running on the ground.

Q5. You spill a bucket of soapy water on a marble floor accidently. Would it make it easier or more difficult for you to walk on the floor? Why?

It would make it more difficult for us to walk on the marble floor since the frictional force would further decrease and thus our grip on the floor while walking would become even lesser.

Q4. Suppose your writing desk is tilted a little. A book kept on it starts sliding down. Show the direction of frictional force acting on it.

The frictional force will be opposite to the direction of motion

Q3. Alida runs her toy car on dry marble floor, wet marble floor, newspaper and towel spread on the floor. The force of friction acting on the car on different surfaces in increasing order will be

(a) wet marble floor, dry marble floor, newspaper and towel.
(b) newspaper, towel, dry marble floor, wet marble floor.
(c) towel, newspaper, dry marble floor, wet marble floor.
(d) wet marble floor, dry marble floor, towel, newspaper

The force of friction acting on the car on different surfaces in increasing order will be wet marble floor, dry marble floor, newspaper and towel. Therefore (a) is the correct answer.

Q2. Four children were asked to arrange forces due to rolling, static and sliding frictions in a decreasing order. Their arrangements are given below. Choose the correct arrangement.

(a) rolling, static, sliding
(b) rolling, sliding, static
(c) static, sliding, rolling
(d) sliding, static, rolling

(c) is the correct arrangement. Static friction > sliding friction > rolling friction.

Q1. Fill in the blanks.

(a) Friction opposes the _____________ between the surfaces in contact with each other.
(b) Friction depends on the _____________ of surfaces.
(c) Friction produces __________.
(d) Sprinkling of powder on the carrom board ________ friction.
(e) Sliding friction is ___________ than the static friction.

(a) Friction opposes the motion between the surfaces in contact with each other. (b) Friction depends on the nature of surfaces. (c) Friction produces heat. (d) Sprinkling of powder on the carrom board reduces friction. (e) Sliding friction is less than the static friction.
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