NCERT Solution For Class 8 Science, Chapter 12 – Friction

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NCERT Solution For Class 8 Science, Chapter 12 – Friction provides us with all-inclusive information on all concepts. As students would have to learn the basics about the subject in class 8, this curriculum for class 8 is comprehensive study material, which explains the concepts in a great way.

Important Formulae

F _{f =} μ F _n

Friction force is a derived force and not like gravitation i.e. a fundamental force. The formula for friction force is,

Where F_f is the friction, μ is the coefficient of friction force and  Fn is the normal force. It is a vector quantity.

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 carom board ________ friction.

(e)        Sliding friction is ________ than the static friction.

Solution:

(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 carom board reduces friction.

(e)        Sliding friction is less than the static friction.

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

Solution:

(c) static, sliding, rolling

Friction comes into play when irregularities present within the surfaces of two objects involved get interlocked with one another. Static friction comes into play after we attempt to move an object which is at rest. Sliding friction comes into play when an object slides over the surface of another object. In sliding friction, the time given for interlocking is incredibly small. Hence, interlocking isn’t strong. Therefore, less force is required to beat this interlocking. Due to this reason, sliding friction is a smaller amount than static friction. Similarly, the realm of contact just in case of rolling friction is smallest as compared to static or sliding friction. This area of contact changes gradually due to rolling. Hence, rolling friction is lesser than both static and sliding friction. Thus, the right sequence is—static, sliding, rolling.

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

Solution:

(a) wet marble floor, dry marble floor, newspaper and towel

Force of friction depends on the character of surfaces connected. The rougher the surface, the more is that the friction between the surfaces connected and vice-versa. Roughness present within the given surfaces are often arranged in an ascending order as wet marble floor, dry marble floor, newspaper and towel. Hence, the right sequence of those surfaces when arranged in keeping with the rise within the force of friction functioning on the car is—wet marble floor, dry marble floor, newspaper and towel.

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.

Solution:

When a book slides on the writing desk, a frictional force acts between the book and therefore the surface of the desk. The direction of frictional force on the book is opposite to the direction of its motion and acts in upward direction, as shown within the following figure.

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

Solution:

We are ready to walk thanks to the friction present between our feet and therefore, the ground. So as to run, we push the bottom within the backward direction with our feet. The force of friction pushes it in the forward direction and allows us to run. The force of friction between the bottom and feet decreases when there’s soapy water spilled on the ground. Hence, it becomes difficult to run on the soapy floor.

Q6. Explain why sportsmen use shoes with spikes.

Solution:

Sportsmen use shoes with spikes because these shoes give them a more robust grip while running. This is due to the reason that the force of friction between the shoes and the ground increases with the help of spikes.

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?

Solution:

Force of friction arises due to interlocking of irregularities on the two surfaces in touch. When a big object is placed on the ground, the interlocking of irregularities on the surfaces of box and floor become strong. This is because the two surfaces in touch are pressed harder. Hence, more force is required to beat the interlocking. Thus, to push the heavier box, Seema needs to apply a greater force than Iqbal.

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

Solution:

Friction comes into play when irregularities present within the surfaces of two objects get interlocked with one another. In sliding, the time given for interlocking is incredibly small. Hence, interlocking isn’t strong. Therefore, less force is required to beat this interlocking. Due to this reason, sliding friction is a smaller amount than static friction.

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

Solution:

Advantages of friction:

(i)         We are able to walk because of friction.

(ii)        Friction between the tip of the pen and a paper allows us to write.

Disadvantages of friction:

(i)         Tyres and soles of shoes wear out because of friction.

(ii)        Friction between the different parts of machines produces heat. This can damage the machines.

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

Solution:

When a body moves through a fluid, it experiences an opposing force which tries to oppose its motion through the fluid. This opposing force is the drag force. This frictional force depends on the form of the body. By giving objects a special shape, the force of friction engaged on it are often minimized. Hence, it becomes easier for the body to move through the fluid.

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