NCERT Solutions For Class 8 Science Chapter 12 Exercise
Chapter 12 – Friction
Questions:
1. 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.
Soln:
(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 lesser than the static friction.
2. 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
The answer is (c) static, sliding, rolling
3. 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
The answer is (a) wet marble floor, dry marble floor, newspaper and towel.
4. 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.
A frictional force is created between the book and the desk surface when it slides down on it. The friction force acting on the book is directed upward and in the opposite direction to the direction of its travel. In the diagram below, it is displayed.
5. 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?
Because of the friction between our feet and the ground, walking is feasible on the floor. When walking, our feet press on the earth in a rearward motion. We are able to walk because of the force of friction pushing it forward. If there is soapy water on the floor, there is less friction between the feet and the ground. This makes walking on the soapy floor challenging.
6. Explain why sportsmen use shoes with spikes.
Because spikes provide a superior grip when running, athletes use shoes with spikes. This is a result of the spikes increasing the force of friction between the shoes and the ground.
7. 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?
The force of friction is created when the imperfections on the two surfaces in contact interlock. The imperfections on the floor and box’s surface interlock strongly when a heavy object is placed on it. This is a result of greater pressure between the two in touch surfaces. As a result, greater force is needed to release the interlocking. Seema must therefore exert more force than Iqbal in order to push the heavier box.
8. Explain why sliding friction is less than static friction.
Friction arises when surface imperfections between two things come into contact and become entangled. There is very little time allotted for slides to interlock. Interlocking is therefore weak. Consequently, less effort is needed to release this entanglement. Sliding friction is therefore less than static friction.
9. Give examples to show that friction is both a friend and a foe.
Benefits of friction
a) Friction allows us to move and walk.
b) The friction created by the pen’s tip and the paper allows us to write.
The drawbacks of friction
a) Tyres and shoe soles deteriorate due to friction.
b) Heat is produced by friction between the machine’s many components. The machines may be harmed by this.
10. Explain why objects moving in fluids must have special shapes.
An opposing force seeks to prevent a body from moving through a fluid in the same way as it does. The drag force is this opposing force, and the frictional force is contingent upon the geometry of the body. It is possible to reduce the amount of friction acting on an object by giving it a unique shape. As a result, a body can travel through the fluid more easily.
