 # Chapter 12 Friction

• Push or pull is known as force.
• Friction is the opposing force which is exerted by a moving body.
• Friction helps in moving, running and stops the body.
• Friction is of different types: static friction, rolling, sliding and dynamic.
• Interlocking of irregularities in two surfaces called as friction.
• Rolling friction is usually less than sliding friction.
• Sliding friction is smaller than the static friction.
• The substance which helps in reducing the friction or make the surface smooth is called as lubricant.
• When one body rolls over another body, this is called rolling friction.
• When one body slides over another body is known as sliding friction.
• Ball bearing are the small balls which are used between hubs and the axles of ceiling fans to reduce the friction.
• The frictional force exerts by the fluids is known as drag.
• Aero-plane have such shape to reduce the friction.
• Friction is more felt on the rough surfaces.
• Friction depends on the point of contact of two surfaces.
• Heat is produced by 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. Ans: (a) Motion Friction is the opening force which change in state of the motion. (b) Nature Friction depends on the nature of the surface. Rough surface have high friction and smooth surface have less friction. (c) Heat Friction produces heat, it is the opposing force acting on the surface. (d) Reduce Powder is used as lubricant to smoothen the surface. (e) Less Sliding friction is less than the static friction, sliding friction is less than the static friction, sliding friction is the friction when one slides over another body. 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 Ans: (c) Static, sliding, rolling Static > sliding > rolling Because in rolling the interlocking time between two surfaces are very less as compared to the sliding and static friction. In static friction, the interlocking time between two surfaces are greater. So, static friction is higher than sliding and rolling friction. The area of contact is very less in rolling friction as compare to static and sliding friction. 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 Ans: (a) Wet marble floor, dry marble floor, towel newspaper Force of friction depends upon the nature of the surface. Friction is more when more irregularities are present on the surface. A smooth surface have less friction. So, as in the increasing order of the force of friction, a wet marble floor have more irregularities so it shows the greater force of friction than the other whereas newspaper have less irregularities on its surface as compared to other. So, it exerts less force of friction. 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. Ans: Frictional force is the opposing force it acts in opposite direction of the object. When the book slides on the desk, then the friction acts on the opposite direction of the book. In sliding friction, the interlocking between two surfaces is less than the static friction. 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? Ans: It is difficult to walk on the floor when the soapy water spills on the marble because it reduces the friction of the floor. Soapy water makes the surface of the marble is smooth and slippery. So, it is difficult to walk on the floor. Q6 Explain why sportsmen use shoes with spikes. Ans: Sportsmen uses shoes with spikes because it increases the force of friction. Spikes makes the grip better of the shoes with the ground. More irregularities plays important role in increase in force of friction. Spikes helps the sportsmen to run and protect them from slipping. 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? Ans: Irregularities increase the surface area for the friction. Force of friction depend on the interlocking of irregularities between two surfaces. Iqbal has to apply less force to push a lighter box because here the interlocking between the floor and the lighter box is very small. But Seema have to apply a greater force to push the heavy box because here the interlocking between the floor and the heavy box is very strong. Seema have to apply the larger force in comparison to Iqbal. Q8 Explain why the sliding friction is less than the static friction. Ans: Sliding friction is less than the static friction because force of friction depends on the interlocking of two surfaces. In sliding, interlocking between two surfaces is weak, so to overcome this we have to apply less force. But in static friction, the interlocking between the irregularities of two surfaces is very strong. So we can say that sliding friction is less than the static friction. Q9 Give examples to show that friction is both a friend and a foe. Ans: Advantages of friction are as follows: We are able to walk. We can write. Friction is used to stop the vehicle. It produces heat. Helps in different types of motion. Disadvantages of friction are as follows: Forest fires occurs due to friction between branches of trees. It is an opposing force, so energy is needed to overcome it. Tyres and soles of shoes tears out. Damage machines. It produces a lot of heat. Q10 Explain why objects moving in fluids must have special shapes. Ans: When a body moves in fluid, it experiences a force of friction called a drag force. It fluids, to overcome the frictional force, objects given special shapes. Frictional force can be minimized by changing in shape of the object. The special shape of the body exerts less friction and move easily in the fluid. That’s why objects moving in fluids must have special shapes.