Summary of friction

Solved exercise of friction

Summary

Friction is such a force which tends to slow down or stop the moving bodies. Frictional force always acts in the direction opposite to motion.

Example: Pencil becomes blunt because of friction between its tip and the surface of the paper. Match stick do not light up due to lack of friction with the worn match box.

Frictional Force

A force that always opposes the relative motion of two surfaces in contact with one another is called frictional force. Example rubbing both hands together to create heat. A washing machine pushing along a force.

How is Friction Caused?

All surfaces experience friction as they rub across each other, when two bodies are placed upon one another the irregularities of the surfaces interlock .When a force is applied on a body sliding over another, its interlocked irregularities collide each other. Mutual forces between them opposes their Sliding.

Measuring Frictional Force

Frictional force can be measured by a spring balance. Example: A stone is place on the table and tie a thread around it. Pull the stone using a spring balance, stone initially does not move. when the applied force is gradually increased for a moment it just begins to slide. This gives the force of friction which opposes the applied force.

Factors affecting Friction

Friction depends partly on the smoothness of the contacting surfaces. Friction actually increases between two extremely smooth surfaces because of increases attractive electrostatic forces between their atoms. When a body is moving over a horizontal surface, it presses down against the surfaces with force equal to its weight  to the pull of gravity upon. Frictional force vary in strength according to how hard the two surfaces are pressed against one another and how slippery or how rough the surfaces are.

Types of Friction

1. Static friction: The force of friction acting between the two bodies does not allow them to start slide upon one another is called static friction.Example: pushing a big heavy box on the floor
2. Sliding friction : The force of friction acting between the two bodies when they are sliding upon one another with a uniform speed is called sliding friction. Example: It is easier to keep the box moving than to get it started, therefore sliding friction is generally less than the static friction.
3. Rolling friction: When the surface of one body rolls over the surface of the other, the force of opposition offered to its motion is called rolling friction. Example: Rolling a pencil between the palms.

Effects of Friction

1. Friction Opposes motion: When force is applied on a body to move over the other body, the irregularities of the surfaces interlock. Frictional force created due to this cause always acts in the opposite direction of the applied force. Therefore Friction always opposes the motion of the body and tries to stop it. example: Bicycle braking system works on the basis of this effect of friction.
2. Friction produces heat: When an iron nail is taken and rubbed in on the stone surface the nail becomes hot because of the fricition. It is for the same reason that your palms get warm when rubbed against each other. The jar of the mixer grinder gets hot when run for a few minutes. In fact, all moving parts of the machines get hot because of friction.
3. Friction causes wear and tear: When the tyres of the vehicles, rubber bushes of furniture wear out due to friction. Moving parts of the machine under friction are frequently replaced due to wear and tear.

Friction – A necessary evil: If there is no friction, the bodies will either slip or will not move. Further more the moving bodies will not stop. Even we can not walk properly without friction. It is very difficult to walk on smooth and wet floor as friction is very less. While we walk, friction between the ground and the feet prevents us from slipping.

Example: Example, Match stick catches fire because of heat produced by the friction

Disadvantageous of friction: Force of friction brings about wear and tear in moving bodies. You might have seen worn out tyres of the automobiles, worn out soles of the shoes, worn out mechanical parts and ball bearings of the machines due to friction. Friction reduces the speed of the machines. The movable parts of a machine produce huge amount of heat, it leads to much loss of energy. Therefore the energy required to operate the machine increases by friction and friction reduces efficiency considerably. So friction is both useful and harmful at different situations and hence it is a necessary evil.

Methods of increasing friction

• In order to increase friction generally the surfaces in contact are made rough.
• Spikes increase friction and provide desired grip in their movement.
• Automobile handles, steering’s, foot rests, the handles of sports bats, suit cases, walkers and ramps. They are generally covered by holding grips of grooved rubber, They provide required friction in handling them.
• Rubber bushes are fixed at the bottom of ladders, walking sticks and furniture to avoid slipping.
• Flooring of toilets, bath rooms, step, They have rough surfaces to provide grip while walking on them.
• Worn out tyres of the vehicles .New tyre with heavy treads and groves may skid on the sharp turns of the roads due to less friction, and can cause serious accidents. It is for the same reason that special kinds of treads and groves are made in new tyres.
• Braking system of the vehicles are provided with rough surfaces to acquire required friction, Grinding stones of the flour mills are designed with rough groves to increase friction in grinding.

Methods of reducing friction

• Friction is undesirable must be reduced so as to increase efficiency. In order to reduce friction generally the surfaces in contact are made smooth and slippery.
• We have felt very comfort able to peddle your bicycle, when its chain and hubs are properly oiled.
• when a few drops of oil are poured on the hinges of a door, the door moves smoothly. Here oil acts as a lubricant to reduce friction.
• A lubricant is a substance which reduces friction between surfaces in contact.
• Oil, grease, graphite and soap solutions are commonly used as lubricants in machines. A thin layer of lubricant separates the two moving surfaces, so that they do not rub against each other directly.
• Modern automobiles, aircrafts, boats ships and submarines are given a streamlined shape so that they offer least friction to the air or water.

Rollers: many heavy objects and luggage’s are fitted with small wheels called rollers. Ball bearings used to run heavy machines and wheels of the automobiles reduces the friction to a large extent.

Example: Pulling a luggage bag with rollers it contain ball bearings

Exercise:

I. Four alternatives are given under each complete/incomplete statement. Choose the correct answer and put a tick () mark against it :

1. Friction is caused mainly due to
   a) softness of the surfaces
   b) hardness of the surfaces
   c) irregularities of the surfaces
   d) smoothness of the surfaces

Ans:c) irregularities of the surfaces

1. Proper arrangement of frictions in their decreasing
   order is
   a) rolling, static, sliding.
   b) rolling, sliding, static.
   c) sliding, static, rolling.
   d) static, sliding, rolling.
   ans:d) static, sliding, rolling.
2. The increasing order of friction in the following
   substances is
   a) sand, glass, wood. b) glass, wood, sand.
   c) glass, sand, wood. d) wood, glass, sand.
   Ans:c) glass, sand, wood.

II. Fill in the blanks by choosing correct words from
the list given in the brackets :
(less, heat, opposite, reduce, greater, more.)

1.Friction always acts in the direction oppoiste to thedirection of the applied force.

2.Friction produces heat

3.Static friction is greater than the sliding friction.

4.Frictional force is less in smooth surfaces.

5.Lubricant reduces the friction between the surfaces in contact

III. Match the friction managing agents given in list
‘A’ with their applications given in list ‘B’ :

IV. Answer the following questions

1. What is friction? Explain how it is caused.

Ans:Friction is such a force which tends to slow down or stop the moving bodies. Frictional force always acts in the direction opposite to motion.

Example: Pencil becomes blunt because of friction between its tip and the surface of the paper. Match stick do not light up due to lack of friction with the worn match box.

2.Name the two important factors which affect friction?

Ans: Friction depends partly on the smoothness of the contacting surfaces. Friction actually increases between two extremely smooth surfaces because of increases attractive electrostatic forces between their atoms. When a body is moving over a horizontal surface, it presses down against the surfaces with force equal to its weight  to the pull of gravity upon

3.Make a list of the effects of friction.

Ans:Friction Opposes motion: When force is applied on a body to move over the other body, the irregularities of the surfaces interlock. Frictional force created due to this cause always acts in the opposite direction of the applied force. Therefore Friction always opposes the motion of the body and tries to stop it. example: Bicycle braking system works on the basis of this effect of friction.

Friction produces heat: When a iron nail is taken and rubed in on the stone surface the nail becomes hot because  of he fricition. It is for the same reason that your palms get warm when rubbed against each other. The jar of the mixer grinder gets hot when run for a few minutes. In fact all moving parts of the machines get hot because of friction.

3. Friction causes wear and tear: When the tyres of the vehicles, rubber bushes of furniture wear out due to friction. Moving parts of the machine under friction are frequently replaced due to wear and tear.

4.Why is friction called a necessary evil?

Ans: If there is no friction, the bodies will either slip or will not move. Further more the moving bodies will not stop. Even we can not walk properly without friction. It is very difficult to walk on smooth and wet floor as friction is very less. While we walk, friction between the ground and the feet prevents us from slipping so it is said to be necessary evil.

5.State two advantages of friction.

Ans:If there is no friction, the bodies willeither slip or will not move.Further more the moving
bodies will not stop. Even we can not walk properly without friction. It is very difficult to walk on smooth and wet floor asfriction is very less. While we walk, friction between the ground and the feet prevents us from slipping

6.State two disadvantages of friction.

Ans: Force of friction brings about wear and tear in moving bodies. You might have seen worn out tyres of the automobiles, worn out soles of the shoes, worn out mechanical parts and ball bearings of the machines due to friction. Friction reduces the speed of the machines.

7.What is a lubricant? Give two examples.

Ans:A lubricant is a substance which reduces friction between surfaces in contact.

Example:Oil, grease, graphite and soap solutions are commonly used as lubricants in machines

8.How does a lubricant reduce friction?

Ans:A lubricant is a substance which reduces friction between surfaces in contact.

Oil, grease, graphite and soap solutions are commonly used as lubricants in machines. A thin layer of lubricant separates the two moving surfaces, so that they do not rub against each other directly to reduce the friction.

9.How can you say that sliding friction is greater than
the rolling friction?

Ans: sliding friction is the force resisting the motion when a body slides n a surface this force of friction depends on the area of contact between the two surface. As the area of contact is less in the case of rolling then in the case of sliding rolling friction is less than the sliding friction

10.Explain why objects moving in fluids must have stream-line shapes.

Ans: The fluid is the name given to a substances which begins to flow when anexternal force is applied to it. Object moving in fluids face the friction of the fluid that opposes their motion so objecs moving in fluids must have unique shpes called streame lined shaes to reduce the fluid friction.

V. Give reasons for the following

1.A ball rolling over the ground slows down.

Ans:Due to the friction from the medium it is moving on the ball comes into rest becuase the friction acts in oppoistes direction of the moving ball slowly the force becomes to rest .

2.The surface of a conveyor belt is made rough and tight.

Ans:because to increase the friction between object and belt.

3.The worn out tyres are discarded.

Ans: As the tyres gets worn out the dynamic friction between the tyres and the road surface reduces thus during braking action the car is likely to slide.