Force
A. Fill in the blanks:
1. When speed of a body is zero, it is considered to be at the state of rest.
2. For the force to act, two bodies must interact with each other.
3. By applying force in the direction opposite to the motion of a body, we can reduce its speed.
4. Friction between the brake shoe and the wheel slows down or stops a moving vehicle.
5. Friction causes wastage of energy.
6. As friction produces heat, different parts of the rotating machine get damaged with time and need periodic replacement.
B. State whether the following statements are true(T) or false(F).
1. The force acting on a body increases the mass of the body. [F]
2. Force can change the direction of motion. [T]
3. If the applied force is not enough, the object may not move. [T]
4. Less force will be required to move a heavier body than a lighter one. [F]
5. Friction causes wear and tear of the surface in contact. [T]
6. Only solid surfaces experience friction. [F]
C. Choose the correct option.
1. Which of the following is an effect of force?
a. Starts motion. [ ]
b. Changes the direction of motion. [ ]
c. Stops motion. [ ]
d. All of them. [✓]
2. Which of the following is not a unit of force?
a. gram-force. [ ]
b. dyne. [ ]
c. joule. [✓]
d. newton. [ ]
3. Kneading dough shows which effect of force?
a. Change in speed. [ ]
b. Change in direction. [ ]
c. Change is shape and size. [✓]
d. change in mass. [ ]
4. Which of the following is not an effect of friction?
a. To produce heat. [ ]
b. To increase the speed of the body. [✓]
c. Change in shape and size. [ ]
d. Change in mass. [ ]
5. Which of the following is not a type of friction?
a. Rolling friction. [ ]
b. Static friction. [ ]
c. Slowing friction. [✓]
d. Sliding friction. [ ]
6. Friction can be reduced by ...............
a. using ball bearing. [ ]
b. polishing. [ ]
c. streaming. [ ]
d. all of them. [✓]
D. Match the following.
=
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Column A |
Column B |
|---|---|
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1.Contanct force |
a. Magnetic force. [3] |
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2. Stretching a rubber band |
b. Opposes relative motion. [4] |
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3. Non-contact force |
c. Reduces friction. [5] |
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4. Friction |
d. Muscular force. [1] |
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5. Polishing |
e. Force as a pull. [2] |
A. Show the given text in form of the flowcharts.
1. Types of forces.
=
2. Types of friction.
=
B. Give reasons for the following statements.
1. Force does not change the mass of the body.
= Because the applied force does not change the composition of matter that present in the body.
2. Squeezing a tube is an example of applying force.
= Squeezing a tube is an example of applying force because it results in decrease in its dimension or volume.
3. The force required to start a motion is maximum as compared to the force required to keep a body in motion.
= Because in the state of rest the speed of the body is zero but in the state of motion, the body already has some speed.
4. The sole of our shoes wears out after using them for a long time.
= Because the friction between the shoe sole and the ground causes wear and tear.
5. Heavy objects are moved using rollers or wheels.
= Because rolling friction is less than sliding friction. It needs less force to roll an object on a surface than to slide it.
6. When we rub our palms, they get warm.
= Because the friction between the palms produces heat.
7. We put talcum powder on a carrom board.
= Because it fills the small spaces on the surface on the board and reduces friction. So that the striker moves easily.
8. Friction is a necessary evil.
= Because friction plays an important role in our daily life yet it causes wear and tear.
9. We slip on the floor, if there is water , oil or soap solution on the floor.
= Because it reduces friction as the water or oil fill the spaces between the floor and feet and form a thin layer on the floor.
C. Answer in short.
1. When is an object is said to at rest?
= When the object is not in motion, it is said to be at rest.
2. Give the names of different types of non-contact forces.
= The names of non-contact force are - i) Magnetic force; ii) Electrostatic force; and iii) Gravitational force.
3. Does a magnet only exert attractive force? Give examples of the materials that can be attracted by a magnet.
= No, a magnet not only attracts it also repels.
Iron and Nickel are the two materials which are attracted by magnet.
4. How does friction depend on the surface of bodies in contact?
= Friction is caused due to the irregularities of surface in contact. If we see with magnifying lenses there are tiny hills and grooves when two surface come in contact these hills and grooved interlock with each other. This oppose motion.
5. Which states of matter exert frictional force?
= All states of matter exert friction. Liquid and gases also exert friction. An object moving through them faces friction which is known as fluid friction or drag.
6. Arrange static friction, rolling friction and sliding friction in increasing order.
= Frictions in increasing order are -
Rolling friction < Sliding friction < Static friction
7. How does a lubricant reduce friction?
= Lubricants form a thin layer between two surfaces in contact and fill the depression present in contact. This reduces the unevenness of the floor and thus reduce friction.
8. How can you increase friction?
= We can reduce friction by grooving and making the surface rough like we spread sand or gravel to increase friction or the treaded tyres helped to not to slip on the road.
9. Why do aeroplanes have a streamlined shape?
= When a body moves through liquid or gas, it experience friction. It is found that resistance or friction is minimum if a body has a streamlined shape i.e. narrow in the front and back and broad in the middle.
At Length
A. Explain the following terms.
1. Force.
= Force is a push or pull which tends to change its state of rest or motion or the speed or direction of motion or its shape or size.
2. Friction.
= It is the force that opposes or resists the relative motion between two bodies or surface in contact to each other.
3. Sliding friction.
= The friction exists between a body or surface when the body slides over it.
4. Static friction.
= The friction which exists when the bodies in contact are at rest with respect to each other.
5. Rolling friction.
= The friction exists between an object or a surface when the body rolls over it.
6. Lubricant.
= The things used to reduce friction are called lubricants. Example:- Oil and talcum powder.
B. Differentiate between the following.
1. Contact and non-contact force.
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Contact Force |
Non-contact Force |
|---|---|
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1. The force in which the two interacting bodies are in physical
contact with each other is called contact force. |
1. The force which do not require a physical contact between two
interacting bodies and act as a distance is called non-contact force. |
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2. Example:- Applied force, normal force, frictional force etc. |
2. Example:- Magnetic force, Electric force etc. |
2. Static and kinetic friction.
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Static Friction |
Kinetic Friction |
|---|---|
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1. Static friction exists when the bodies in contact are at rest. |
1. Kinetic friction exists when they are moving. |
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2. Static friction is more than kinetic friction. |
2. Kinetic friction is less than static friction. |
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3. There is no subdivision of static friction. |
3. It is of two types – rolling friction and sliding friction. |
3. Sliding and rolling friction.
=
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Sliding Friction |
Rolling Friction |
|---|---|
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1. Sliding friction exists when a body slides over a surface. |
1. Rolling friction exists when an object rolls over a surface. |
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2. Sliding friction is more than rolling friction. |
2. Rolling friction is less than sliding friction. |
C. Read the given information carefully. Identify the incorrect parts of the information.
1. If there is water on the floor it is easier to walk.
= If there is water on the floor it is easier to walk.(wrong)
= If there water on the floor it is harder to walk.(right)
2. On putting sand or gravel on the ground, the slipperiness of the ground increases.
= On putting sand or gravel on the ground, the slipperiness of the ground increases.(wrong)
= On putting sand or gravel on the ground, the friction of the ground increases.(right)
3. This is because the sand decreases the friction between our foot and ground.
= This is because the sand decreases the friction between our foot and ground.(wrong)
= This is because the sand increases the friction between our foot and ground.(right)
D. Answer in details.
1. Give some examples of push and pull that we see in our daily life.
= Some examples of push and pull in our daily lives are -
i) When we go to school, we lift our school bag, this is pull.
ii) When we go to a shop we pull the door and push the door to get out.
iii) Football players often stop the ball travelling at a great speed.
2. Discuss the various effects of force.
= The various effects of force are -
i) Force can produce motion:- Force when applied to an object by pushing, pulling or hitting it with another object can make first object move.
ii) Force can stop motion:- We can stop motion by producing motion opposite to direction motion of an object.
iii) Force can change speed of motion:- We can increase its speed by applying force to its direction of motion of the body.
iv) Force can change the direction of motion:- We can change the direction of the moving body by applying force in the desired direction.
v) Force can change the shape and size of an object applying force.
3. Explain gravitational force.
= The earth attracts all bodies towards its centre. The attractive pull of the earth is an example of gravitational force. In fact all bodies that posses mass exerts this force. Hence gravitational force can defined as the force with which any two object pull each other. Its due to gravitational force or gravity that all object either remain on the ground or always fall back on the ground.
4. Why do two bodies in contact exert frictional force? What will be the direction of friction?
= Friction is caused due to irregularities on the surface in contact. If we carefully observe with magnifying glasses we will see that there are tiny hill and grooves when two surface come in contact with each other these hills and grooves interlock with each other and oppose motion.
5. Discuss various properties of friction.
= Friction always oppose relative motion. Hence is slows down and stop a moving vehicle.
Friction produce heat that is why we rub our hands together and touch them with our cheeks we feel warm.
Friction causes wear and tear of surfaces in contact. It results in wearing out of soles of our shoes.
6. What are the factors affecting friction?
= The factors affecting friction are:-
Nature of surface in contact.
Weight of the body.
Area of surface in contact.
7. Discuss the types of friction.
= Friction is of two types -
Static Friction:- The word static means stationary or not moving. The friction exists when the body is in rest is called static friction.
Kinetic Friction:- The word kinetic means moving. The type of friction exists when the body is in motion is called kinetic friction. This is of two types - Rolling Friction and Sliding Friction.
8. State the advantages and disadvantages of friction.
= The advantages of friction are -
i) Friction between our feet, ground and foot wear help us to walk on ground without slipping.
ii) The treaded tyres and rough surface helps the vehicle move safely on roads.
iii) Friction between the break shoe and the wheels slows down and stop a moving vehicle.
iv) We are able to hold things due to friction.
The disadvantages of friction are -
i) Friction causes wear and tear of rubbing surfaces.
ii) As friction produces heat, different parts of machines gets damaged.
iii) friction causes wastage of energy.
9. Explain some methods of reducing friction between bodies.
= Some methods of reducing friction between bodies are -
i) Polishing:- By polishing the surface we reduce its irregularities and make it smooth and reduce friction.
ii) Lubricating:- We can reduce friction by lubricating two surfaces.
iii) Streamlining:- When a body moves though liquid or gas it experience friction. It is found that streamlining the body reduces friction.