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# Notes - Force & Pressure Class 8 Notes | EduRev

## Class 8 : Notes - Force & Pressure Class 8 Notes | EduRev

``` Page 1

BAL BHARATI PUBLIC SCHOOL, PITAMPURA, DELHI – 110034

Class-VIII  { U.T. - 1}

NOTES : FORCE & PRESSURE

Q.1. What is force? State the various effects of force.
Ans. A push or pull on an object is called force.

Effects of force:
(i) Force can change the speed of a moving object.
(ii)  Force can change the direction of a moving object.
(iii)  Force can change the shape of an object.

Q.2. Give one example where force can changes the

(a)  Speed of a moving object
(b) Direction of a moving object.
©  the shape of an object.

Ans. (a)  The force applied by a hockey player on a moving ball changes the speed of ball.
(b) The force applied by a batsman changes the direction of moving cricket ball.
©  Force applied on a spring can change its shape.

Q.3.  Identify the actions involved in the following situations as push or pull, or both:
(a) Opening a drawer.
©  A cricket ball hit by a batsman.
(d) Drawing a bucket of water from a well.

Ans. (a)  Pull.
(b)  Push; Pull.
(d)  Pull.

Note: a)  At least two objects must interact for a force to come into play.

b) Force applied on an object in the same direction add to one another.

c) If the forces act in the opposite directions on object, the net force acting on it is the
difference between the two forces.

d) The state of motion of an object is described by its speed & the direction of motion. So we
can say that a force can change the state of motion of an object.

e) S.I unit of force is Newton ( N).

Q.4.  In a tug of war, when the two teams are pulling the rope, a stage comes when the rope
does not move to either side at all. What can you say about the magnitudes & directions
of the forces being applied to the rope by the two teams at this stage?

Ans. At this point of time, the two teams are applying equal but opposite forces to the rope due to
which the net force acting on the rope is zero & hence the rope does not move.
Page 2

BAL BHARATI PUBLIC SCHOOL, PITAMPURA, DELHI – 110034

Class-VIII  { U.T. - 1}

NOTES : FORCE & PRESSURE

Q.1. What is force? State the various effects of force.
Ans. A push or pull on an object is called force.

Effects of force:
(i) Force can change the speed of a moving object.
(ii)  Force can change the direction of a moving object.
(iii)  Force can change the shape of an object.

Q.2. Give one example where force can changes the

(a)  Speed of a moving object
(b) Direction of a moving object.
©  the shape of an object.

Ans. (a)  The force applied by a hockey player on a moving ball changes the speed of ball.
(b) The force applied by a batsman changes the direction of moving cricket ball.
©  Force applied on a spring can change its shape.

Q.3.  Identify the actions involved in the following situations as push or pull, or both:
(a) Opening a drawer.
©  A cricket ball hit by a batsman.
(d) Drawing a bucket of water from a well.

Ans. (a)  Pull.
(b)  Push; Pull.
(d)  Pull.

Note: a)  At least two objects must interact for a force to come into play.

b) Force applied on an object in the same direction add to one another.

c) If the forces act in the opposite directions on object, the net force acting on it is the
difference between the two forces.

d) The state of motion of an object is described by its speed & the direction of motion. So we
can say that a force can change the state of motion of an object.

e) S.I unit of force is Newton ( N).

Q.4.  In a tug of war, when the two teams are pulling the rope, a stage comes when the rope
does not move to either side at all. What can you say about the magnitudes & directions
of the forces being applied to the rope by the two teams at this stage?

Ans. At this point of time, the two teams are applying equal but opposite forces to the rope due to
which the net force acting on the rope is zero & hence the rope does not move.

Q.5. What is meant by a contact force?

Ans. A force that can be applied only, when an object comes in direct contact with an another object
is called contact force. Eg. Muscular force & Frictional force.

Q.6. What is muscular force?

Ans. The force resulting due to the action of muscles is called muscular force. Eg pushing an object
or lifting a bucket of water.

Note: Since muscular force can be applied to an object only when our body is in contact with
the object, therefore, muscular force is a contact force.

Q.7. What is meant by non-contact force?

Ans. A force that can be applied by an object on another object even from a distance (without
touching) is called a non-contact force. Eg. Magnetic force, Electrostatic force & Gravitational
force.

Q.8. A small device pulls iron nails from a distance. Which type of force is involved in this
process?

Ans. Magnetic force

Q.9. What is electrostatic force?

Ans. The force exerted by a charged body on another charged or uncharged body is known as
electrostatic force.

Q.10. When a plastic pen is rubbed in dry hair, it attracts tiny pieces of paper. Which force is
involved in this process?

Ans. Electrostatic force.

Q.11. What is meant by gravitational force?

Ans.  Every object in this universe attracts every other object with a force, called gravitational force.

Q.12. What is meant by Force of gravity?

Ans. The force, with which the earth pulls the objects towards it, is called the force of gravity or
simply gravity.

Q.13.  A coin falling to the ground on slipping from hand. Name the type of force involved in
this process?

Ans. Gravitational force.

Q.14.  Define pressure.

Ans. The force acting on unit area of surface is called pressure.

Pressure = Force / Area.
Page 3

BAL BHARATI PUBLIC SCHOOL, PITAMPURA, DELHI – 110034

Class-VIII  { U.T. - 1}

NOTES : FORCE & PRESSURE

Q.1. What is force? State the various effects of force.
Ans. A push or pull on an object is called force.

Effects of force:
(i) Force can change the speed of a moving object.
(ii)  Force can change the direction of a moving object.
(iii)  Force can change the shape of an object.

Q.2. Give one example where force can changes the

(a)  Speed of a moving object
(b) Direction of a moving object.
©  the shape of an object.

Ans. (a)  The force applied by a hockey player on a moving ball changes the speed of ball.
(b) The force applied by a batsman changes the direction of moving cricket ball.
©  Force applied on a spring can change its shape.

Q.3.  Identify the actions involved in the following situations as push or pull, or both:
(a) Opening a drawer.
©  A cricket ball hit by a batsman.
(d) Drawing a bucket of water from a well.

Ans. (a)  Pull.
(b)  Push; Pull.
(d)  Pull.

Note: a)  At least two objects must interact for a force to come into play.

b) Force applied on an object in the same direction add to one another.

c) If the forces act in the opposite directions on object, the net force acting on it is the
difference between the two forces.

d) The state of motion of an object is described by its speed & the direction of motion. So we
can say that a force can change the state of motion of an object.

e) S.I unit of force is Newton ( N).

Q.4.  In a tug of war, when the two teams are pulling the rope, a stage comes when the rope
does not move to either side at all. What can you say about the magnitudes & directions
of the forces being applied to the rope by the two teams at this stage?

Ans. At this point of time, the two teams are applying equal but opposite forces to the rope due to
which the net force acting on the rope is zero & hence the rope does not move.

Q.5. What is meant by a contact force?

Ans. A force that can be applied only, when an object comes in direct contact with an another object
is called contact force. Eg. Muscular force & Frictional force.

Q.6. What is muscular force?

Ans. The force resulting due to the action of muscles is called muscular force. Eg pushing an object
or lifting a bucket of water.

Note: Since muscular force can be applied to an object only when our body is in contact with
the object, therefore, muscular force is a contact force.

Q.7. What is meant by non-contact force?

Ans. A force that can be applied by an object on another object even from a distance (without
touching) is called a non-contact force. Eg. Magnetic force, Electrostatic force & Gravitational
force.

Q.8. A small device pulls iron nails from a distance. Which type of force is involved in this
process?

Ans. Magnetic force

Q.9. What is electrostatic force?

Ans. The force exerted by a charged body on another charged or uncharged body is known as
electrostatic force.

Q.10. When a plastic pen is rubbed in dry hair, it attracts tiny pieces of paper. Which force is
involved in this process?

Ans. Electrostatic force.

Q.11. What is meant by gravitational force?

Ans.  Every object in this universe attracts every other object with a force, called gravitational force.

Q.12. What is meant by Force of gravity?

Ans. The force, with which the earth pulls the objects towards it, is called the force of gravity or
simply gravity.

Q.13.  A coin falling to the ground on slipping from hand. Name the type of force involved in
this process?

Ans. Gravitational force.

Q.14.  Define pressure.

Ans. The force acting on unit area of surface is called pressure.

Pressure = Force / Area.
Unit of pressure is N/m
2
(Newton per square metre) which is also called Pascal (Pa).

Note: Pressure is inversely proportional to area. Therefore smaller the area, larger the
pressure on a surface for the same force.

Q.15. Explain why, school bags are provided with wide straps to carry them?

Ans. As a result the area of contact between the shoulders & the straps increases, thereby
decreasing the pressure on the shoulders, as pressure is inversely proportional to area.

Q.16. Explain why, wooden sleepers are kept below the railway line?

Ans. So as to increase the area of contact between the rails & the ground, thereby decreasing the
pressure on the ground, as pressure is inversely proportional to area. This prevents the sinking
of rails into the ground.

Q.17. Why is it easier for a camel to run in a desert?

Ans. Camel has board feet causes an increase in the area of contact between the sand & its feet. As a
result pressure exerted by camel on the sand decreases because pressure is inversely
proportional to area. This prevents the sinking of its feet’s into the sand.

Q.18. How does the pressure of a liquid depends on its depth?

Ans The pressure exerted by a liquid increases with increasing depth inside the liquid..

Q.19. Where is the pressure greater, 10 m below the surface of the sea or 20 m below the
surface of sea?

Ans. 20 m below the surface of sea.

Q.20. One student says that water exerts pressure on the bottom of the bucket but another
student says that water exerts pressure on the sides of the bucket. What would you like
to say?

Ans. Both the students are correct as water exerts pressure on the bottom as well as on the side
walls of the container.

Q21. Two tiny holes are made in a plastic bucket, one near the middle part & the other just
above the bottom. When this bucket is filled with water, the water rushes out from the
bottom hole much faster than from the upper hole. What conclusion do you get from
these observations?

Ans The pressure exerted by a liquid increases with increasing depth inside the liquid..

Q22. Explain why, the walls of dam are thicker near the bottom than at the top?

Ans The pressure exerted by a liquid increases with increasing depth inside the liquid. That’s why
the wall of a dam is made thicker at the bottom (than at the top) so as to tolerate very high
sideways pressure exerted by deep water.

Page 4

BAL BHARATI PUBLIC SCHOOL, PITAMPURA, DELHI – 110034

Class-VIII  { U.T. - 1}

NOTES : FORCE & PRESSURE

Q.1. What is force? State the various effects of force.
Ans. A push or pull on an object is called force.

Effects of force:
(i) Force can change the speed of a moving object.
(ii)  Force can change the direction of a moving object.
(iii)  Force can change the shape of an object.

Q.2. Give one example where force can changes the

(a)  Speed of a moving object
(b) Direction of a moving object.
©  the shape of an object.

Ans. (a)  The force applied by a hockey player on a moving ball changes the speed of ball.
(b) The force applied by a batsman changes the direction of moving cricket ball.
©  Force applied on a spring can change its shape.

Q.3.  Identify the actions involved in the following situations as push or pull, or both:
(a) Opening a drawer.
©  A cricket ball hit by a batsman.
(d) Drawing a bucket of water from a well.

Ans. (a)  Pull.
(b)  Push; Pull.
(d)  Pull.

Note: a)  At least two objects must interact for a force to come into play.

b) Force applied on an object in the same direction add to one another.

c) If the forces act in the opposite directions on object, the net force acting on it is the
difference between the two forces.

d) The state of motion of an object is described by its speed & the direction of motion. So we
can say that a force can change the state of motion of an object.

e) S.I unit of force is Newton ( N).

Q.4.  In a tug of war, when the two teams are pulling the rope, a stage comes when the rope
does not move to either side at all. What can you say about the magnitudes & directions
of the forces being applied to the rope by the two teams at this stage?

Ans. At this point of time, the two teams are applying equal but opposite forces to the rope due to
which the net force acting on the rope is zero & hence the rope does not move.

Q.5. What is meant by a contact force?

Ans. A force that can be applied only, when an object comes in direct contact with an another object
is called contact force. Eg. Muscular force & Frictional force.

Q.6. What is muscular force?

Ans. The force resulting due to the action of muscles is called muscular force. Eg pushing an object
or lifting a bucket of water.

Note: Since muscular force can be applied to an object only when our body is in contact with
the object, therefore, muscular force is a contact force.

Q.7. What is meant by non-contact force?

Ans. A force that can be applied by an object on another object even from a distance (without
touching) is called a non-contact force. Eg. Magnetic force, Electrostatic force & Gravitational
force.

Q.8. A small device pulls iron nails from a distance. Which type of force is involved in this
process?

Ans. Magnetic force

Q.9. What is electrostatic force?

Ans. The force exerted by a charged body on another charged or uncharged body is known as
electrostatic force.

Q.10. When a plastic pen is rubbed in dry hair, it attracts tiny pieces of paper. Which force is
involved in this process?

Ans. Electrostatic force.

Q.11. What is meant by gravitational force?

Ans.  Every object in this universe attracts every other object with a force, called gravitational force.

Q.12. What is meant by Force of gravity?

Ans. The force, with which the earth pulls the objects towards it, is called the force of gravity or
simply gravity.

Q.13.  A coin falling to the ground on slipping from hand. Name the type of force involved in
this process?

Ans. Gravitational force.

Q.14.  Define pressure.

Ans. The force acting on unit area of surface is called pressure.

Pressure = Force / Area.
Unit of pressure is N/m
2
(Newton per square metre) which is also called Pascal (Pa).

Note: Pressure is inversely proportional to area. Therefore smaller the area, larger the
pressure on a surface for the same force.

Q.15. Explain why, school bags are provided with wide straps to carry them?

Ans. As a result the area of contact between the shoulders & the straps increases, thereby
decreasing the pressure on the shoulders, as pressure is inversely proportional to area.

Q.16. Explain why, wooden sleepers are kept below the railway line?

Ans. So as to increase the area of contact between the rails & the ground, thereby decreasing the
pressure on the ground, as pressure is inversely proportional to area. This prevents the sinking
of rails into the ground.

Q.17. Why is it easier for a camel to run in a desert?

Ans. Camel has board feet causes an increase in the area of contact between the sand & its feet. As a
result pressure exerted by camel on the sand decreases because pressure is inversely
proportional to area. This prevents the sinking of its feet’s into the sand.

Q.18. How does the pressure of a liquid depends on its depth?

Ans The pressure exerted by a liquid increases with increasing depth inside the liquid..

Q.19. Where is the pressure greater, 10 m below the surface of the sea or 20 m below the
surface of sea?

Ans. 20 m below the surface of sea.

Q.20. One student says that water exerts pressure on the bottom of the bucket but another
student says that water exerts pressure on the sides of the bucket. What would you like
to say?

Ans. Both the students are correct as water exerts pressure on the bottom as well as on the side
walls of the container.

Q21. Two tiny holes are made in a plastic bucket, one near the middle part & the other just
above the bottom. When this bucket is filled with water, the water rushes out from the
bottom hole much faster than from the upper hole. What conclusion do you get from
these observations?

Ans The pressure exerted by a liquid increases with increasing depth inside the liquid..

Q22. Explain why, the walls of dam are thicker near the bottom than at the top?

Ans The pressure exerted by a liquid increases with increasing depth inside the liquid. That’s why
the wall of a dam is made thicker at the bottom (than at the top) so as to tolerate very high
sideways pressure exerted by deep water.

Q23.  What is meant by atmospheric pressure?

Ans. The envelop of air around us is known as atmosphere. Now the pressure exerted by this air is
known as atmospheric pressure.

Note:
(a) The atmospheric pressure is maximum at the sea level.
(b) As we go up in the atmosphere from the surface of earth, the atmospheric pressure goes on
decreasing.

Q24. What is the cause of atmospheric pressure?

Ans. The atmospheric pressure is due to the weight of air present in the atmosphere above us.

Q25. Why are our bodies not crushed by the large pressure exerted by the atmosphere?

Ans. The reason that we are not crushed is that the pressures inside our bodies is also equal to the
atmospheric pressure & cancel the pressure from outside.

Note:  (a)  At high altitudes, the atmospheric pressure becomes much less than our blood
pressure. Therefore some of the blood vessels in our body burst & nose bleeding
take place at high altitudes.

(b)  Rubber suckers are usually used to hold objects with the help of suctions. When
we press the sucker, most of the air between its cup & the surface escapes out.
The sucker sticks to the surface because the pressure of atmosphere acts on it. If
there is no air between the cup & the surface, than it would be impossible to pull
the sucker off the surface.

NUMERICALS:

1. Calculate the pressure when a force of 200 N is exerted on an area of:
(a)  10 m
2

(b)  5 m
2

2.  What force acting on area of 0.5 m
2
will produce a pressure of 500 Pa?

3. The tip of drawing pin has an area 1.0 x 10
-8

m
2
. Find the pressure exerted if force applied is
10 N.

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