Lakhmir Singh & Manjit Kaur: Force and Pressure- 2 | Lakhmir Singh & Manjit Kaur Solutions: Class 8 Science PDF Download

Short Answer Type Questions

Q.31. Define ‘state of motion’ of an object. Name the ‘agent’ which can change the state of motion of an object.

The state of motion of an object is defined by the speed acquired by it in a particular direction. Force can change the state of motion of an object.

Q.32. Give two examples of situations where you push or pull to change the state of motion of objects.

• Pushing a carom board coin
• Moving a glass placed on table

The force can change the state of motion on applying on it.

Q.33. What is meant by saying that force is due to an interaction’? Give an example to illustrate your answer.

It means for force to come into play, at least two object must interact with each other. For example: If the man pushes the car with his hands due to which the car starts moving, there is an interaction between man and a car. During this interaction force arises which Acts on the car and makes it move in the direction of applied force.

Q.34. 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 and directions of the forces being applied to the rope by the two teams at this stage?

Same magnitude of force are acting in opposite direction. Therefore they cancel each other.

Q.35. What is force? State the various effects of force.

A force is a push or pull upon an object resulting from the object’s interaction with another object. Various effects of force are:
(a) It can change shape
(b) It can change direction
(c) It can change position

Q.36.
(A) Give one example where force moves a stationary object.

Pushing a table
The force can change the state of motion of object.

(B) State one example where force stops a moving object.

A brake applied on bicycle to stop due to force.

Q.37.
(A) Give one example where force changes the speed of a moving object.

Pushing a slow moving bicycle increases its speed.

(B) Give one example where force changes the direction of a moving object.

Hitting a moving ball change its direction.

Q.38. Why does the shape of an ointment tube change when we squeeze it?

The shape changes because our finger applies force on the tube that is why the ointment tube’s shape changed.

Q.39. What happens to the springs of a sofa when we sit on it?

The shape and size of the spring changes, the force due to sitting on the sofa changes the shape and size of the spring.

Q.40. Name the various types of forces.

• Contact Forces (Frictional Force, Tension Force, Normal Force, Air Resistance Force, Applied Force and Spring Force)
• Action-at-a-Distance Forces (Magnetic Force, Electrical Force, Gravitational Force)

Q.41. What is muscular force? Give one example of muscular force.

Muscular force is a force which is generated by our muscles during heavy works, exercises etc which produce lactic acid. The example of muscular force is bouncing a ball.

Q.42. Which of the following are non-contact forces?
Magnetic force, Frictional force, Gravitational force, Muscular force, Electrostatic force.

Non-Contact Forces are the forces exerted by an object that can be experienced by an object even from a distance without any contact with each other magnetic, Electrostatic and Gravitational Forces are Non-Contact Forces

Q.43. Give two examples from everyday life which show that air exerts pressure.

• Drinking Straw
• Vacuum Cleaner

Q.44. What is a rubber sucker? How does it work? State any one use of a rubber sucker.

Rubber sucker a device used in toys, sunlight blocker – suction cups, suction clamps and various other places where “sticking” is required for temporary period and when you don’t want to use glue.
The principle on which Rubber suckers work: When the sucker is pressed against the surface, it squeeze out the air that is behind it. The atmospheric pressure on the outside of the sucker holds its rim firmly against the smooth surface.

Q.45. Why do mountaineers usually suffer from nose-bleeding at high altitudes?

At higher altitude the atmospheric pressure decreases therefore the blood vessels exceed the outside pressure which causes blood vessels to get rupture.

Q.46. Describe one activity to show the existence of atmospheric pressure.

Crushing can experiment
When a can filled with hot water is closed and is cooled down rapidly by pouring cold water on it, it will crush instantly. This experiment proves that there is a huge atmospheric pressure exerts on everything on the surface of the earth.

Q.47. Explain why, water comes out more slowly from an upstairs tap than from a similar tap downstairs.

As the depth of water increases the pressure also increases.

Q.48. What is meant by gravitational force (or force of gravity)? Give its one example.

The force of attraction between all masses in the universe, especially the attraction of the earth’s mass for bodies near its surface. For example: When you hold a ball up in the air, the mass of Earth allows the ball to fall to the Earth.

Q.49. Calculate the pressure when a force of 200 N is exerted on an area of:
(a) 10m²
(b) 5m²

Given: Force applied is given as: 200N
Now, we know that the Pressure is the force applied perpendicular to the surface of an object per unit area. or Pressure = Force/ Area
(a) Area = 10m²
P = 200N/10m²
or P = 20 Nm² or 20 Pascal
(b) Area = 10m²
P = 200N/5m²
or P = 40 Nm² or 40 Pascal

Q.50. Which force do the animals apply while moving, chewing and doing other activities?

Muscular force

Q.51. Which force is responsible for raising our body hair when we try to take off a terylene or polyester shirt in the dry weather?

Electrostatic force
The force acting on object due to its charged.

Q.52. Name the type of forces involved in the following:
(a) A horse pulling a cart.
(b) A sticker attached to steel almirah without glue.
(c) A coin falling to the ground on slipping from hand.
(d) A plastic comb rubbed in dry hair picking up tiny pieces of paper.
(e) A moving boat coming to rest when rowing is stopped.

(a) Muscular force is a contact force due to action of muscles.
(b) Magnetic force
(c) Gravitational force
Objects fall towards the earth due to its gravity.
(d) Electrostatic force
The force acting on an object due to its charged particles.
(e) Frictional force
Stream of water opposed to moving boat to stop.

Q.53. Why does a sharp knife cut objects more effectively than a blunt knife?

Lesser the area, larger the pressure for the same force applied. Therefore it is easier to cut with sharp knives than blunt ones, with same force.

Q.54. Explain why, wooden (or concrete) sleepers are kept below the railway line.

There are many reasons. Some of them are

• To keep the lines equidistant from each other.
• To reduce unwanted vibrations in the track.
• The sleepers have a large surface area compared to the lines; this larger surface area decreases the pressure due to the weight of the train on the track. If only the rails were used without slippers; they being thinner would extent greater pressure over the bed of stone and move deeper into the ground

Q.55. Explain why a wide steel belt is provided over the wheels of an army tank.

The weight of the tank (its mass being acted upon by the force of gravity) is spread over a larger surface area (the big steel belt), giving it a smaller pressure and therefore it makes it sink in less, allowing it to travel faster over more surfaces where others would with less surface area.

Q.56. Explain why the tip of a sewing needle is sharp. 

The tip of sewing needle is sharp because with the decrease of area, pressure increases and the needle can easily penetrate the stitching material.

Q.57. Explain why snow shoes stop you from sinking into snow. 

The snow shoes have large, flat soles so they exert less pressure (= force /area) on the soft snow and stop the wearer from sinking into it.

Q.58. Explain why, when a person stands on a cushion, the depression is much more than when he lies down on it. 

When a man stands on a cushion then only his two feet (having small area) are in contact with the cushion. Due to this the weight of man falls on a small area of the cushion producing a large pressure. This large pressure causes a big depression in the cushion.

Q.59. Explain why, porters place a thick, round piece of cloth on their heads when they have to carry heavy loads. 

The round piece of cloth on their heads increases the area on which the force (weight of the load) will act. We know that, the pressure is inversely proportional to area. Therefore, increase in area reduces the pressure and this helps the porter to carry the heavy load easily.

Q.60. Give one practical application of magnetic force.

Magnetic forces are the force due to magnetic field of a magnet. The closing of door of refrigerator works on the application of magnetic forces.