Lakhmir Singh & Manjit Kaur: Force and Pressure- 3 | Lakhmir Singh & Manjit Kaur Solutions: Class 8 Science PDF Download

Q.61.
(A) What is meant by a contact force? Explain with the help of an example.

A Contact force as the name suggests acts on a point of an object by direct contact. The Contact may be Continuous or sometimes momentary. The former case is known as Continuous force and the latter case is known as impulse force.
Example: Pushing an object along the floor: This is an example of a continuous force. Here, continuously, the direct force is put on the object to move it along the floor.

(B) What is meant by a non-contact force? Explain with the help of an example.

An imaginary force can pull everything towards the earth surface. All the objects are pulled towards the earth without any physical contact with the objects and bodies. These are non contact forces in the most basic sense.
Example

• Gravitational force
• Magnetic force
• Electrostatics
• Nuclear force

Q.62. 
(A) Define frictional force (or friction).

Frictional Force refers to the force generated by two surfaces that contacts and slide against each other. These forces are mainly affected by the surface texture and amount of force impelling them together. The angle and position of the object affect the amount of frictional force. If an object is placed flat against an object, then the frictional force will be equal to the weight of the object. If an object is pushed against the surface, then the frictional force will be increased and becomes more than the weight of the object.

(B) Explain why frictional force is said to be a contact force.

If life an object which is moving over a horizontal surface does not continue with constant velocity when the accelerating force is removed. Instead, the object slows down and eventually stops. The deceleration is caused by a frictional force between the object and the surface on which it travels. A frictional force always acts to oppose the motion of an object over a surface and is an example of a contact force. When we try to slide one object over another, we find that there is a force which opposes the motion. This force is called friction. Whenever there is relative motion of two surfaces in contact, there is frictional resistance. When a body moves over another, it has to overcome friction. This needs energy. Hence, some energy is always wasted in overcoming friction. When two parts of a machinery rub against each other heat are produced, and the efficiency of the machine decreases. There will be a greater wear and tear in the different parts of a machine due to friction.

(C) Explain, why magnetic force is said to be a non-contact force.

Magnetic force is acted upon a body by the magnetic lines of force. These are imaginary closed line of lines of force which emanate from the N pole f the magnet and goes into the S pole of the magnet. So, a magnet can influence a magnetic material like a piece of iron even when they are not in contact thus magnetic force is a non contact force.

Q.63.
(A) Define pressure. What is the relation between pressure, force and area? State the units in which pressure is measured.

Pressure is defined as a measure of the force applied over a unit area. Pressure often is expressed in units of Pascals (Pa), newtons per square meter (N/m² or kg/m.s²), or pounds per square inch. Other units include the atmosphere (atm), torr, bar, and meters sea water (msw).
In equations, pressure is denoted by the capital letter P or the lowercase letter p.
Pressure is a derived unit, generally expressed according to the units of the equation:
P = F / A
where P is pressure, F is force, and A is area

P is pressure
F is the component of force perpendicular to the surface

A is the area of the surface

When a force is constant over an area, the pressure acting on that area is simply

Pressure is a scalar quantity, thus it acts in all directions at any given point. In order for pressure to create a force, the pressure must be integrated over some area.

(B) Explain why school bags are provided with wide straps to carry them. 

A wider strap distributes the force throughout a greater surface area. You can do a comparison using a simple physics equation.
Pressure = Force/Area
So take two different straps and measure the area of them.
Then find out how much weight the pack will be holding.
Then divide the Weight by the Area and you end up with pressure.

Q.64.
(A) What is meant by atmospheric pressure? What is the cause of atmospheric pressure?

Atmospheric Pressure: The air around you has weight, and it presses against everything it touches. That pressure is called atmospheric pressure, or air pressure. It is the force forced on a surface by the air above it as gravity pulls it to Earth.
Causes:

• Height of altitude
• Density of air

(B) Why are our bodies not crushed by the large pressure exerted by the atmosphere?

Because the atmosphere also pressurize the body. Diving down 100 meters will pressurize the gasses in the body and they will compress, like the air in our lungs and if the lung volume gets below half a litre the lungs can get partly filled with blood plasma. But sorry no bone crushing. But if you take a big breath and hold it while you ascend from 100 meters to the surface, your ribs may start to break.

(C) Explain why atmospheric pressure decreases as we go higher up above the earth’s surface.

The air molecules at lower altitudes are more compressed by gravity than the ones at the higher altitudes. The pressure level is highest right at the surface of the earth because the air at this level is supporting the weight of all the air above it. More weight above means a greater downward gravitational force. As we move up through levels of the atmosphere, the air has less air mass above it and gravity isn’t strong enough to pull down a greater number of particles. So the balancing pressure decreases. This is why atmospheric pressure drops as we rise in altitude.

Q.65.
(A) How does the pressure of a liquid depend on its depth? Draw a labelled diagram to show that the pressure of a liquid (say, water) depends on its depth.

All the liquids exert pressure on the base or bottom and walls of their container. All the liquid have weight. When we pour liquid into a vessel, then the weight of liquid pushes down on the base of the vessel producing a pressure. The liquid exerts pressure on the base of the vessel. The pressure exerted by a liquid changes with depth in the liquid. The pressure exerted by a liquid increases with increasing depth inside the liquid. The pressure exerted by a liquid is small just under the surface of the liquid. But as we go we deeper in a liquid the pressure of liquid increases. As the depth of liquid increases the weight of liquid column pushing down from above increases and hence the pressure also increases.

(B) Explain why the walls of a dam are thicker near the bottom than at the top.

A dam is thicker at the bottom than at the top because the forces exerted are strongest close to the ground. Water pressure increases proportionally with depth, so the lower portion of the dam has greater water pressure exerted upon it. The increasing thickness of the lower dam also helps it support its own weight.

Q.66. Which of the following is not an example of muscular force?
(a) A porter carrying a load on a wheel-barrow.
(b) An apple falling from a tree.
(c) A child riding a bicycle.
(d) A person drawing water from a well.

Correct Answer is Option (b)
An apple falling from a tree is an example of gravitational force.

Q.67. Which of the following is not an example of the force of gravity?
(a) A leaf falling from a tree.
(b) A boy pushing a cart on a level plane.
(c) A diver jumping into a swimming pool.
(d) A stone falling from the top of a cliff.

Correct Answer is Option (b)
A boy pushes a cart on a level of plane due to force.

Q.68. When we press the bulb of a dropper with its nozzle kept in water air in the dropper is seen to escape in the form of bubbles. Once we release the pressure on the bulb, water gets filled in the dropper. The rise of water in the dropper is due to:
(a) Pressure of water
(b) Gravity of the earth
(c) Shape of rubber bulb
(d) Atmospheric pressure

Correct Answer is Option (d)
Atmospheric pressure exerted by air.

Q.69. A rectangular wooden block has length, breadth and height of 50 cm, 25 cm and10 cm, respectively. This wooden block is kept on ground in three different ways, turn by turn. Which of the following is the correct statement about the pressure exerted by this block on the ground?
(a) The maximum pressure is exerted when the length and breadth form the base
(b) The maximum pressure is exerted when length and height form the base
(c) The maximum pressure is exerted when breadth and height form the base
(d) The minimum pressure is exerted when length and height form the base

Correct Answer is Option (c)
The maximum pressure is exerted when breadth and height form the base Pressure = force / area.

Q.70. Which of the following are contact forces ?
(A) Friction
(B) Gravitational force
(C) Magnetic force
(D) Muscular force
(a) A and B
(b) B and C
(c) A and D
(d) B and D

Correct Answer is Option (c)
Force applied when it is in contact with the object.

Q.71. If we release a magnet held in our hand, it falls to the ground. The force responsible for this is:
(a) Muscular force
(b) Magnetic force
(c) Electrostatic force
(d) Gravitational force

Correct Answer is Option (d)
Force applied when it is in non contact with the object.

Q.72. Which of the following force is utilized in reducing air pollution by removing dust, soot and fly-ash particles from the smoke coming out of chimneys of factories ?
(a) Magnetic force
(b) Gravitational force
(c) Electrostatic force
(d) Frictional force

Correct Answer is Option (c)
Charged particles gets removed by electrostatic force .

Q.73. The same force Facts on four different objects having the areas given below, one by one. In which case the pressure exerted will be the maximum?
(a) 20 m²
(b) 50 m²
(c) 10 m²
(d) 100 m²

Correct Answer is Option (c)

Q.74. Which of the following represent correct values for the normal atmospheric pressure?
(A) 101.3 kilopascals
(B) 76 mm of mercury
(C) 101.3 pascals
(D) 76 cm of mercury
(a) A and B
(b) B and C
(c) A and D
(d) B and D

Correct Answer is Option (d)

Q.75. Which of the following does not work on the existence of atmospheric pressure?
(a) Rise of iodine solution in the glass tube of dropper
(b) Rise of cold drink in a long plastic straw
(c) Sticking of suction hook on the wall of a room
(d) Rise of mercury in glass tube of thermometer

Correct Answer is Option (a)
P = height × density × gravity

Q.76. The magnitude of force is expressed in the unit of force called:
(a) Pascal
(b) Kelvin
(c) Newton
(d) Magdeburg

Correct Answer is Option (c)
The unit of force is newton.

Q.77. Which of the following change appreciably when a batsman hits a moving cricket ball?
(A) Shape
(B) Direction
(C) Size
(D) Speed
(a) A and B
(b) B and C
(c) A and C
(d) B and D

Correct Answer is Option (d)
The force can change the direction and Speed of motion.

Q.78. Which of the following is not an effect of force?
(a) A force can change the speed of a moving object
(b) A force can change the direction of a moving object
(c) A force can change the composition of a moving object
(d) A force can change the shape and size of an object

Correct Answer is Option (b)
The force can change the direction of motion.

Q.79. Which of the following is not a non-contact force?
(a) Electrostatic force
(b) Gravitational force
(c) Frictional force
(d) Magnetic force

Correct Answer is Option (b)
Friction force is an example of contact force.

Q.80. Which of the following scientists gave the idea of the existence of gravitational force?
(a) Einstein
(b) James Watt
(c) Faraday
(d) Newton

Correct Answer is Option (d)
Newton propound the universal laws of gravitational

Q.81. Some mustard oil is kept in a beaker. It will exert pressure:
(a) Downwards only
(b) Sideways only
(c) Upwards only
(d) In all directions

Correct Answer is Option (d)
Fluid exerted pressure in all direction

Q.82. A pressure of 10 kPa acts on an area of 0.3 m². The force acting on the area will be :
(a) 3000 N
(b) 30 N
(c) 3 N
(d) 300 N

Correct Answer is Option (a)
Force = pressure × area
= 10 × 1000 × 0.3 = 3000N

Q.83. The magnitude of atmospheric pressure is equal to the pressure exerted by a:
(a) 76 mm tall column of mercury(
(b) 760 mm tall column of alcohol
(c) 76 cm tall column of mercury
(d) 760 cm tall column of mercury

Correct Answer is Option (c)
The standard temperature and pressure (STP), refers to nominal conditions in the atmosphere at sea level, which supports 760 millimeters in a mercurial barometer (760 mm Hg)

Q.84. The atmospheric pressure is usually measured in the unit of :
(a) Newtons per square meter
(b) Pascal
(c) Cm of mercury
(d) Mm of mercury

Correct Answer is Option (d)

The level of mercury is measured in mm.

Q.85. When a force of 5 N acts on a surface, it produces a pressure of 500 Pa. The area of surface then must be
(a) 10 cm²
(b) 50 cm²
(c) 100 cm²
(d) 0.01 cm²

Correct Answer is Option (c)
Pressure = thrust / area
Area = Pressure / Thrust = 500/ 5 = 100 cm²

Q.86. Two tiny holes are made in a plastic bucket, one near the middle part and the other just above 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 this observation? 

Pressure exerted by water increases with creasing depth

Q.87. What is common in the working of the devices such as a drinking straw, a syringe, a dropper and a rubber sucker? 

All these devices work on the existence of atmospheric pressure

Q.88. A rocket has been fired upwards to launch a satellite in its orbit. Name the two forces acting on the rocket immediately after leaving the launching pad (Ignore the frictional force due to air resistance). 

Upward force applied by the rocket engine and downward gravitational force applied by the earth

Q.89. 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? 

Water exerts pressure on the bottom of the bucket as well as on the sides of the bucket

Q.90. Name the forces acting on a plastic bucket containing water held above ground level in your hand. Discuss why the forces acting on the bucket do not bring a change in its state of motion.

Upward muscular force applied by hand and downward gravitational force applied by earth ; The two forces being equal and opposite balance each other and hence do not bring a change in the state of motion.