Class 8 SELINA Solutions Physics Chapter 3: Force and Pressure

(iv) N m

The moment of a force is equal to the product of the magnitude of the force and the perpendicular distance of the force from the pivoted point.

Moment of force = force × perpendicular distance of force from the point O.

i.e., the S.I unit of moment of force is N m

(iii) applying the force farthest from the pivoted point.

The moment of a force is equal to the product of the magnitude of the force and the perpendicular distance of the force from the pivoted point.

Moment of force = force × perpendicular distance of force from the point O.

Thus, to decrease the amount of force we have to increase the perpendicular distance from the pivoted point.

(i) kgf

The unit of gravitational force is kgf.

(iii) N m^-2

As we know,

(iii) Pressure = Thrust/Area

Pressure is defined as thrust acting per unit area.

When placed on a surface, a body exerts a thrust equal to its own weight.

∴ Thrust = weight of the body = 5 kgf.

(iii) suction pads

Lizards can stay and move on walls because their feet behave like suction pads, allowing them to remain pressed against the wall due to atmospheric pressure.

(i) weight

The pressure exerted by a liquid is due to its weight.

(i) increase in depth

The pressure inside a liquid at a point increase with the height of the liquid column above it, and as the depth increases, so does the pressure inside the liquid.

The atmospheric pressure on the earth's surface at sea level is about one hundred thousand Pascal.

(i) the atmospheric pressure decreases.

As we move above the earth’s surface, the atmospheric pressure decreases. Hence, nose bleeding may occur at higher altitudes since external atmospheric pressure is less than internal temperature.

(a) 1 kgf = 10 N (nearly).

(b) Moment of force = force × distance of force from the point of turning.

(c) In a door, handle is provided farthest from the hinges.

(d) The unit of thrust is newton.

(e) Thrust is the normal force acting on a surface.

(f) Pressure is the thrust acting on a surface of unit area.

(g) The unit of pressure is pascal.

(h) Pressure is reduced if area of surface increases.

(i) Pressure in a liquid increase with the depth.

(j) The atmospheric pressure on earth surface is nearly 10⁵.

False.

The S.I. unit of force is newton (N) whereas kgf is the force by with earth pulls a 1 kg object towards itself.

False

A force can either produce linear motion or a turning motion.

True

True

False

Pressure is defined as thrust per unit area.

i.e., Pressure ∝ 1/Area

Hence, for a given thrust, pressure is less on a large surface.

True

False

As we know,

Hence, a man will exert more pressure on the ground when he is standing than when he is lying on the ground.

False

In the case of a blunt nail, the thrust applied during hammering acts on a larger area, resulting in less pressure on the piece of wood, making it difficult to hammer the nail.

True

False

Water in a lake exerts pressure in every direction.

True

True

True

False

The higher we go, the lower the air pressure is.

Force is the cause of change in the state of the body (either the state of rest or the state of motion) or in its size or shape.

The S.I. unit of force is newton (N).

When a force is applied to a body, it produces two effects:

(i)It has the ability to change the shape and size of a body.

(ii) It can either stop a moving body or move a stationary body.

(a) Applying a force to a rigid body causes only a change in motion. The force has no effect on the distance between its constituent particles.

(b) When a force is applied to a non-rigid body, it changes its size or shape as well as moves it.

(i) As shown in figure (a), when force F is applied to the ball, it begins to move in a straight line in the direction of force F and continues to do so. This is known as linear motion.

(ii) Figure (b) shows that when a force F is applied to a wheel pivoted at a point, the force turns the wheel in an anticlockwise direction about the axis of rotation, producing turning motion.

The moment of a force is equal to the product of the magnitude of the force and the perpendicular distance of the force from the pivoted point.

i.e., Moment of force = force× Perpendicular distance from the pivoted point.

As we know,

Moment of force = force × Perpendicular distance from the pivoted point.

Therefore, the S.I unit of moment of force is newton metre.

The two factors on which moment of force dependent are:

• Magnitude of force.
• Perpendicular distance of force from the pivoted point.

The expression for the moment of force (τ about a given axis of rotation is:

Moment of force (τ) = force × Perpendicular distance from the pivoted point.

The moment of a given force about a given axis of rotation is reduced by either decreasing the force or reducing the force's perpendicular distance from the axis of rotation.

One way to increase the moment of a given force is to move it away from the axis of rotation.

∵ Moment of force ∝ perpendicular distance from the axis of rotation.

If the effect on the body causes it to turn clockwise, the moment of force is referred to as the clockwise moment.

If the effect on the body causes it to turn anticlockwise, the moment of force is referred to as the anti-clockwise moment.

Thrust is the term used to describe a force applied to a surface in a perpendicular direction. In other words, thrust refers to the normal force acting on a surface.

The units of thrush are newton (N), kilogram force (kgf) and gram force (gf).

Thrust has an effect that is proportional to the surface area on which it acts. Thrust has a greater effect on a smaller surface area than on a larger surface area.

Pressure is defined as the thrust acting per unit area.

The S.I. unit of pressure in newton per metresquare (N/m²) which is also called pascal (Pa).

Pressure is thrust acting per unit area. Thrust is directly proportional to pressure.

Greater the thrust, greater is the pressure and smaller the thrust, smaller is the pressure.

Pressure on a surface depends on two factors:

• The area of the surface on which thrust acts.

• The amount of thrust acting on the surface.

The bottom part of a building's foundation is made wider so that its weight acts on a larger surface area, exerting less pressure on the ground (pressure is inversely proportional to surface area). This avoids the sinking of buildings into the ground.

A knife with sharp edges has a larger area of contact than a blunt knife. So the pressure exerted by a sharp knife will be greater, making it easier to cut than a blunt knife.

(i) A gum bottle has a broader base and a narrower neck, and when turned upside down, there is no change in thrust because thrust is the weight of the body exerted on the surface.

(ii) Because pressure is inversely proportional to surface area, when a gum bottle is turned upside down, the surface area decreases and the pressure increases.

The pressure at a point in a liquid depends on the following two factors:

The height of the liquid column: Liquid pressure increases as the liquid column rises above the point.

The density of the liquid: As the liquid's density increases, so does its pressure.

The thrust on unit area of the earth surface due to the column of air is called the atmospheric pressure.

The blood in our veins exerts slightly more pressure than atmospheric pressure, rendering atmospheric pressure ineffective. So, despite the enormous atmospheric pressure, we do not feel uneasy.

The atmospheric pressure on Earth's surface is 1.013 x 10⁵ Pa.

(a) A spanner used to tighten or loosen a nut, has a long handle to produce larger turning moment.

(a) It is easier to turn a steering wheel of large diameter because the larger the diameter, the greater the perpendicular distance from the axis of rotation, and thus less force is required to generate the same amount of turning effect as a steering wheel of small diameter.

(b) The hand flour grinder has a handle near the rim that is at a maximum distance from the axis of rotation, allowing it to be easily rotated at its centre with a small force on the handle.

(c) It is easier to open the door by pushing on its free end because the greater the perpendicular distance from the axis of rotation, the less force is required to open it.

(d) A potter's wheel pivots in the centre.

(e) When the potter applies a force through the stick near the rim of the wheel, the point of application of force is at the greatest distance from the axis of rotation, requiring less force to rotate the wheel.

(i) Wide wooden sleepers are installed beneath railway tracks to increase the area of the tracks in contact with the ground. This reduces the pressure exerted by the rails against the ground.

(ii) A tall building's foundation is made wider so that its weight acts on a larger surface area, exerting less pressure on the ground (pressure is inversely proportional to surface area). This prevents the building from sinking into the earth.

• Take a glass tube and tie a balloon to the lower end.
• Hold it vertically and straight, as shown in the figure above.
• Pour some water into the tube.

Observations:

Over time, we will notice that unlike its initial state the balloon bulges out, as shown in figure (b) above.

Conclusion:

The balloon bulges because the water column exerts pressure at the bottom. And the force on the balloon equals the weight of the water column, which is known as thrust.

Thus, the pressure exerted by a liquid at the bottom of the container in which it is kept is given as:

• Take a glass tube with one end closed and an opening near the bottom.
• Tie a deflated balloon to the side opening of the tube.
• Hold the tube vertically straight, as shown in the figure above.
• Pour some water into the tube.

Observations: Over time, we will notice that the balloon bulges out, as shown in figure (b) above.

Conclusion: Based on the above experiment, it can be concluded that a liquid exerts pressure both horizontally and vertically.

• Take a glass tube and open both ends. Hold it vertically.
• Tie a balloon to its lower end.
• Pour some water into the tube.
• Figure (a) above shows how the balloon begins to bulge out.
• Add more water to the tube.
• The balloon will expand more, as shown in figure (b) above.

Conclusion:

The above experiment demonstrates that as the height of the water column increases, more pressure is exerted on the balloon, causing it to bulge out more. This demonstrates that the liquid pressure at a given point rises in proportion to the height of the liquid column above it.

The experiment below shows that at a given depth, a liquid exerts the same pressure in all directions.

• Fill a balloon with water.
• Tie the mouth of the balloon.
• Make holes in the balloon by inserting pins in multiple direction.

Observation:

The water flows out through each holes.

Conclusion:

Water comes out of each hole, demonstrating that liquid exerts equal pressure in all directions.

• Take two identical glass tubes and open both ends. Mark them as A and B.
• Hold both tubes vertically and secure an inflated balloon to their lower ends.
• In tube A, pour some water.
• Pour some concentrated sugar solution into tube B until it reaches the same height as the water in tube A.

Observation:

The balloon attached to tube B bulges out more than the balloon attached to tube A, as shown in the figure above.

Conclusion:

Sugar solution has a higher density than water, so it exerts more pressure, this shows that pressure exerted by a liquid is directly proportional to its density.

The following experiment shows how air exerts pressure:

• Fill a glass with water to the brim and place a postcard on top.
• Place the palm of one hand on top of the post card, then invert the filled glass.
• Now, gently remove your hand from the postcard to release it.

Observation:

It is observed that the post card does not fall from the glass.

Conclusion:

The post card does not fall from the glass because atmospheric pressure acts upwards on the post card from outside the glass, overcoming the pressure on the post card caused by water in the glass.

• Take a thin-walled tin can with an airtight stopper. Remove the stopper.
• Fill the can partially with water. Heat the can over the flame of a hob until the water boils.
• Now, the air pressure inside and outside the can is the same.
• When steam comes out of the opening, place the stopper.
• Remove the can from the hob.
• Place the can in the tub and pour cold water on it.

Observation:

It is observed that the tin can collapses after the cold water is poured on it.

Conclusion:

The tin can collapse because when steam is released, it removes the majority of the air from the can. When cold water is poured over the can, steam condenses into water, creating a partial vacuum inside. The air pressure outside is higher than inside, causing the can to collapse. This shows that air exerts pressure.

(a) When air is removed from the balloon, the pressure inside becomes much lower than the outside atmospheric pressure, causing the balloon to collapse.

(b) There are two forces acting on the water inside the dropper:

1.       Liquid pressure from inside and atmospheric pressure from outside.

2.       The atmospheric pressure acting from the outside balances the liquid pressure from the inside, preventing water from coming out of the dropper.

When the bulb is pressed, the liquid pressure rises above atmospheric pressure, and water emerges from the dropper.

(c) Two holes are drilled in a sealed oil tin to extract oil because atmospheric pressure acts through one hole as air enters through it, and oil easily exits through the other.

The atmospheric pressure decreases as we go higher. Thus, at higher altitude the atmospheric pressure decreases.

No, the body will not rotate because the moment of force equals the force multiplied by the perpendicular distance from the pivot point.

i.e., Moment of force = force × Perpendicular distance from the pivoted point.

τ = F x 0 = 0.

Since the perpendicular distance is zero, or the force is parallel to the point of application, thus the body will not rotate.

A man exerts more pressure on the surface while walking than when he is standing because while walking, only one foot is in contact with the floor, resulting in a smaller surface area.

Whereas when standing, both feet are in contact with the floor, providing a larger surface area. And since pressure is inversely proportional to surface area, pressure exerted while walking is greater than pressure exerted while standing.

Camels and elephants have broad feet, which increases the surface of the feet in contact with the ground. This reduces the pressure applied to the ground, as pressure is inversely proportional to surface area.

As a result, camels can walk more easily on the desert sand without sinking. And elephants' large body weight is supported by their broad feet, allowing them to move around comfortably.

A sharp pin has a less surface area compared to a blunt pin; hence it exerts more pressure for a given applied force and can easily penetrate a surface such as wood or a wall. As a result, it performs better than a blunt pin.

This diagram illustrates that pressure of liquid increases with depth. Hole at greater depth has more pressure and hence flow of liquid from there is more. Other hole is at less depth so the pressure of liquid there is less and hence flow of liquid from there is less. So, the diagram shows that pressure increases with increase in depth.

As we know, the pressure at a point due to a liquid increases with the height of the liquid column above it.

Hence, a dam is built with a much greater thickness of bottom walls than top walls to withstand the increasing pressure of water.

As we know, pressure inside fluids increases with depth. Thus, the pressure exerted by water deep beneath the sea is significantly greater than at sea level. As a result, deep sea divers wear special suits designed to protect them from the extreme pressure of water. These suits are known as diving suits. These suits include buoyancy compensators to counteract the weight of their diving equipment and the pressure of water at great depths.

Given that,

Force, f = 20 N

Distance, d = 0.5 m

Moment of force, τ =?

As we know,

Moment of force, τ = force (f) × distance (d)

= 20 × 0.5

= 10 N m

Given that,

Force f = 25 N

Moment of force = 2.5 N m

Perpendicular distance, d =?

As we know,

Moment of force = force× Perpendicular distance from the pivoted point.

2.5 = 25 × d

∴ d = 0.1 m = 10 cm

Given that,

Force f = 5.0 N

Distance d = 10 cm = 0.1 m

Moment of force =?

As we know,

Moment of force = force× Perpendicular distance from the pivoted point.

= 5 × 0.1 = 0.5 N m

Given that,

Moment of force, τ = 2.0 N m

Diameter = 2 m

i.e., Distance, d = radius = 1 m

Force f =?

As we know,

Moment of force = force× Perpendicular distance from the pivoted point.

∴ Force = 2 N

Given that,

Thrust, f = 200 N

Area, A = 0.02 m²

Pressure =?

As we know,

Given that,

Pressure = 50000 Pa

Area = 0.05 m²

Thrust =?

As we know,

-

Given that,

Pressure, P = 50,000 Pa

Trust, F = 100 N

As we know,

Given that,

Force, F = 300 N

Area, A = 30 cm² = 30 × (10^-2)² = 0.003 m²

As we know,

Given that,

Pressure, P = 20,000 Pa

Area, A = 1 cm² = 1 × (10^-2)² = 10^-3 m²

As we know,

∴Force=2 N

2000 Pa

Given that,

Force, F = 60 N

Area, A = 15 cm × 20 cm = 300 cm² = 0.03 m²

As we know,

∴ Pressure = 2000 Pa

Given that,

Pressure, P = 0.5 m²

Trust, F = 100 kgf

As we know,

Given that,

Weight or Force, F = 60 kgf

Area, A = 2:5 cm × 0.5 cm = 1.25 cm² = 1.25 × 10^-4 m²

As we know,

(a) 0-01 kgf cm² (b) 0-04 kgf cm (c) 0-02 kgf cm-²

Given that,

Thrust or Weight, F= 2 kgf

(a)

Area, A₁= 20 cm × 10 cm = 200 cm²

Pressure =?

As we know,

(b)

Area, A₂ = 5 cm × 10 cm = 50 cm²

Pressure, P =?

Now,

(c)

Area, A₃ = 20 cm × 5 cm = 100 cm²

Pressure, P =?

Now,