Understanding Force & Pressure Around Us

• P - Point → In the above paragraph, I point out to the theory of: How force and pressure govern the movement and behavior of objects in our world.

• E - Evidence → The evidence that supports the theory is:

  • Isaac Newton explained how force moves objects.
  • Archimedes showed how force rotates things.
  • Blaise Pascal demonstrated that liquids transmit pressure equally.
  • Torricelli proved that gases like air exert pressure.

• E - Explanation → Now I explain my point: Different scientists discovered specific rules about how nature works. Together, these ideas explain how force causes motion and how pressure behaves in liquids and gases.

• L - Link → I link my knowledge force and pressure to: A complete understanding of how things move and work around us daily.

1. A longer spanner increases the perpendicular distance from the bolt (pivot). Since the moment of force, M = F × d, increases with distance, the same force produces a greater turning effect and turns the bolt more easily.

2. The turning effect of force is called the moment of force (torque).

3. The tip of the nail has a very small area of contact. Gunjan's body weight acting on this small area produced a very high pressure (as pressure is inversely proportional to area), which is why the nail hurt her foot.

4. The pressure exerted by water increases with depth. Deep in the sea, water exerts a very large pressure on a diver's body. So divers need special suits to protect themselves from this large liquid pressure.

5. Gases exert pressure, and heating increases this pressure. The heat from the heater increased the pressure of the gas inside the balloon, so the balloon expanded.

1. Mass
   Reason — Force, thrust, friction and pressure are all forces or are related to force, whereas mass is the amount of matter in a body and is not a force.
2. Glass
   Reason — Gas, air, liquid and water are fluids (they can flow), while glass is a solid.
3. Light
   Reason — Surface area, pressure, depth and thrust are all related to pressure, whereas light has no connection with pressure.
4. Spin
   Reason — Thrust, push, pull and force are all forces or the way a force acts, while spin is a turning (rotational) motion, not a force.
5. Friction
   Reason — Depth, area, gravity and density are all linked to pressure (pressure depends on area; liquid pressure depends on depth and density; pressure arises due to gravity), whereas friction does not affect pressure.

Note: In part 5, the grouping is based on factors connected with pressure. Since both gravity and friction are forces, the question is slightly open to interpretation; "friction" has been chosen as it has no role in determining pressure in this chapter.

1. A force is a push or pull that can change the state of rest or motion, direction of motion, shape or size of a body.

2. Squeezing a toothpaste tube (or pulling a rubber band, pushing a wheelbarrow).

3. One newton is the force which, when applied on a body of mass 1 kilogram, increases its speed by 1 metre per second in every second (1 N = 1 kg m/s²).

4. Newton-metre (Nm).

5. The pressure exerted by a liquid on the base and walls of the container in which it is kept is called liquid pressure.

1. The SI unit of moment of force is newton-metre (Nm).
2. The pressure due to a liquid at a point inside it increases with the height of the liquid column above it.
3. The pressure exerted by liquids on the walls of the container is called lateral pressure.
4. Atmospheric pressure is defined as the force exerted by atmosphere on a unit area of the surface of the earth.
5. Manometer consists of a U-shaped tube filled with some liquid (water or mercury).

1. True

2. True

3. False
   Corrected Statement — It is easier to open the lid of a container with a spoon than with a coin, because a spoon has a longer arm (greater distance from the pivot), which gives a greater turning effect.

4. False
   Corrected Statement — The SI unit of thrust is the newton (N). Pascal (Pa) is the SI unit of pressure.

5. False
   Corrected Statement — The pressure due to a liquid at a point inside it increases (not decreases) with the height of the liquid column above it.

1. The walls of the dams are made thicker at the bottom to withstand the greater lateral pressure of water.

2. Pressure is inversely proportional to area.

3. Pressure in gases is more in a closed container than in an open container.

4. A narrower tip of a needle increases pressure on the skin.

5. The turning effect of force is maximum when the force is applied farthest from the pivot.

A wrench with a long arm increases the perpendicular distance (d) of the point of application of force from the pivot. Since the moment of force, M = F × d, a larger distance means a smaller force (F) is needed to produce the required turning effect. So unscrewing the nut becomes easier.

A sharp knife has a very thin edge, so it makes a small area of contact with the object. As pressure is inversely proportional to the area of contact, the same force produces a very high pressure at the edge, and the object is cut easily. A blunt knife has a larger area of contact and so exerts less pressure.

Snowshoes have a larger surface area than ordinary shoes. A larger area of contact reduces the pressure exerted on the snow. Due to this lower pressure, the feet do not sink into the snow, making walking easier.

A sharpened pencil has a very small area at its tip. The same force acting over this small area produces a high pressure (as pressure is inversely proportional to area). This high pressure makes it easy to push the pencil into the soil.

On over-inflating, more and more air is filled inside the balloon. The gas exerts pressure in all directions on the walls of the balloon, and this pressure keeps increasing. When the pressure becomes greater than the balloon walls can withstand, the balloon bursts.

A force can produce the following effects.

• It can stop a moving body.
• It can produce motion in a stationary body.
• It can change the shape and size of a body.
• It can change the direction of motion of a body.
• It can change the speed of a moving body.

When a force is applied on a body that is pivoted (fixed) at a point, the body turns about that point instead of moving in a line. This is called the turning effect of force.

For example, a door is fixed at one end with hinges. When we push it, it turns about the hinges and opens. Similarly, when force is applied on the handle of a wrench, the bolt turns and gets tightened.

Moment of force is the product of the magnitude of force and the perpendicular distance between the point of application of force and the pivoted point. It gives the turning effect of force about a fixed point, and the formula is written as M = F × d.

It depends on two factors.

• Magnitude of the applied force — the moment of force is directly proportional to the applied force.
• The perpendicular distance between the point of application of the force and the pivot — the turning effect increases with this distance.

Pressure depends on two factors.

• Magnitude of force — pressure is directly proportional to the force. The greater the force, the greater the pressure.
• Area of contact — pressure is inversely proportional to the area of contact. Pressure increases when the force acts on a smaller area and decreases when it acts on a larger area.

Some applications of the turning effect of force are as follows.

• A handle is provided at the free edge of a door. By applying force at this maximum distance from the hinges, it is easiest to open or close the door.
• Unscrewing a nut tightly fitted to a bolt is easier with a wrench having a long arm, as less force is needed to produce the required turning effect.
• A potter's wheel is pivoted at the centre and is turned with a stick applied at the outer rim. Similarly, a hand flour grinder (chakki) is moved by applying force at its handle.
• A driver turns the steering wheel of a car by applying force at its rim.
• It is easier to open the lid of a container with a spoon rather than a coin.

Some physical examples of pressure are as follows.

• When a person dives deep into a pool, the ears start hurting, because the pressure of water increases towards the bottom.
• A sharp knife cuts fruits easily, as its thin edge makes a small area of contact and produces a very high pressure.
• Shopping bags and carry bags have wide straps to increase the area of contact and reduce the pressure exerted on the hands and shoulders.
• Nails, awl pins and thumb pins have pointed tips. Their small area produces a very high pressure that enables them to pierce a surface.
• Porters and construction workers place a round piece of cloth (turban) on their heads while carrying heavy loads, to increase the area of contact and reduce the pressure on the head.

A manometer is a simple device used to measure the pressure exerted by a liquid. It consists of a U-shaped tube filled with some liquid (water or mercury). One end of the tube is open to air, and the other end is connected to a thistle funnel which has a rubber membrane stretched across its mouth.

When the funnel is placed in the liquid whose pressure is to be measured, the levels of liquid in the two arms of the tube change. The difference in the levels of liquid in the two arms is a measure of the pressure exerted by the liquid on the rubber membrane. The greater this difference, the greater is the pressure exerted.

The pressure exerted by liquids on the walls of the container is called lateral pressure.

The factors affecting liquid pressure are as follows.

• Height of the liquid column — the pressure at a point inside a liquid increases with the height of the liquid column above it.
• Density of the liquid — the pressure is directly proportional to the density of the liquid. The greater the density, the more the pressure exerted.

Some applications of atmospheric pressure are as follows.

• Ink rises in the tube of a fountain pen due to the pressure of the atmosphere.
• We can drink liquids through a straw. Sucking lowers the pressure inside the straw, and the atmospheric pressure outside pushes the liquid up.
• Rubber suction pads stick to a wall. When the pad is pressed, the air between the wall and the pad is expelled, lowering the pressure there, and the atmospheric pressure presses the pad firmly against the wall.
• At high altitudes, the atmospheric pressure is low while the blood pressure inside the body remains the same, so blood may ooze out of the nose (nose bleeding).
• When the piston of a syringe is pulled out, the pressure inside it decreases, and the atmospheric pressure forces the liquid to enter the syringe.

1. The iron rod is used as a lever. A longer rod gives a greater perpendicular distance from the pivot, which increases the moment of force (turning effect). So heavy stone slabs can be moved with less effort.

2. Broad shoes have a larger area of contact. Since pressure is inversely proportional to area, a larger area reduces the pressure on the soft muddy ground. So the feet do not sink into it.

3. Pressure in a liquid increases with depth. The bottom pipe is at a greater depth, so the water there is under greater pressure and flows out more strongly than from the top pipe.

4. Gases exert pressure in all directions and spread quickly in all directions. So a leaked gas spreads fast everywhere, which makes the leak dangerous.

5. The turning effect of force (moment of force) explains why the workers use the iron rod as a lever.

1. A push or pull acting on a body which tends to change its state of rest or motion, or its direction of motion, or its shape or size, is called force.

2. The SI unit of force is the newton (N).

3. Gravity is an attractive force that draws two objects together. Its strength increases with the masses of the two objects and decreases with the square of the distance between them.

The second boy opens the door easily because he applies force at the edge of the door, far from the hinges (pivot), producing a greater turning effect. Since moment of force, M = F × d, a larger distance (d) gives a greater moment of force. The first boy applies force near the hinges, where d is very small, so the turning effect is small and the door hardly moves.

Heating increases the kinetic energy of the air molecules inside the balloon. As a result, they collide more frequently and forcefully with the balloon walls, increasing the gas pressure. When the pressure exceeds the elastic limit of the balloon, it bursts. The balloon kept in the cool room remains at lower pressure, so it does not burst.

Given,

Force(F) = 200 N

Area (A) = 0.5 m²

$$\begin{aligned}P = \frac{F}{A} \\[1em] = \frac{200}{0.5} \\[1em] = 400 \text{ N m}^{-2}\end{aligned}$$

⇒ P = 400 Pa

So, the pressure exerted is 400 Pa.

Given,

Force(F) = 50 N

Distance (d) = 0.3 m

Moment of force (M) = Force × Distance

= 50 × 0.3

= 15 Nm

So, the moment of force is 15 Nm.

Steps

• Place a brick on the ground that is to be lifted.
• Take a strong stick and rest it over a small support (such as a stone), which acts as the pivot, with one end of the stick under the edge of the brick.
• First, press down on the free end of the stick at the point farthest from the pivot, and lift the brick.
• Now press the stick at a point closer to the pivot using the same effort.

Observation The brick is lifted much more easily when force is applied at the far end of the stick (greater distance from the pivot) than when force is applied closer to the pivot. This shows that the turning effect of force increases as the distance from the pivot increases.

Poster title: Pressure in Everyday Life

• Sharp knives — A sharp knife has a thin edge with a small area of contact, so the same force produces a very high pressure that cuts easily.
• Tractors — Tractors have broad tyres which increase the area of contact and reduce the pressure on the ground, so they do not sink into soft soil.
• Diving suits — Deep-sea divers wear special suits to protect themselves from the large water pressure, which increases with depth.

1. A push or a pull acting on an object is called force.

2. The force applied by Sahil alone was less than the force needed to move the heavy cupboard. So he could not move it on his own.

3. When two people apply force in the same direction, the forces add up. So the net (total) force increases, and the object can be moved more easily.

4. When the door is pushed at its edge (farthest from the hinges), it swings open easily because of a greater turning effect. When it is pushed at its centre (nearer the hinges), the turning effect is smaller, so it hardly moves.

5. The turning effect of force is called the moment of force.

1. When the force remains constant, pressure is inversely proportional to the area of contact. As the area decreases, the pressure increases, and as the area increases, the pressure decreases.

2. The tip of the pencil applies more pressure. This is because the tip has a much smaller area of contact, and the same force acting over a smaller area produces a greater pressure.

3. The force acting per unit area is called pressure.

4. Decreasing the contact area increases the pressure (for the same force).

5. A sharp knife has a thin edge with a small area of contact, so the same force produces a very high pressure and the object is cut easily. A blunt knife has a larger area of contact and so exerts less pressure.

1. The lowest hole had the greatest water pressure. This is because pressure in a liquid increases with depth.

2. From this activity, we can conclude that the pressure exerted by a liquid increases with depth.

3. When a balloon is inflated, it expands (grows in size) equally in all directions.

4. The balloon experiment shows that gases also exert pressure, and they exert it equally in all directions.

5. A manometer is used to measure the pressure in liquids.

1. As depth increases, the pressure in a liquid increases.

2. Deep-sea divers wear special suits to protect themselves from the large water (liquid) pressure, which increases with depth in the deep ocean.

3. As we go higher above the ground, the atmospheric (air) pressure decreases with altitude.

4. Knowing that water pressure increases with depth, engineers design submarines with strong and thick walls (hull) that can withstand the large pressure deep underwater.

5. Thrust is the force that acts perpendicular (normal) to a surface, whereas force is any push or pull that may act in any direction.

1. The product of force and the perpendicular distance from the pivot point is called moment of force.
2. The turning effect of a force about a fixed point is called moment of force.
3. The SI unit of pressure is pascal (Pa).
4. The force acting perpendicular to the surface of a body can be defined as thrust.
5. Atmospheric pressure at sea level is approximately 101325 pascal.

1. True

2. False
   Corrected Statement — Moment of force depends on both the magnitude of force applied and the perpendicular distance from the pivot.

3. True

4. True

5. False
   Corrected Statement — Pressure increases (not decreases) with depth in a liquid.

A push or pull acting on a body which tends to change its state of rest or motion, or its direction of motion, or its shape or size, is called force.

Two of its effects are:

• It can produce motion in a stationary body or change the speed of a moving body.
• It can change the shape and size of a body.

The force exerted by the atmosphere on a unit area of the surface of the earth is called atmospheric pressure.

• Sharp knives, needles and pins have small areas of contact, so they exert a high pressure that helps in cutting or piercing.
• Heavy vehicles like tractors and trucks have broad tyres, which increase the area of contact and reduce the pressure on the ground.

Wide tyres have a large area of contact with the ground. A larger area reduces the pressure exerted by the tractor on the ground. Due to this lower pressure, the tractor does not sink into the soft ground.

When a force is applied on a body that is pivoted (fixed) at a point, the body turns about that point instead of moving in a line. This is called the turning effect of force, and it is measured by the moment of force, M = F × d.

For example, a door is fixed at one end with hinges. When we push it at its edge (away from the hinges), it turns about the hinges and opens easily, because the large distance from the pivot gives a greater turning effect.

Thrust is the force acting perpendicular (normal) to a surface, and pressure is the thrust acting per unit area.

If a thrust F acts perpendicular to a surface of area A, then

$\text{P} = \frac{\text{F}}{\text{A}}$

That is, Pressure = $\frac{\text{Thrust}}{\text{Area}}$

Thrust = Pressure × Area.

This shows that for a given thrust, pressure is more on a smaller area and less on a larger area.

Aim: To confirm the presence of atmospheric pressure.

Materials required: A cardboard, a glass and water.

Procedure

• Fill the glass with water up to the brim, so that there is no air gap between the water level and the brim of the glass.
• Cover the glass with the cardboard piece and press one palm tightly against the cardboard.
• Invert the glass quickly and then gently remove the hand below the cardboard.

Observation: The cardboard remains in its place and does not fall down, and the water does not spill.

Conclusion: Some force acts on the cardboard from below, strong enough to hold the weight of the water above it. This upward force is produced by atmospheric pressure. Hence, atmospheric pressure exists around us.

A manometer is a simple device used to measure the pressure exerted by a liquid. It consists of a U-shaped tube filled with some liquid (water or mercury). One end of the tube is open to air, and the other end is connected to a thistle funnel having a rubber membrane stretched across its mouth.

To use it, the funnel is placed in the liquid whose pressure is to be measured. The levels of liquid in the two arms of the tube change, and the difference in these levels gives a measure of the pressure exerted by the liquid on the rubber membrane. The greater the difference in the levels, the greater is the pressure.

Given,

Force (F)= 200 N

Area (A) = 0.5 m²

$$\begin{aligned}\text {Pressure} = \frac{\text{Force}}{\text{Area}} \\[1em] = \frac{200}{0.5} \\[1em] = 400 \text{ N m}^{-2}\end{aligned}$$

⇒ P = 400 Pa

So, the pressure exerted is 400 Pa.