# Chapter 3 – Force and Pressure

The following Topics and Sub-Topics are covered in this chapter and are available on MSVgo:

Introduction

A push or pull occurs under force. When watching sports, you might have wondered what makes a ball become fast or slow and even change directions after coming in contact with a player. The simple answer is- Force. But what determines the subsequent path that the ball follows? We will try to answer all such questions here.

While force is subjected, there are two entities engaged in the process. The first is the object employing the force, and the other is the object experiencing this force. The S.I. unit of force is Newton. Here we analyze the effects of force and the factors that determine a force’s direction and magnitude.

When several forces are applied to an object, the resultant force is calculated with its magnitude and direction. The direction of the net force determines the direction of acceleration of the item.

Some forces are magnetic force, friction force, gravitational force, buoyancy, tension force, and spring force.

#### Turning Effect of Force

The moment of force or torque on a fixed object at a point or axis is the turning effect of force. The product of force and the pivot’s vertical distance from the force’s line of action is used to calculate torque.

The moment of force is equal to the force multiplied by the pivot’s perpendicular distance from the force.

So, the moment of force = force x perpendicular distance of the object from its axis.

The S.I. unit of moment of force is Newton metre or Nm.

#### Pressure

The pressure is the force exerted on a unit area. You must have experienced that it is easier to use a sharp knife instead of a blunt knife. The use of a sharp knife is better because the sharp end of a knife has a smaller area and thus exerts more pressure. Similarly, it is easier to push a nail’s pointed side into a wall than its flat side.

So, Pressure= force/Area or P =F/A.

Not only solids but liquids and gases also exert pressure. The S.I. pressure unit is Pascal. The force of 1 Newton applied on an area with a one-metre square dimension is one Pascal.

#### Factors Affecting Pressure

We know that pressure is measured by dividing the amount of force by the area experiencing that force. So, the factors affecting pressure are –

• Force
• Area

#### The Pressure Exerted by Liquids

Just as solids, liquids also exert pressure. Their container mostly experiences the pressure exerted by liquids. This pressure arises from several factors such as gravitational force, acceleration, or the force exerted by a closed container.

The pressure in liquids is applied in all directions as all the molecules are tightly packed, and liquids are shapeless.

P = hρg or

Pressure = Depth of liquid X Density of the liquid X Gravitational force (9.81m/s).

When the depth of a fluid column increases, so does the pressure. Pressure tends to increase because, as you go to a greater depth, liquid at a lesser depth must also sustain liquid above it. To describe fluid pressure, we can say that it is the pressure that arises at a point within a liquid due to the fluid’s density or the liquid’s weight above it.

#### Upthrust

An item that is wholly or partially immersed in liquid experiences a higher pressure on its lower portion than on its upper layer, which creates the resulting upward force. This force is called buoyancy and is also known as upthrust.

#### The Pressure Exerted by Gases

All fluids exert pressure on their containers. Just like liquids, gases also exert pressure on the walls of their container. Gases are homogeneous and hence exert the same pressure in all directions.

The pressure exerted by gases is affected by temperature, gas volume, and the number of molecules. With the increase in temperature, the gases gain more energy and thus exert increased force and pressure. As the volume of the gases decreases, the pressure exerted by them increases. As the number of molecules of gas rises, the gas pressure also increases.

#### Atmospheric Pressure

The atmosphere or the air surrounding us also exerts pressure. The force exerted by a column of air on a unit area of any surface is known as the atmospheric pressure on Earth’s surface. The value of atmospheric pressure is about 1Kg/cm2 or 105 Pascal, which is very high. But we do not experience this pressure as our blood pressure counters and balances it.

The value of atmospheric pressure is maximum at sea level, and it gradually decreases with elevation or height. As we move towards hilly areas, the atmospheric pressure becomes significantly less. Working of a dropper, syringe, pump, and even straws are all examples of atmospheric pressure.

#### Conclusion

There is a direct connection between force and pressure. As force is divided amongst the area, the area is inversely proportional to pressure. So, with an increase in area, the pressure decreases provided the force remains constant.

#### FAQs

1. What is meant by thrust?
A force that is applied perpendicular to the surface is known as thrust.
2. The moment of force is a scalar quantity or a vector quantity?
The moment of force is a vector quantity as it has both magnitude and direction.
3. How does force affect pressure?
With an increase in force, Pressure increases provided that the area remains constant.
4. Which instrument measures atmospheric pressure?
The measurement of atmospheric pressure takes place with the help of an instrument called a Barometer.

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