Thrust and Pressure: Have you ever noticed how our feet get trapped in loose sand, but a camel can effortlessly move on it? Why do we feel more at ease carrying a school bag with broad straps than one with small straps? To answer these questions, we must first understand two words that appear to be extremely similar: thrust and pressure. On the surface, the two names appear to be the same and are used interchangeably. However, we can see how different they are if we look closely. In this post, we will look at examples of thrust and pressure to see how they vary.

What is Thrust and Pressure?

Thrust is a kind of force acting on an object. An object can experience force from many directions. The force which acts on an object perpendicular to its surface is called thrust, and the effect of thrust per unit area on an object is called pressure. When an object is kept on a surface, it will exert a force on it. The effect of force on that surface depends on the area of contact with the surface. The more the area, the smaller the effect of the force (less pressure), and the smaller the area, the greater the effect of the force (more pressure).

Thrust Definition

The force acting perpendicular to the surface of an object is called thrust. It is represented by the letter ‘\(T\)’. The SI unit of thrust is the same as the SI unit force. It is measured in newton \({\rm{(N)}}\). Thrust is a vector quantity because both its magnitude and direction describe it.

Thrust Examples

When an object is immersed in a liquid, it will lose some of its weight. This happens because the liquid exerts an upward force on the object, due to which the net downward force reduces. This upward force acts perpendicular to the surface of the object immersed in the liquid. It is also known as upthrust.

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When we fix a drawing pin on a display board, we apply a force perpendicular to the surface of the display board.

Pressure Definition

Pressure is defined as the thrust acting per unit area on an object. It is denoted by the letter ‘\(P\)’. Let us assume that a thrust \(T\) is acting on area \(A\), then the pressure exerted on the object will be given by
\(P = \frac{T}{A}\)
The SI unit of pressure is the pascal. It is denoted by the symbol \(‘{\rm{Pa}}’\). One pascal is the pressure exerted by a thrust of one newton acting on a surface of an area of one square meter. \({\rm{1}}\,{\rm{Pa}}\,{\rm{ = }}\,\frac{{{\rm{1}}\,{\rm{N}}}}{{{\rm{1}}\,{{\rm{m}}^{\rm{2}}}}}\)
Therefore, \(1\,\rm{Pa}\) is equal to \(1\,\rm{N}\,\rm{m}^{-2}\).

Factors Affecting Pressure

The pressure exerted on an object depends on the following factors:

1. The area of the surface in contact: The pressure on an object increases with a decrease in the surface area on which it is exerted and vice versa.
2. The magnitude of thrust: The pressure on an object is directly proportional to the thrust acting on the object. The greater the thrust, the more will be the pressure exerted on the object.

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Pressure Examples

1. If we try to cut fruit with the flat or blunt side of the knife, it will exert less pressure on the surface of the fruit, and it will be difficult to cut the fruit. However, if we use the sharper side of the knife, it will exert more pressure, and we can easily cut the fruit.
2. Camels can easily walk on the sand because they can have broad soles. They exert less pressure on the sand and make it easy for them to walk without sinking in the sand.
3. Wide concrete sleepers are kept below the railway tracks to reduce the pressure created by the trains on the ground.
4. Heavy trucks and buses have wide tyres because the larger area of tyres reduces the pressure on the road.

Pressure in a Liquid

Liquids exert pressure on the surface of an object which is immersed in it. The cause of the pressure exerted by a liquid is the weight of the liquid column at different heights of the liquid and the collision of particles of liquid in a random motion. A liquid exerts pressure in all directions and on the walls of the container in which it is kept. It exerts pressure on the bottom of the container. The pressure at a point inside the liquid depends directly on the following three factors:

1. Depth of the point from the surface of the liquid \((h)\)
2. Density of the liquid \((ρ)\)
3. Acceleration due to gravity \((g)\)

Therefore, the pressure inside a liquid, \(P = hρg\).
The pressure inside a liquid does not depend on the shape and size of the container and the surface area on which it is acting.

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Pressure Exerted by Air

Air exerts pressure in all directions. All objects on the surface of the earth experience a force exerted by the atmosphere of the earth. The thrust exerted by the air above the earth’s surface due to its gravity is called atmospheric pressure. Atmospheric pressure reduces with an increase in altitude from the ground level. It is measured by a device called a barometer. The height of the mercury column in a barometer indicates the atmospheric pressure. The pressure exerted by the atmosphere at sea level is around \(760\,\rm{mm}\) of mercury.

Difference Between Thrust and Pressure

Let us look at the difference between thrust and pressure:

Thrust	Pressure
Thrust is the force acting perpendicular to the surface of an object.	Pressure is the force acting per unit area on an object.
The SI unit of thrust is newton \({\rm{(N)}}\).	The SI unit of pressure is newton per square meter \({\rm{N}}\,{{\rm{m}}^{ – 2}}\).
Thrust does not depend on the area of the surface.	Pressure depends on the area of the surface on which it acts.

Solved Examples Based on Thrust and Pressure

Let us look at some of the solved examples based on Thrust and Pressure:

Q.1. The base area of a bottle is \(10\,\rm{cm}^2\). Water is filled in it up to a height of \(20\,\rm{cm}\). If its density be \(1000\,\rm{kg}\,\rm{m}^{-3}\). Calculate the pressure and thrust exerted by water at the bottom of the bottle. Assume that \(g = 9.8\,\rm{m}\,\rm{s}^{-2}\).
Ans: Given, the area of the bottle, \(A = 10\,{\text{c}}{{\text{m}}^2} = 10 \times {10^{ – 4}}\,{{\text{m}}^2}\)
The height up to which water is filled, \(h = 20\,\rm{cm} = 0.2\,\rm{m}\)
The density of water, \(ρ = 1000\,\rm{kg}\,\rm{m}^{-3}\)
To find the pressure, we use the formula, \(P = ρgh\).
\(P = 1000 \times 0.2 \times 9.8 = 1960\,\rm{Pa}\)
Now, the thrust exerted by the water will be given by
\(T = P \times A\)
\(T = 1960 \times 10 \times 10^{-4} = 1.96\,\rm{N}\)
Therefore, the pressure and thrust exerted by water at the bottom of the bottle are \(1960\,\rm{Pa}\) and \(1.96\,\rm{N}\), respectively.

Q.2. A box of mass \(100\,\rm{kg}\) is kept on the ground. Find the thrust exerted by the box on the ground.
Ans: Given, the mass of the box, \(m = 100\,\rm{kg}\)
The acceleration due to gravity, \(g = 9.8\,\rm{m}\,\rm{s}^{-2}\)
In this case, the thrust exerted by the box on the ground will be equal to the weight of the box.
Therefore, the thrust exerted by the box, \(T = mg\)
\(T = 100 \times 9.8 = 980\,\rm{N}\)

Q.3. Calculate the pressure exerted by an object of weight \(2000\,\rm{N}\) kept on the ground. The area of the surface of the object which is in contact with the ground is \(4\,\rm{m}^2\).
Ans: Given, the weight of the object or thrust acting on the ground, \(T = 2000\,\rm{N}\)
The area of the surface in contact with the ground, \(A = 4\,\rm{m}^2\)
The pressure exerted by the object on the ground is given by
\(P{{=}}\frac{T}{A}\)
\(P = \frac{{2000}}{4} = 500\,{\text{Pa}}\)

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Summary

We know that pressure is the thrust per unit surface and that it is important in every real-world circumstance. With broad straps on the handles, the force acting per unit area decreases when the luggage is heavy. As a result, a person carrying this luggage feels less stressed.

Similarly, the smaller surface area of the nails, or simply the pointed tips, shift a bigger force applied on them, allowing for easier wall fastening. Pressure is measured in newtons per metre square because it is force per unit area. One metre per square metre is referred to as Pascal (Pa) after the physicist Blaise Pascal. Solids apply pressure based on their weight and the surface area through which the weight operates.

From this article, we can conclude that thrust is the force that acts on an object perpendicular to its surface, whereas pressure is the effect of this force. Pressure is the force acting per unit area on an object. The pressure on an object decreases with an increase in the area of the surface in contact. We studied the thrust and pressure formulas and understood their calculation.

Thrust and Pressure – FAQs

Let us look at some of the frequently asked questions about Thrust and Pressure:

Q.1: What is the formula of thrust and pressure?
Ans: Thrust \((T)\) is the product of pressure \((P)\) and area \((A)\). The formula of thrust is given by
\(T = P \times A\)
Pressure is the thrust or force acting on a unit area of the surface of an object. The formula of pressure is given by
\(P = \frac{T}{A}\)

Q.2: What is pressure?
Ans: Pressure is defined as the thrust or force acting per unit area on an object.

Q.3: What is the symbol of thrust? Name the SI unit of thrust.
Ans: Thrust is denoted by the letter ‘\(T\)’. Its SI unit is newton \({\rm{N}}\).

Q.4: Give an example of thrust.
Ans: When a person stands on the ground, his weight acts as the thrust because it acts perpendicular to the ground.

Q.5: How can we decrease the effect of pressure?
Ans: The effect of pressure can be decreased by decreasing the magnitude of the force acting perpendicular to the surface in contact or by increasing the area of contact with the surface experiencing thrust.

Q.6: How is pressure related to the area?
Ans: The pressure exerted on an object is inversely proportional to the area of the surface on which it is acting. If thrust works on a larger area, the pressure exerted on the area reduces. However, when thrust acts on a smaller area, the pressure exerted on the area increases.

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