Thrust and Pressure: Definition, Difference, Formula

Thrust and Pressure are the types of forces that can be applied to a particular object. The key difference between the two is the direction of force applied. A force can be exerted in any direction to a surface. Thrust is the force applied to a surface in a direction that is normal or perpendicular to the surface, whereas pressure is the thrust or force applied to a unit area of surface. Thrust and pressure are both affected by the earth's gravity and gravitational pull. On the surface, the two terms appear to be the same and are often used interchangeably. However, if we look closely, we can see how unlike them they are.

• The majority of you have probably noticed that small vehicles, such as cars, have small and thin tyres, whilst larger vehicles, such as buses and trucks, have thick tyres.
• The thicker the tires, the heavier the vehicle.  

Key Terms: Force, Area, Pressure, Thrust, Air, Liquid, Velocity, Gravitational Acceleration

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What is Thrust and Pressure?

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Thrust is a force that acts on an object. Force can be applied to an item from a variety of directions. Thrust is the force that operates perpendicular to an item's surface, while pressure is the effect of thrust per unit area on an object. When you keep an object on a surface, it exerts a force on it. The force's effect on that surface is determined by the area of contact with it. The greater the area, the smaller the force's influence (less pressure), and the smaller the area, the greater the force's effect (more pressure).

• Thrust is the force that acts ordinarily on a surface. It is a vector quantity. 
• Pressure is a force that can be exerted in any direction on a surface. 
• When a body is placed on a surface, it produces a thrust on the surface equal to its weight. Regardless of how the body is positioned on the surface, the thrust is the same. 
• Pressure is defined as the amount of force (push) applied to a surface per unit area. It can alternatively be described as the force divided by the region across which the force acts. 
• Pressure has different effects depending on the area of the surface it affects on. The pressure effect is measured in units of force per unit area. It is a scalar quantity.

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Formula & Unit of Pressure and Thrust

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Thrust is a vector quantity whereas pressure is a scalar quantity. Let’s look at the formula and unit of both thrust and pressure.

Formula for Thrust

From the definition of thrust, we can conclude that,

Thrust = Pressure * Area

It is expressed in units of force. The newton (N) is the SI unit of thrust, while the dyne is the CGS unit. Also,

1N = 10⁵ dyne

In the MKS system, the gravitational unit of thrust is kgf, but in the CGS system, it is gf. Hence,

1 kgf = 9.8N 

1gf = 980 dyne

Formula for Pressure

If an area A is affected by a thrust F, then

Pressure = Thrust / Area

or 

Pressure (P) = Force (F) / Area (A)

Pascal (Pa) or Newton per meter² (N/m²) is the SI unit of pressure. The pressure exerted on a surface of size 1m by a force of 1N acting naturally on it is one pascal. The CGS unit of pressure is dyne/cm2. Also,

1 N/m² = 10 dyne/cm²

There are other units of pressure such as bars and millibar. 

1 bar = 10⁵ N/m²

1 millibar = 10^-3 bar = 10² N/m²

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Factors Affecting Pressure

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Some of the major factors that affect pressure are given below.

• According to the pressure formula, it is influenced by two key factors: force and the region over which the force is delivered. When the force and pressure both rise, the area stays the same, and vice versa.
• The density of the liquid, the depth of the liquid or the height of the liquid column, and the acceleration due to gravity on the liquid are all elements that affect liquid pressure. The formula for liquid pressure, P = ρgh, explains it.
• The temperature and volume of a gas have an impact on its pressure. The molecules of the gas travel at a high velocity as the temperature rises, transferring heat to other molecules by collision. As a result, the pressure of the gas rises. The kinetic energy of gas molecules grows as the capacity of the gas container shrinks, and it strikes all of the containers that increase the gas pressure.

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Pressure In a Liquid

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The surface of an object immersed in liquid exerts pressure on it. The weight of the liquid column at different heights of the liquid and the collision of liquid particles in a random motion are the causes of the pressure exerted by a liquid. A liquid exerts pressure in all directions as well as against the container's walls. It exerts pressure on the container's bottom. The following three factors have a direct impact on the pressure inside the liquid:

• The distance between the tip and the liquid's surface (h)
• The liquid's density (ρ)
• Gravitational acceleration (g)

As a result, P = ρhg is the pressure inside a liquid.

The pressure inside a liquid is independent of the container's shape and size, as well as the surface area on which it acts.

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Pressure Exerted By Air

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The pressure of air is exerted in all directions. The atmosphere of the earth exerts a force on all objects on the surface of the planet. Atmospheric pressure is the force exerted by the air above the earth's surface owing to gravity. With an increase in height above ground level, atmospheric pressure decreases. A device known as a barometer is used to measure it. The atmospheric pressure is measured by the height of the mercury column in a barometer. At sea level, the atmosphere exerts roughly 760mm of mercury pressure.

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Relation Between Thrust and Pressure

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The measure of force per unit area is known as pressure. It's also known as the ratio of force applied to the area on which it's applied. Thrust is the response force acting on an accelerated item due to the force applied to that object. Force simply refers to the push or pull on an object, but thrust is the reaction force acting on an accelerated object due to the force applied to that object. The magnitude of the force is also known as thrust. The S.I unit for both force and thrust is Newton (N).

The equation for the relationship between pressure and thrust is:

Pressure = Thrust / Area

We can see from the previous equation that the pressure is directly proportional to the pressure, i.e., 

Thrust ∝ Pressure

As a result, the relationship between thrust and pressure is that the thrust is directly proportional to the pressure.

Examples of Thrust

Some of the real life examples of thrust are:

• A fish being thrown out of the water by a big wave is an example of push.

• It is simple to carry or walk in high heels, causing thrust to act here (a perpendicular force).

• A pistole engine has the ability to generate thrust on its own.

• The thrust of a vehicle is the force produced by the engine that propels the vehicle forward.

• Thrust is the unseen yet extremely powerful force that propels an aeroplane forward. Every aeroplane has at least one engine. All aeroplanes rely on push to go in the air, from small privately owned and operated propeller planes to massive jet-powered commercial and military jets. The engines (or engines) produce it, which allows the plane to move forward.

Examples of Pressure

Some of the real life examples of thrust are:

• The pointed points of nails or pins are used to apply a lot of pressure to the pointed ends, allowing them to be driven in with little effort.

• The cutting tools also have either sharp or pointed edges, thus a smaller push can result in more pressure at the edges, allowing for easier cutting.

• You've noticed that while sipping beverages using a straw, you're sucking the air out of the straw. This happens because you're applying pressure while sipping your beverage.

• Hammering a sharp pin is easier than hammering a dull pin. This is because the area at the sharp pin's end is less than the area at the blunt pin's end. This causes an increase in pressure, making it easier to pound the sharp pin.

• Your feet sink into loose sand if you stand on it, but your body does not sink into it if you lie down on it. The thrust exerted on the sand is the same in both situations. However, when you lie on the sand, the thrust is distributed over a broad area, and when you stand, the force is distributed over a small area. This happens due to the pressure.

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Applications

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Applications of Thrust

Some of the real life applications of thrust are:

• Thrust is the force that propels an aeroplane through the air. Through some form of propulsion system, thrust is generated by the aircraft's engines.

• Thrust is used to help an aeroplane overcome its drag and a rocket overcome its weight.
• Propeller engines are used in certain planes, whereas jet engines are used in others. A turbine blade spins in propeller engines, whereas a combustion-powered propeller spins in jet engines. Regardless, all engines provide thrust, which propels aeroplanes through the air. We need to look at Newton's Third Law of Motion to properly understand thrust which states that “Every action has an equal and opposite reaction”. When the engines of an aeroplane are turned on, they produce "action." This action is met by movement, which propels the plane ahead.

Applications of Pressure

Some of the real life applications of pressure are:

• Bags and luggage have broad handles for added comfort. The area covered by broad handles is quite large. As a result, the pressure placed on the hands and shoulders when carrying the bags and suitcases would be minimal.

• More than four tyres are used on trucks carrying big loads. In the case of trucks, having more tyres increases the area of contact with the road. As a result, the pressure on the tires is lessened.

• The air is forced out by squeezing the rubber top on the dropper. When you dip the dropper tip into the liquid and stop pressing the top, the low pressure inside the dropper flows out and fills the void. Because the pressure inside the tube is lower than the pressure outside, the ink stays in the dropper.

• The railway tracks are separated from the ground by wide wooden sleepers, which reduce the pressure exerted by the rails.

• Building foundations are designed broader than the walls to reduce the pressure produced by the structure on the ground.

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Difference Between Thrust and Pressure

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Tabulated below are the major differences between Thrust and Pressure:

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Solved Examples

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Example 1: The given mass of the box kept on the ground is 100kg. Calculate the thrust of the box kept on the ground.

Solution: Given,

Mass of the box kept on the ground = 100 kg

g = 9.8 m/s2

The thrust exerted by the box on the ground in this situation will be equal to the box's weight.

As a result, the thrust of the box will be 

T = mg = 100 * 9.8 = 980 N

Example 2: A 60-kilogram girl wearing high-heeled shoes balances herself on a single heel. The heel is 1.5 cm in diameter and is circular. Calculate the amount of pressure the heel exerts on the horizontal floor.

Solution: Given,

Mass, m = 60 kg

Radius, r = D/2 = (1.5 * 10-2) / 2 m

Pressure = Force / Area

Force, F = mg and Area = πr2

P = mg/πr2 = (60 * 9.8) / (3.142 * (0.75 * 10-2))2 = 3.32 * 106 Pa

Example 3: A tank filled with water can reach a height of up to 1 metre. Determine the pressure applied on the tank's bottom. Given density of water = 1000kg/m3 and g = 9.8 m/s2.

Solution: Given,

g = 9.8 m/s2

Density = 1000 kg / m3,

Pressure is formulated as P = ρ × g × h

P = 1000 × 9.8 × 1 = 9800 Pa.

Example 4: Calculate the pressure produced on the ground by an item weighing 2000 N. The object's surface that is in contact with the ground has a surface area of 4 m2.

Solution: Given,

 the force exerted on the ground by the weight of the object = 2000 N

Surface area of the object = 4 m2

Pressure = Force / Area

Pressure = 2000 / 4 = 500 N/m2

Hence the pressure produced on the ground is 500 N/m2.

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Things to Remember

• Pressure is the thrust or force applied to a unit area of surface, whereas thrust is the force applied to a surface in a direction that is normal or perpendicular to the surface.
• When a body is placed on a surface, it creates a thrust equal to its weight on the surface. Pressure has a less effect when applied to a big region, but a larger effect when applied to a tiny area.
• Pressure is a scalar quantity measured in newton per meter2, whereas thrust is a vector quantity measured in newton.
• The newton (N) is the SI unit of thrust while the pascal (Pa) or newton per meter2 (N/m2) is the SI unit of pressure.
• The relationship between thrust and pressure is that the thrust is directly proportional to the pressure.

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