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Physics > Gravitation > Thrust, Pressure and Buoyancy
Gravitation

Thrust, Pressure and Buoyancy

In your schools, you must have seen bulletin boards. So when you try to fix a notice or a paper on the notice board, what you do is you try to fix the pin on the board. Here you need to apply the force to fix the pin on the board. What is the phenomenon behind this? The answer to this is thrust. Let us study about thrust in detail.

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Thrust & Pressure

Most of you must have noticed that the small vehicle like car have small and thin tyres, while the vehicle like the bus and trucks have thick tyres. It’s heavier the vehicle, the thicker are the tyres. Why is it so? This can be all explained by thrust and pressure. Thrust and pressure are both related to the gravity and gravitational pull of the earth.

Let us first try to understand what is thrust:

Thrust

The force acting on the object perpendicular to the surface is called as thrust. Thrust is a vector quantity and its unit is same as that of the force i.e is Newton.

Have you ever noticed that when you stand on the sand at the beach, your feet go deep into the sand? But on the other hand, if you lie down on the sand your body does not go that deep into the sand. Here in both the cases, the force is exerted by the body but still, its effect is different in both the cases.

Why is that so? When you stand on the loose sand, the force that is the weight of your body is acting on the area, area equal to your feet. When you lie down, in that case, the same force acts on an area equal to the area of your whole body. Here we see that effect of the thrust of the same magnitude is different on the different areas. This means the effects of thrust depends on an area it acts.

Hence the physical quantity that explains the dependence of thrust on the area is the pressure. It is thrust per unit area. The SI unit of pressure is Pascal or N/m²

P =   \( \frac{Thrust}{Area} \)

P = \( \frac{F}{A} \)

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What is Buoyancy?

There are objects that float on the surface of the water while some of them sink into the water. Why does this happen? Why do certain objects float and some objects sink? The floating or sinking of the objects is based on the concept of buoyancy.

Lets us see an example to understand this.Suppose you throw a stone in the water, the stone goes deep into the water, on the other hand, if you throw a plastic bottle in the water, you see that the bottle starts floating on the surface of the water. So whenever any object is put into the water, the object will float or sink depends on the upward force exerted by the water.

An upward force is applied by the water on any object floating on it which is called as Buoyancy. Thus, if any object is sinked inthe water it is experiencing buoyancy.

So here there are two forces acting on the stone and the plastic bottle. One is the gravitational pull which acts in downwards direction and other is the upthrust that acts in upwards direction. In this case, the density of the bottle is less, while the density of the stone is more. In case of the stone, the gravitational pull is more and so the stone sinks into the water.

Solved Question For You

Q1. Why can camels easily run in deserts but not other animals or we humans?

Ans: Every time we walk on the sand, our feet go inside the sand. In case of camels, they have broad and flat feet. Since their feet are broad the area is more. As we know when the area is more, the pressure is less and so they do not sink into the sand. So as the area is more they are able to run or walk more easily as compared to us or other animals.

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