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Force and Pressure - Atmospheric Pressure and its Measurement

Grade 8CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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The envelope of air surrounding the Earth is known as the atmosphere, which extends up to many kilometers above the surface.

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Atmospheric pressure is defined as the pressure exerted by the weight of the air column over a unit area of the Earth's surface.

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The magnitude of atmospheric pressure is enormous; at sea level, it is approximately 101,325 Pa101,325 \text{ Pa} or 1.013×105 N/m21.013 \times 10^{5} \text{ N/m}^{2}.

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We do not feel this huge pressure because the internal pressure of our bodies (exerted by blood and other fluids) is equal to the atmospheric pressure and cancels it out from within.

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Atmospheric pressure decreases with an increase in altitude because the column of air above a point becomes shorter and less dense as we go higher.

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A simple device used to measure atmospheric pressure is called a Barometer, originally designed by Evangelista Torricelli using Mercury (HgHg).

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At sea level, the atmosphere can support a column of mercury approximately 760 mm760 \text{ mm} high.

📐Formulae

P=FAP = \frac{F}{A}

Patm≈1.013×105 PaP_{atm} \approx 1.013 \times 10^{5} \text{ Pa}

1 atm=760 mm of Hg1 \text{ atm} = 760 \text{ mm of Hg}

1 Pascal (Pa)=1 N/m21 \text{ Pascal (Pa)} = 1 \text{ N/m}^{2}

💡Examples

Problem 1:

Calculate the force exerted by the atmosphere on a rectangular rooftop of dimensions 10 m×5 m10 \text{ m} \times 5 \text{ m} at sea level (take Patm=105 PaP_{atm} = 10^{5} \text{ Pa}).

Solution:

Given: Area A=10 m×5 m=50 m2A = 10 \text{ m} \times 5 \text{ m} = 50 \text{ m}^{2}. Pressure P=105 PaP = 10^{5} \text{ Pa}. Using the formula F=P×AF = P \times A, we get F=105×50=5×106 NF = 10^{5} \times 50 = 5 \times 10^{6} \text{ N}.

Explanation:

The atmosphere exerts a downward force proportional to the surface area of the object. Even though this force is massive (55 million Newtons), the roof does not collapse because air inside the building exerts an equal upward force.

Problem 2:

Why do mountaineers sometimes suffer from nosebleeds at high altitudes?

Solution:

As altitude increases, the atmospheric pressure decreases significantly. The blood pressure inside the human body remains relatively higher than the thin outside air, causing small blood vessels (capillaries) in the nose to burst.

Explanation:

This demonstrates that Pinternal>PexternalP_{internal} > P_{external} at high altitudes, creating a pressure imbalance.

Problem 3:

A rubber sucker pressed against a smooth surface sticks to it firmly. Explain the physics behind this using pressure concepts.

Solution:

When the sucker is pressed, most of the air between the cup and the surface is forced out. The atmospheric pressure acting on the outside of the sucker is much greater than the residual air pressure inside, holding it tightly against the surface.

Explanation:

To pull the sucker off, the applied force must be large enough to overcome the atmospheric pressure (PatmP_{atm}).