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Factors Affecting Pressure Guide

PhysicsBeginnerReading time: 3 min

Overview

Why can a cicada easily pierce hard tree bark? Why do people sink when walking on snow, but float on it when wearing skis? These phenomena involve the effect of pressure. This experiment uses the control variable method to take you through an exploration of how the magnitude of force and the surface area jointly determine "pressure".

Background

  • In the 17th century, French scientist Blaise Pascal conducted in-depth research on pressure and discovered Pascal's law.
  • To commemorate his contributions, the unit of pressure in the International System of Units was named "Pascal", abbreviated as "Pa".
  • In 1654, the Magdeburg hemispheres experiment shocked the world, vividly demonstrating the immense power of atmospheric pressure.

Key Concepts

Force (FF)

FF

The force acting perpendicularly on a surface. It is usually represented by FF in formulas.

Area (SS)

SS

The actual area where two objects are in contact and squeezing each other. It is represented by SS in formulas.

Pressure (PP)

P=FSP = \frac{F}{S}

The ratio of the magnitude of the force to the surface area. It characterizes the effect of the force.

Formulas & Derivation

Pressure Definition

P=FSP = \frac{F}{S}
The unit of FF is Newton (N), the unit of SS is square meter (m2m^2), and the unit of PP is Pascal (Pa).

Experiment Steps

  1. 1

    Exploring the Effect of Force Magnitude

    Keep the "Surface Area (S)(S)" constant (e.g., 5cm25cm^2). Gradually increase the "Force (F)(F) ". Observe how the indentation depth hh of the sponge changes. What does this indicate?
  2. 2

    Exploring the Effect of Surface Area

    Keep the "Force (F)(F)" constant (e.g., 50N50N). Decrease the "Surface Area (S)(S) ". Predict: Will the indentation of the sponge become deeper or shallower? Verify your hypothesis.
  3. 3

    Comprehensive Analysis

    Observe the pressure PP calculated on the right panel. You will find that: the value of pressure PP directly reflects the effect of the force (indentation depth). The larger PP is, the deeper the indentation.

Learning Outcomes

  • Understand that pressure is a physical quantity representing the effect of force.
  • Master the pressure formula P=F/SP = F/S and its unit conversion.
  • Be able to use the control variable method to explain pressure phenomena in daily life.
  • Learn to decrease pressure by increasing surface area, or increase pressure by decreasing surface area.

Real-world Applications

  • Increasing Pressure: Kitchen knives are sharpened (decreasing area), needles are very fine, cicada's mouthparts.
  • Decreasing Pressure: Skis are wide (increasing area), schoolbag straps are wide, tank tracks, railway sleepers.

Common Misconceptions

Misconception
The greater the force, the greater the pressure.
Correct
Incorrect. You must also consider the surface area. A tank is much heavier than a person (larger force), but due to the large area of its tracks, its pressure on the ground may be less than that of a person wearing high heels.
Misconception
Pressure is gravity.
Correct
Incorrect. Pressure is a squeezing force, while gravity is the attraction of the Earth. The magnitude of pressure equals gravity only when an object rests on a horizontal surface and the force is applied vertically.

Further Reading

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