Shanghai Textbook Edition - Science for Grade 8, Semester 1: The Balance of Efficiency: Decoding the Pressure Formula and Wisdom of Increase and Decrease in Daily Life

The Essence of Efficiency: Density of Force Distribution

In physics,pressure (Pressure) is not solely determined by how much force you apply, but by how that force is 'distributed.' Pressure is a physical quantity describing the effect of force (degree of deformation or compression), and its essence isthe density of force distribution. The force acting perpendicularly on a surface is calledforce.

Quantitative Definition of Pressure

Pressure equals the force applied per unit area on an object. It is defined by the formula:
$$P = \frac{F}{S}$$
with units ofPascal (Pascal, Pa), where 1 Pa = 1 N/m². This formula reveals a strategic balance: by changing the area of force application $S$, humans can artificially amplify or reduce the 'destructive power' or 'supporting strength' of pressure.

Logic of the Wisdom of Increase and Decrease

Increase Pressure: When the applied force $F$ is constant, reduce the area of force application $S$. For example: hypodermic needles used by doctors, kitchen knives. This facilitates penetration and cutting.
Decrease Pressure: When the applied force $F$ is constant, increase the area of force application $S$. For example: skis (Photo 3-1-3), tank treads. This aids load-bearing and protection, preventing sinking into soft surfaces.

Professor's Insight

Pressure is not a rigid formula—it’s wisdom about 'distribution.' With limited resources (force), by adjusting the point of contact (area), we can create razor-sharp precision capable of piercing through anything, or achieve profound stability capable of bearing the weight of the world.

QUESTION 1

Take a look at your kitchen knife—why do you need to sharpen it on a whetstone after some time?

To reduce resistance and make the blade smoother.

To reduce the area of force application $S$, thereby increasing pressure $P$.

To increase the knife’s weight, thus increasing the force $F$.

To increase the area of force application $S$, thereby reducing pressure $P$.

QUESTION 2

Of the following four situations, which one is designed to 'increase pressure'?

Wearing a thimble on your finger while sewing

Laying railway tracks on sleepers

Increasing the width of backpack straps

Driving a nail into wood with a hammer (using the nail tip)

QUESTION 3

Use a tape measure to estimate: approximately how many times greater is the pressure you exert on the ground when standing compared to lying down?

About 1 times (same pressure)

About 5–10 times

About 20–50 times

About 500 times

QUESTION 4

Large trucks typically have more and wider wheels than small cars—the underlying physical principle is:

Increasing the number of wheels to increase friction

Increasing the number of wheels to distribute the massive pressure, reducing pressure on the ground

Reducing the contact area between wheels and the ground

For aesthetics and load balancing

QUESTION 5

When walking on deep snow, what measures should you take to move freely without sinking in?

Walking on your tiptoes

Running quickly across

Wearing wide skis or snowshoes

Carrying heavy loads to increase stability