MYP Physics Pressure Notes

Why Does The Same Force Sometimes Cause Very Different Effects?

1. Pressure describes how concentrated a force is when it acts on a surface.
2. The effect of a force depends not only on how large the force is, but also on the size of the area over which it is applied.
3. A force acting on a small area produces a larger effect than the same force acting on a larger area.
4. Pressure helps explain everyday experiences such as cutting, piercing, and sinking.

Example

• Walking barefoot on sand feels easier than walking on sharp shingle.
• Your weight (force) is similar in both situations, but the contact area is much smaller on the sharp stones, so the pressure on your skin is larger.

Calculating Pressure

In the simplest situations, pressure is calculated using

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

where $P$ is pressure, $F$ is the force acting perpendicular (normal) to the surface, and $A$ is the contact area.

Relationship Between Pressure, Force, and Area

1. Pressure increases when a force is applied over a smaller contact area.
2. Pressure decreases when the same force is spread over a larger contact area.
3. Both force and area must be considered together to understand pressure.

Units And Conversions Matter

1. The SI unit of pressure is the pascal (Pa): $$1\ \mathrm{Pa}=1\ \mathrm{N\ m^{-2}}$$
2. In everyday contexts, you often see:
   1. kPa: $1\ \mathrm{kPa}=10^3\ \mathrm{Pa}$
   2. MPa: $1\ \mathrm{MPa}=10^6\ \mathrm{Pa}$
   3. hPa (hectopascal): $1\ \mathrm{hPa}=100\ \mathrm{Pa}$ (common in weather maps)
   4. bar: $1\ \mathrm{bar}=10^5\ \mathrm{Pa}$ (so $1\ \mathrm{mbar}=1\ \mathrm{hPa}$)

A Small Area Creates A Huge Pressure

1. When a force is concentrated onto a tiny area, the pressure can become enormous.
2. This is why sharp tools work as they create high pressure at the cutting edge.

What Is Atmospheric Pressure?