# Pressure

Physics Lesson 29 - Pressure in General
10:17 — by Seth O'Day

Tip: This isn't the place to ask a question because the teacher can't reply.

## Key Questions

• Pressure is the force acting on an unit area ,

• It can be given as a formula, it's

.

$F$ = Net force acting on the body .
$A$= Area which is force is exerted.

.

• Pressure is acting on all three states.

• Solid.
• Liquid.
• Gas.
• Below image states how area is affect for prssure,

• Pressure is defined as Force per unit area.
Thus $P r e s s u r e$$=$$\frac{F}{A}$ , where $F$ is the Force acting normally on a surface , and $A$ is the surface area on which i tis acting.

The $S . I .$ unit of Pressure is Newtons per meter squared or $N {m}^{-} 2$. This unit is also called Pascals.

• Pressure is the force divided by the area.

If somebody stand on you with both bare feet, it may not be so bad, but if they put all of their weight on a very small heel, the force is concentrated over a very small area, so the pressure would be very high.

This is why it is so easy for a needle to break the skin. The tip of the needle has a very small area, so the pressure can be very high.

A physics teacher may lay down on a bed with many, many nails, but never on just one nail. Why?

If we apply same amount of force in a sharp and a blunt knife, then sharp one cuts easier while the blunt one does not. This is a result of pressure.

#### Explanation:

For sharp knife force is applied on a small region. While for blunt knife though force is same but is being applied on a larger area. So the pressure from the sharper one is higher. When the area applying the pressure is larger, like a blunt knife, it provides a greater amount of resistance when the same amount of force is applied than the smaller area.

• ${P}_{2} = {P}_{1} + \left(\rho\right) g h$

Where ${P}_{2}$is the pressure that is at the lower point in the fluid
${P}_{1}$is the pressure at the higher point in the fluid. It could also be at the surface of the fluid. Often this is the case and you are asked to used standard air pressure which is 101325 pa.

$\rho$, (pronounced rho, my students call it curly p or fancy p) is the density of the fluid.
g is gravity, $9.8 \frac{m}{{s}^{2}}$
and h is the vertical distance from the point of ${P}_{1}$ to the point of ${P}_{2}$, this should be a positive number.

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