3 1 Understanding pressure P PRESSURE Pressure is

3. 1 Understanding pressure (P)

PRESSURE

Pressure is defined as force per unit area. Therefore, the relation between force and pressure can be expressed as F = the magnitude of the force A = the area over which it is spread.

Pressure depends on two things: 1. the Force (in Newtons) 2. the Area it's pressing on (in square metres)

UNITS FOR PRESSURE Many units are used for pressure depending on the situation but the fundamental (S. I. ) units for pressure are Nm-2, (1 Nm-2 = 1 Pa). (Pa = pascal) Non SI unit for pressure include mm Hg.

The relationship between pressure, force and area Notice that a large force might only create a small pressure if it's spread out over a wide / bigger area. Eg: 100 N/ 50 m 2 = 2 Nm-2 100 N/ 10 m 2 = 10 Nm-2

Also, a small force can create a big pressure if the area is tiny. u Eg: 2 u 20 N / 0. 4 m 2 = 50 Nm 10 N / 0. 4 m 2 = 40 Nm-2

Example An office safe has a weight of 500 N. If the area of the base is 1. 25 square metres, what is the pressure on the floor of the office? Ans. 400 Nm-2

Example A physics teacher has a weight of 800 N. If his feet have an area of 0. 025 square metres each, what pressure does he exert on the ground when he stands on one foot / both feet? Ans: on one foot = 32000 Nm-2 On two feet = 16000 Nm-2

Feel the pressure!!! PRESS THE SHARP END ONTO YOUR THUMB PRESS THE BLUNT END ONTO YOUR THUMB

Objects and Pressure

Think!!!!!!! Can you figure out why camels have large feet ? In an hydraulic car braking system, why do we make the piston down by the wheel BIGGER than the piston by the brake pedal?

Using Pressure

Using Pressure

Using Pressure When the area is small, a moderate force can create a very large pressure. This is why a sharp knife is good at cutting things: when you push the very small area of the sharp blade against something, it creates a really large pressure.

Using Pressure

Using Pressure Ice skates have sharp edges, and thus a small area in contact with the ice. This means that your weight creates a very large pressure on the ice, far more than if you were standing in ordinary shoes.

Pressure and People

Using Pressure Even a slender supermodel can damage floors by walking on then in high-heeled shoes. This is because the area of the heel is small, so you can easily create enough pressure to cause a dent in the floor. The pressure can be greater than if an elephant was standing there, even though the force is much less. So you should be able to figure out why elephants and camels have large feet.

Pressure in liquids

Pressure in liquid acts in all directions

Static Fluid Pressure The pressure exerted by a static fluid depends only upon the depth of the fluid, the density of the fluid, and the acceleration of gravity. Size and shape of the container does not influence the pressure in the liquid

Density of liquid, ρ kgm-3 Base surface area, A m 2 Depth, h m The column of liquid exerts a pressure on the base due to its weight Column of liquid h Volume of liquid = A h meter cube (m 3) Mass of liquid = A h ρ kilogram (kg) Weight of liquid = A h ρ g Newton (N) Surface area, A Pressure in liquid P = (weight of liquid) /(surface area) =Ahρg /A =hρg

Example A huge metal tank is used to store acertain type of oil. What is the pressure exerted by the oil at a depth of 3. 5 m if the density of the oil is 840 kgm -3 and g is 9. 8 Nkg-1. P=hρg = 3. 5 x 840 x 9. 8 = 28, 812 Pa

Applications of pressure in liquids Thickness of of walls of swimming pools, dams

Applications of pressure in liquids Window of submarines / aquariums

Divers suit Constructions of deepsea exploring vehicles

It is important to remember that vertical distance, or elevation, determines water pressure; physical distance from a storage tank does not. This is why customers with properties in low-lying areas experience higher water pressure than customers located on the hilltops and ridges in our service territory.

THINK !!!!!! Explain why the aluminium can crumples when it is brought to the ocean floor but, It does not crumple if a few holes are made on the can

THANK YOU! THE END Physics is simply fun!