This page covers the following topics:
1. Pressure and force
2. Pressure in a fluid
3. Archimides' principle
4. Atmospheric pressure
Pressure is defined as the effect of a force on a given area. It can be calculated by dividing the force over the area it is acting on. Pressure is measured in pascals, Pa.
Pressure is inversely proportional to the area a force is exerted on. This means that a force exerted on a lower area causes a higher pressure.
Liquids and gases are both fluids, since they can change shape and flow. Fluids exert forces on any surface perpendicular to it. These forces cause a pressure to act on the given surface.
Fluids exert pressure on objects which depends on the depth, caused by the weight of the column of liquid above the object and it acts in all directions. The pressure in liquids can be calculated by multiplying the density of a liquid, the height of a column and the gravitational field strength.
A hydraulic press is a mechanism that allows lifting objects with a higher force than is supplied just at a lower speed. This is achieved by using the fact that a pressure within a liquid is consistent and by modifying the area different values of force can be achieved. The force on the smaller piston exerts a pressure on the fluid which is trasmitted to the other piston. This pressure produces an upwards force on the greater piston, which causes the platform to be raised.
Archimides' principle states that an object either partially or fully submerged in a fluid feels an upwards force called upthurst, which is equal to the weight of the fluid displaced by the object. This force arises due to the fact that an object experiences a greater pressure from below in comparison to above. If the upthrust is greater than the weight, the object will float. If it is not, the object will sink.
Atmospheric pressure is the pressure that the air molecules exert on everything around us and it changes depending on the altitude. Since going up the atmosphere the mass of air above decreases, the pressure at higher altitude is lower.
Anna's hand has a surface area of 0.06 m². She pushes a circular button of surface area 0.0018 m² with a force of 7 N. Calculate the pressure Anna exerts on the button.
The force exerted by Anna is acting over the area of the button rather than her hand; thus, 0.0018 m² should be used in calculations.
p = F ÷ A
p = 7 N ÷ 0.0018 m² = 3890 Pa (3 s. f.)
Explain why a sharper knife is better at cutting than a blunt knife.
The force being exerted on what is being cut is spread over a smaller surface area when using a sharp knife. Since pressure is inversely proportional to the area a force is exerted on and the area is smaller for a sharper knife, the sharper knife exerts a greater pressure on what is being cut, which makes it better at cutting things than a blunt knife.
A sharper knife has a lower area which results in a higher exerted pressure.
A wooden block with a base area of 0.08 m² and a weight of 70 N is placed on the same ground as an aluminium block with a base area of 0.23 m² and a weight of 200 N. Find which block exerts a greater pressure on the ground below them.
p = F ÷ A
for the wooden block: pressure = 70 N ÷ 0.08 m² = 875 Pa
for the aluminium block: pressure = 200 N ÷ 0.23 m² = 870 Pa (3 s. f.)
Therefore, the wooden block exerts a greater pressure on the ground.
A boat toy is moved from 0.2 m below sea level to 0.45 m below it. Given that the density of sea water is 1025 kg/m³ and g = 9.8 m/s², calculate the pressure difference between the two positions.
p = ρgh
0.2 m below sea level: pressure = 1025 kg/m³ × 9.8 m/s² × 0.2 m = 2009 Pa
0.45 m below sea level, pressure = 1025 kg/m³ × 9.8 m/s² × 0.45 m = 4520.25 Pa
pressure difference = 4520.25 Pa − 2009 Pa = 2511.25 Pa
Explain how atmospheric pressure changes with an increasing distance from the Earth’s surface.
Since going up the atmosphere the mass of air above decreases, the pressure at higher altitude is lower.
lower mass of air above → lower pressure
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