Thrust on a Surface Area

NOTES:

Magnitude of pressure due to solid:

• pressure is a measurement of effect of force on a surface area.

• pressure is defined as the thrust (force) per unit area.

(Force acting perpendicular to the surface is thrust, therefore pressure is calculated for the force that is perpendicular to the area.)

Pressure = Thrust /Area

Unit: Pascal (Pa) or N/m²

Factors influencing pressure due to solid:

1. Magnitude of force

2. Contact area

• Pressure is directly proportional to magnitude of force and inversely proportional to area.

Magnitude of pressure due to fluid:

• Pressure in a liquid increases with depth below its surface as a result of the weight of the overlying fluid.

• Pressure in a liquid at a particular depth is the same in all directions.

• Pressure in a liquid increases with the density of the liquid.

Volume= base area x depth= Ah

Mass of liquid=density x volume = ρAh

Weight of liquid = mass x g = ρAhg

(take g=10N/kg)

Since weight of liquid is the force pushing down on the bottom of the container holding the liquid,

Pressure= ρAhg/A=ρhg

Pressure = h x ρ x g

Pressure due to a liquid is directly proportional to depth and density of the liquid.

Q1. (Class work): A cup of mass 0.5 kg with its base radius of 5 cm is kept on a table. Calculate the pressure exerted by the cup on the table. (g=10m/s²)

Q2. (Class work): A diver is 10 m below the surface of water in a dam. If the density of water is 1000kg/m³ , determine the pressure due to water on the diver. (g=10N/kg)

Q3. A solid of dimension 25 cm x 25 cm x 25 cm and weighing 625 N is placed on a table. Calculate the pressure exerted on the table. ( Ans: 10⁴ Pa)

Q4. The pressure of water on ground floor is 50,000 Pa and at first floor it is 20,000 Pa. Find the height of the first floor. Take the density of water =1000 kg/m³ and g = 10 m/s². (Ans: 3 m )

Hint: Pressure on the ground floor = Pressure on first floor + hρg

Q5. Kinley weighing 22kg is sitting on a chair of surface area 0.5 m2. What is the pressure exerted by Kinley on the chair? (BCSEA 2020) (Ans: 440 Pa or 431.2 Pa)

Q6. Liquid A and liquid B exert the same pressure. What would be the density of liquid A if the height of liquid B is twice the height of liquid A and density of liquid B is 250 kg/m^3? (BCSEA 2020) (Ans: 500 kg/m³ )

HINT: ρ_A x g x h_A = ρ_B x g x h_B

Q7. The pressure exerted by a lady of weight 45 kg, wearing a circular heel of area 0.025m² is ______.(BCSEA 2019)

Q8. A barrel of mass 20,000kg with its base area of 12 m³ lies on the ground. What is the pressure exerted on the surface? (Take g = 10 m/s² ) (BCSEA 2018)

1. The normal force that acts on a person sitting on a chair is known as. (bcsea:2020)

A area

B thrust

C upthrust
D pressure

2. The magnitude of same force acts on a circle of radius 2 m and rectangle of dimension 2 m x 1 m. Which of the following is CORRECT? (bcsea:2020)

A No pressure will be exerted on the circle and rectangle.

B Pressure exerted on the circle = pressure exerted on the rectangle.

C Pressure exerted on the circle > pressure exerted on the rectangle.

D Pressure exerted on the circle < pressure exerted on the rectangle.

Q3. Explain with an example to show that when the area of contact increases, pressure decreases. (BCSEA 2020)

Q4. Is it easier to swim in a sea water than in a river water? Give reasons (BCSEA 2019)

Q5. Which one of the following is NOT a factor affecting the pressure exerted on a body by a fluid? (BCSEA 2018)

A. depth of the fluid

B. density of the fluid

C. volume of the fluid

D. acceleration due to gravity

Q6. Explain why water tank is made high in a town to supply water?

Q7. Explain why the walls of a dam are made broader at the bottom.

Q8. Pressure in liquid increases as depth increases. Explain this statement with a diagram.

Fill in the blanks:

1. pressure at a point at a depth h in a liquid = __________

2. pressure is _________ in all directions about a point in a liquid.

3. pressure at a point inside the liquid is _________ to its depth.

4. thrust is the force __________ to the surface.

5. Pressure is __________ proportional to magnitude of force and ________ proportional to contact area.

6. A blunt knife exerts ___________________________ pressure than a sharp knife. (BCSEA 2018)