## 10 Common Problems of Pressure - Junior Physics

- Category: Junior High
- Written by fisikastudycenter

physics.fisikastudycenter.com - Learning pressure of a body, a gas and a fluid. The application of pascal law, and measuring the pressure of fluid in a tube will be discussed, junior high school physics grade 8.

**Problem 1**

A block with lenght of p = 1.5 m, width l = 1 m, height t = 0.5 m and mass m = 300 kg lays on the table.

What is the pressure at the bottom surface of the block?

**Answer**

The bottom area of the block:

A = p x l

A = 1.5 x 1 = 1.5 m^{2}

The weight of the block as a force,

F = w = m x g = 300 x 10 = 3000 Newton

So then the pressure

P = F/A = 3000/1.5 = 2000 Pascal

**Problem 2**

A jar of water with 15 cm of height. Find the pressure of water at the bottom of the jar, ignore the atmospheric pressure and use the acceleration due to gravity g = 10 m/s^{2} and the density of water 1000 kg/m^{3}!

**Answer**

The pressure at the depth of 15 cm:

P = ρ x g x h

P = 1000 x 10 x 0,15 = 1500 Pascal

**Problem 3**

A student measures the pressure of a gas in a tube using below equipment.

If the atmospheric pressure is 76 cm Hg, and h = 3 cm, find the pressure of the gas in the tube!

**Answer**

The gas pressure in the tube

P = Po − h

P = 76 cmHg − 3 cm Hg = 73 cm Hg

**Problem 4**

What is the pressure of the gas in the tube on this experiment below?

Use the atmospheric pressure Po = 76 cm Hg, and given h = 3 cm!

**Answer**

The gas pressure in the tube

P = Po + h

P = 76 cmHg + 3 cm Hg = 79 cm Hg

**Problem 5**

Given U-pipe filled with water to measure the density of a fluid that then is dropped into the right side of U-pipe.

Find the density of the fluid!

**Answer**

ρ1h1 = ρ2h2

1000 x 5 = ρ2 x 8

h2 = 5000 / 8 = 625 kg/m^{3 }

**Problem 6**

A city locates at 300 m above sea level. If the atmospheric pressure at sea level is 76 cmHg, find the air pressure in the city show in cmHg!

**Answer**

Every 100 m of height increasing, the atmospheric pressure decrease at 1 cm Hg

So then the pressure at the city A,

P = 76 cm Hg − 3 cm Hg = 73 cm Hg

**Problem 7**

Given a hydraulic lift as below!

The narrow piston area is 5 cm^{2} and the wide piston area is 50 cm^{2}. If the weight of the block w = 1800 N, find the magnitude of force F to lift up the block W!

**Answer**

F_{1}/A_{1} = F_{2}/A_{2}

F / 5 = 1800 / 50

F = (1800/50) x 5 = 180 Newton

**Problem 8**

A hydraulic lift with piston diameters D_{1} and D_{2} as shown below

D_{1} = 5 cm^{2} and D_{2} = 15 cm^{2}. If the weight of the block w = 1800 N, find the minimum force F to lift up the weight w!

**Answer**

F_{1}/(D_{1})^{2} = F_{2}/(D_{2})^{2}

F / 5^{2} = 1800 / 15^{2}

F = 1800 x (5/15)^{2} = 1800 / 9 = 200 Newton

**Problem 9**

Argon gas with a volume of 2 m^{3} and the pressure of 6 atm to be pump into a vacuum tube of 8 m^{3}of volume. What is the pressure of Argon gas in the tube?

A. 24atm

B. 14 atm

C. 4 atm

D. 1,5 atm

**Answer**

P_{1}V_{1} = P_{2}V_{2}

(6 atm)(2 m^{3}) = P_{2} (8 atm)

P_{2} = 12 / 8 = 1,5 atm

**Problem 10**

Given four bodies with each mass is 100 kg.

The most little pressure at the bottom surface of the bodies is...

B. 2

C. 3

D. 4

**Answer**

The pressure the force divided by the area:

P =

^{F}/

_{A}

The most little pressure is (2)