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10 Common Problems of Pressure - Junior Physics

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 m2

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/s2 and the density of water 1000 kg/m3!



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/m3

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 cm2 and the wide piston area is 50 cm2. If the weight of the block w = 1800 N, find the magnitude of force F to lift up the block W!

Answer
F1/A1 = F2/A2
F / 5 = 1800 / 50
F = (1800/50) x 5 = 180 Newton

Problem 8
A hydraulic lift with piston diameters D1 and D2 as shown below



D1 = 5 cm2 and D2 = 15 cm2. If the weight of the block w = 1800 N, find the minimum force F to lift up the weight w!

Answer
F1/(D1)2 = F2/(D2)2
F / 52 = 1800 / 152
F = 1800 x (5/15)2 = 1800 / 9 = 200 Newton

Problem 9
Argon gas with a volume of 2 m3 and the pressure of 6 atm to be pump into a vacuum tube of 8 m3of volume. What is the pressure of Argon gas in the tube?
A. 24atm
B. 14 atm
C. 4 atm
D. 1,5 atm

Answer
P1V1 = P2V2
(6 atm)(2 m3) = P2 (8 atm)
P2 = 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...

A. 1
B. 2
C. 3
D. 4

Answer
The pressure the force divided by the area:
P = F/A
The most little pressure is (2)
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