EK 1001 Physics: Q563

[Pose]

Advertisement - Members don't see this ad

A cylindrical vacuum pump with a cross-sectional area of 1 cm^2 is used to lift a mass. What is the greatest mass the vacuum can lift?

The answer given is 1kg.

The explanation is: "Vacuums don't suck. They prevent atmospheric pressure from pushing down. Thus the difference in atmospheric pressure above and below the block is equal to the force applied by atmospheric pressure over 1 cm^2. P = F/A. Thus F = 10^5 * 10^-4. 10N will lift 1 kg."

Whaaaaat?

 

[milski]

Imagine a piston in the middle of the cylinder. On one side, you have atmospheric pressure, on the other side, if the pump is ideal, the pressure is 0. The force acting from the 0 pressure side in F=P.a=0.a=0 N, on the atmospheric pressure side F=P.a=10000 Pa*(0.01)^2=10000*0.0001=10N which is good enough to lift a tiny bit more than 1 kg.

 

[sdm33]

Lets break it down one thing at a time
Vacuums don't suck is like saying physics never suck, I know it sound funny, but what they are saying here is that if you sucking juice via a straw , you are not sucking the juice, instead it is the atmospheric pressure on the surface of the liquid pushing it to the straw to your mouth , now imagine if you are blowing in the straw the fluid is going to pushing the atmospheric pressure.

Review Pascal's law - pressure applied anywhere to a body of fluid causes a force to be transmitted equally in all directions; the force acts at right angles to any surface in contact with the fluid; "the hydraulic press is an application of Pascal's law"

Now for P = F / A ( F = 10^5 pa) Area = we you convert cm to m , make sure you square it as well
F = P x A = 10^5 * 10^-4. = 10 N ( remember they are asking for the mass) F= ma so m = 1kg

 

[premed1001]

Imagine a piston in the middle of the cylinder. On one side, you have atmospheric pressure, on the other side, if the pump is ideal, the pressure is 0. The force acting from the 0 pressure side in F=P.a=0.a=0 N, on the atmospheric pressure side F=P.a=10000 Pa*(0.01)^2=10000*0.0001=10N which is good enough to lift a tiny bit more than 1 kg.

i dont understand that part. isnt the^2 meaning area cm? as a unit of area? why are you actually squaring the value? wouldnt you simply convert 1 cm^2 to .01 m^2?

 

[milski]

i dont understand that part. isnt the^2 meaning area cm? as a unit of area? why are you actually squaring the value? wouldnt you simply convert 1 cm^2 to .01 m^2?

^2 would be area or meters or cm squared, not cubed.
You need to square it to do the unit conversion correctly. A square meter has 10,000 square centimeters, not a 100.

 

[premed1001]

^2 would be area or meters or cm squared, not cubed.
You need to square it to do the unit conversion correctly. A square meter has 10,000 square centimeters, not a 100.

ok so in the equation p=F/A
the area is in units of cubic meters?

 

[milski]

ok so in the equation p=F/A
the area is in units of cubic meters?

The area can never be in cubic anything, it will always be in square meters/feet/cm, etc. Anything cubed will be volume.

 

[premed1001]

nevermind thanks!

 

[milski]

so then why do they use .0001 instead of .01?

A linear meter has a hundred centimeters in it. Now picture a square meter - that is a square which has a side of one meter. Since each side has a hundred little centimeters, they form a 100 x 100 grid of little squares and 1 m^2 = 100 x 100 cm^2 = 10,000 cm^2. Thus, 1 cm^2 is 1/10,000 or 0.0001 m^2.

 

[premed1001]

A linear meter has a hundred centimeters in it. Now picture a square meter - that is a square which has a side of one meter. Since each side has a hundred little square centimeters, they form a 100 x 100 grid of little squares and 1 m^2 = 100 x 100 cm^2 = 10,000 cm^2. Thus, 1 cm^2 is 1/10,000 or 0.0001 m^2.

thankyou milski you are the best!