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## Physics library

### Course: Physics library>Unit 9

Lesson 1: Density and Pressure

# Pressure and Pascal's principle (part 2)

Sal finishes the calculation of work to determine the mechanical advantage in a U-shaped tube. He also explains pressure and Pascal's Principle. Created by Sal Khan.

## Want to join the conversation?

• Paradox: Take two infinitely powerful pistons and place each piston onto the ends of an indefinitely resistant cylindrical container full of liquid. What would happen to the system? Would the liquid turn to gas from all the pressure, or would the system be at a deadlock?
• that wouldn't really work, they would be at a deadlock( assuming the pistons have the same force and area). Think about it, a solid or liquid becomes a gas after it has enough energy to break the bonds between molecules. Compressing a gas may make it a solid OR liquid eventually, but compressing a solid, well the solid stays a solid, and same for liquid.
• Is this how car brakes work? One presses on the brake pedal and the pressure of the pressing is translated to all the brake pads?
• ya!when you press the break you are actually pushing the small piston and when this happens the pressure is transmitted equally and undiminished in all directions and this will lead the liquid level to rise leading to the uplift of the other piston leading to the transmit of the pressure to all the break pads
• at 6.05 it is told that balloon expands uniformly but in the real situation sometime wen we blow a balloon...just one part of it expands first i.e the right side or the left side of it and then the whole balloon starts expanding uniformly. why does this happen?
• I'm not sure, but it may be because the thickness (and thus the k-constant in N/m) of the elastic surface of the balloon is not uniform. This would make it so that it takes less effort to stretch one side than the other, making that one side expand first.
• At around the denominator is 4m when it should be 4 m^2, but I suppose it doesn't matter since we all get it anyway.
• You are right. Thank you for pointing out this mistake because it might help other confused people
• This just blowed my mind
What if we had a machine like this and in place of the piston2 u would have an elevator(or some kind of box in which people would fit in ,)
So if you apply a force on piston1 you would get double the force you applied
Wouldn't that save energy
• Good question, but the answer is NO.
Because work is force times distance. You need to push the piston twice the distance which the second one is moving upward (consider the volumes of liquid!).
By the way: The same phenomenon applies to a lever or a lifting block.
• Can some water be magnetic? I'm just wondering.
• conservation of energy tells us that the work we put into the system is equal to the work that we get out of the system.and we also know that gas is compressible.so if we put work into gas how does it come out?
• You've misstated conservation of energy. It doesn't say anything at all about work in and work out. If you do work on a gas, the gas can just get hotter.
• This concept seems to make sense to me, except when i think about a hose. Specifically a hose, with a nozzle, or tapered end where the water comes out. The area is smaller than the input side. So if the pressure going in (larger area) is equal to the pressure coming out (smaller area) then the force of the water coming out is less than the force of the water pushing in. This seems extremely counterintuitive to me. Can someone please clarify this for me? Thanks for the help!
• P1= P2=F1/A1=F2/A2
F1=hose force
A1=hose area
F2=nozzle force
A2=nozzle area
A2< A1
Therefore, F2< F1. The force at the nozzle is less than the force in the hose.

Momentum is what is increasing at the nozzle. Momentum is mass x velocity. Momentum can change by varying the mass or velocity, or both. The nozzle example increases the momentum by increasing the velocity of the fluid. Consider boxing: a heavyweight has more mass than a lightweight and can transfer more momentum than a lightweight, given the same force and distance of the punch.