Water in straw, then cut straw

  • Thread starter Thread starter platina
  • Start date Start date
  • Tags Tags
    Cut Water
AI Thread Summary
When a straw filled with water is lifted with a finger covering the top, the water remains inside due to air pressure differences. Gravity causes a slight decrease in the water level as the air pressure above the straw lowers. If the straw is cut while submerged, the remaining water may rise higher in the shorter straw to balance the new air pressure with the reduced water weight. This occurs because the external atmospheric pressure pushes up on the lighter column of water, potentially raising it further. The discussion highlights the relationship between air pressure, gravity, and the behavior of liquids in confined spaces.
platina
Messages
21
Reaction score
0
A straw is in a glass of water. I then cover the top of the straw with my finger and lift the straw out of the water. The water stays in the straw.

Is this correct?...

The water in the straw falls ever so little because gravity pulls it down. The air pressure above the straw decreases as a result, until the combination of that lowered pressure and the pressure of the water in the straw equal the atmospheric pressure.

If I were to then cut the straw somewhere in the liquid region so now the straw was shorter, and so there was now less water than before...

Would the water that remained in the straw be pushed up higher into the remaining segment of straw...in order to increase the air pressure above the water? So that the combination of the new air pressure inside the straw and the pressure of the decreased amount of water equalled the atmospheric pressure?
 
Physics news on Phys.org
If I read this correctly, you are reducing the weight of water that has to be supported by the difference in pressure (and so force on the cross-section area of the straw). Since the outside air is pushing upward on a lower weight of water, it should push it farther up the straw until the pressure from the 'compressed' air in the upper part of the straw balances the upward pressure
 

Thread '1:1 versus 2:1 Mechanical Advantage debate'

The rope is tied into the person (the load of 200 pounds) and the rope goes up from the person to a fixed pulley and back down to his hands. He hauls the rope to suspend himself in the air. What is the mechanical advantage of the system? The person will indeed only have to lift half of his body weight (roughly 100 pounds) because he now lessened the load by that same amount. This APPEARS to be a 2:1 because he can hold himself with half the force, but my question is: is that mechanical...
View full post »

Thread 'Newton's first law?'

Some physics textbook writer told me that Newton's first law applies only on bodies that feel no interactions at all. He said that if a body is on rest or moves in constant velocity, there is no external force acting on it. But I have heard another form of the law that says the net force acting on a body must be zero. This means there is interactions involved after all. So which one is correct?
View full post »
Thread 'Beam on an inclined plane'

Thread 'Beam on an inclined plane'

Hello! I have a question regarding a beam on an inclined plane. I was considering a beam resting on two supports attached to an inclined plane. I was almost sure that the lower support must be more loaded. My imagination about this problem is shown in the picture below. Here is how I wrote the condition of equilibrium forces: $$ \begin{cases} F_{g\parallel}=F_{t1}+F_{t2}, \\ F_{g\perp}=F_{r1}+F_{r2} \end{cases}. $$ On the other hand...
View full post »

Similar threads

I What Keeps Water in a Straw When It's Removed from a Glass?
Replies
12
Views
5K
Question about pressure inside closed flask
Replies
7
Views
4K
Understanding the Water Pump: Gravity and Atmospheric Pressure
Replies
2
Views
5K
Height of Water in Straw vs. Time
Replies
3
Views
2K
Working mechanism of straws and siphon
Replies
1
Views
731
Make 2 launchers that shoot a straw with clay at the end
Replies
3
Views
1K
Does carbonation cause straws to float in water?
Replies
2
Views
4K
Why Is There a Limit to Drinking Water Through a Straw?
Replies
11
Views
2K
Pressure of liquid and evapouration of water
Replies
3
Views
2K
Water pressure affects air pressure
Replies
9
Views
3K

Hot Threads

Recent Insights

Back
Top