Water in straw, then cut straw

  • Context: Undergrad 
  • Thread starter Thread starter platina
  • Start date Start date
  • Tags Tags
    Cut Water
Click For Summary
SUMMARY

The discussion centers on the physics of water retention in a straw when submerged and subsequently cut. When a straw filled with water is lifted with a finger covering the top, the water remains due to the balance of atmospheric pressure and the pressure exerted by the water. Cutting the straw while it contains water alters the pressure dynamics; the reduced weight of the water allows the remaining water to rise higher in the shorter straw, as the air pressure above the water adjusts to maintain equilibrium with atmospheric pressure.

PREREQUISITES
  • Understanding of basic fluid dynamics
  • Knowledge of atmospheric pressure principles
  • Familiarity with concepts of air pressure and buoyancy
  • Basic grasp of gravity's effect on liquids
NEXT STEPS
  • Research the principles of fluid dynamics in closed systems
  • Study the effects of atmospheric pressure on liquids
  • Explore buoyancy and its relationship with fluid weight
  • Investigate experiments demonstrating pressure changes in fluids
USEFUL FOR

Students of physics, educators explaining fluid dynamics, and anyone interested in the principles of pressure and buoyancy in liquids.

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
 

Similar threads

Undergrad What Keeps Water in a Straw When It's Removed from a Glass?
  • · Replies 12 ·
Replies
12
Views
6K
Undergrad Question about pressure inside closed flask
  • · Replies 7 ·
Replies
7
Views
5K
Graduate Height of Water in Straw vs. Time
  • · Replies 3 ·
Replies
3
Views
2K
High School Understanding the Water Pump: Gravity and Atmospheric Pressure
  • · Replies 2 ·
Replies
2
Views
5K
Undergrad Filling a pool with the hose end above or in the water
  • · Replies 11 ·
Replies
11
Views
4K
Undergrad What Forces Encourage Liquid Down a Straw?
  • · Replies 1 ·
Replies
1
Views
7K
Why Is There a Limit to Drinking Water Through a Straw?
  • · Replies 11 ·
Replies
11
Views
3K
High School Why does the liquid stay in the straw?
  • · Replies 8 ·
Replies
8
Views
60K
Undergrad Water pressure affects air pressure
  • · Replies 9 ·
Replies
9
Views
4K
High School Water held in straw by vacuum. Is this possible on grand scale?
  • · Replies 8 ·
Replies
8
Views
14K