5.4.6

Upthrust

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Floating and Upthrust

A partially submerged object (an object that’s not fully in a liquid) will experience greater pressure on the bottom surface than on the top surface. This creates a resultant force upwards. We call this force upthrust.

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Upthrust = weight of liquid displaced

  • The upthrust that acts on an object is equal to the weight of the liquid that has been forced away (displaced) by that object.
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Floating

  • If the object’s weight is equal to the upthrust, then the forces balance and the object will float in the liquid.
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Sinking

  • If the object’s weight is greater than the upthrust, then the object will sink.

Depth Control of Submarine

Submarines use the balance between upthrust and weight to control their depth in water.

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Sinking

  • When a submarine wants to sink, it fills its tanks with water to increase its weight.
  • This means the submarine’s weight is greater than the upthrust and it sinks.
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Rising

  • When a submarine wants to come up to the surface, it fills its tanks with compressed air to reduce its weight.
  • Weight becomes less than upthrust so the submarine rises.

Jump to other topics

1Energy

2Electricity

3Particle Model of Matter

4Atoms & Radiation

5Forces

5.1Basics of Motion

5.2Forces

5.3Effects of Forces

5.4Pressure

6Waves

7Magnetism

8Astrophysics

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