Pressure in a Liquid (AQA GCSE Physics): Revision Notes
📚 Revision Notes
5.5.3 Pressure in a Liquid
Pressure in a Liquid (Physics Only)
Factors that Influence Floating and Sinking
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Example: A 1000kg boat will sink into the water until it has displaced 1000kg of water.
- Provided the boat doesn't completely submerge before it displaces this amount, it will float.
- An object floats if its weight is less than the weight of the water it displaces.
Pressure in Liquid
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Key Points:
- Varies with depth and density, leading to an upward force on a partially submerged object.
- The buoyancy force is the upward force that counteracts the weight of the floating object.
- This is equal to the weight of the fluid displaced by the object.
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Example: A ping pong ball floats on water as its density is less than the density of the water, so for the volume displaced, the weight of the equivalent amount of water is greater than the weight of the ping pong ball, so the resultant force is buoyancy, and it floats.
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Increasing depth increases the weight of the water above you, resulting in greater force felt and thus greater pressure.
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Formula
Where pressure is in pascals Pa, the height of the column in metres m, the density in kilogrammes per metre cubed kg/m³, and the gravitational field strength in newtons per kilogramme N/kg, which is normally 10.
Concepts
- Upthrust:
- A partially (or totally) submerged object experiences greater pressure on the bottom surface than on the top surface. This creates a resultant force upwards, known as upthrust.
- Earth's Atmosphere:
- A thin layer of air around the Earth. The atmosphere gets less dense with increasing altitude.
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Key Notes:
- The total weight of the air above a unit area at a certain altitude causes the pressure.
- With higher elevation, there are fewer air molecules above the unit area than the same area at lower heights, resulting in smaller weight and thus less pressure.