Contents
Key Stage 3
Meaning
Upthrust is a force that acts upwards in a fluid due to the fluid being displaced.
About Upthrust
- Upthrust is a force so it is measured in Newtons.
- The force of upthrust is equal to the weight of fluid that has been displaced by an object.
- An object in the water experiences more upthrust than an object in air because water is more dense than air.
- Upthrust is weak in the air because the air is not very dense. For most objects in air upthrust can be ignored because it is so small.
- Upthrust is caused by the pressure in a fluid. Pressure increases with depth so the bottom of an object will experience more pressure than the top. This difference in pressure results in a difference between the force on the bottom and the top, known as upthrust.
Examples
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| A boat floats on the water because the weight is balanced by the upthrust. | A submarine can change the amount of upthrust in order to rise or sink in the water. |
Key Stage 4 Foundation
Meaning
Upthrust is a force that acts upwards in a fluid due to the fluid being displaced.
About Upthrust
- Upthrust is a force so it is measured in Newtons.
- The force of upthrust is equal to the weight of fluid that has been displaced by an object.
- An object in the water experiences more upthrust than an object in air because water is more dense than air.
- Upthrust is weak in the air because the air is not very dense. For most objects in air upthrust can be ignored because it is so small.
- Upthrust is caused by the pressure in a fluid. Pressure increases with depth so the bottom of an object will experience more pressure than the top. This difference in pressure results in a difference between the force on the bottom and the top, known as upthrust.
Key Stage 4 Higher
Calculating Upthrust
NB: You don not need to remember the following equation.
Pressure = (Density of the fluid) x (Gravitational Field Strength) x (Depth)
\( P = \rho g h\)
Where\[ P\] = Pressure caused by the fluid.
\(g\) = Gravitational Field Strength
\(h\) = Depth of the object in the fluid.
NB: You should know the following equation.
Upthrust = (Difference in pressure between top and bottom) x (Cross sectional area perpendicular to depth)
\( F_u = \Delta P A\)
Where\[ F_u\] = The force of upthrust.
\( \Delta P\) = Pressure difference between the top and bottom of the object.
\(A\) = Cross sectional area of the object perpendicular to the height.
Example Calculations
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| A cube of 1m sides is submersed in water which has a density of 1000kg/m3 on Earth with a gravitational field strength of 9.8N/kg. | A cube of 20m sides is submersed in water which has a density of 1000kg/m3 on Earth with a gravitational field strength of 9.8N/kg. | A cube of 4m sides is submersed in water which has a density of 1000kg/m3 on Earth with a gravitational field strength of 9.8N/kg. |
Examples
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| A boat floats on the water because the weight is balanced by the upthrust. | A submarine can change the amount of upthrust in order to rise or sink in the water. |