AQA · GCSE · Physics
Course version: 8463 · Version from Sept 2016
Pressure in a fluid 2 (HT only): This creates a resultant force upwards.
The downward push on the top face and the upward push on the bottom face do not cancel, because the bottom lies deeper and the pressure there is greater.
Checked against the AQA 8463 specification (opens in a new tab) on .
- Point 4 of 6
- 2 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The downward push on the top face and the upward push on the bottom face do not cancel, because the bottom lies deeper and the pressure there is greater. Subtracting one from the other leaves a resultant that points straight up. Its size is the pressure difference multiplied by the area of the face, and that works out to be the weight of the fluid the object has pushed out of the way. This is why a stone feels lighter under water and why a boat with a large submerged volume can support a heavy load. The upward resultant does not depend on what the object is made of, only on the volume submerged and the density of the fluid, so a steel block and a wooden block of the same size receive exactly the same upward push.
What earns the marks
- The upward force on the base exceeding the downward force on the top.
- The resultant of those two forces acting upwards.
- Its size found from the pressure difference multiplied by the area.
- Linking that resultant to the weight of the fluid displaced.
Common mistakes and how to put them right
Saying the water only pushes upwards, ignoring the downward force on the top surface.
Assuming the upward resultant depends on the material the object is made of.
Calculating the resultant from the pressure at the bottom alone.
Confusing the resultant of the fluid forces with the overall resultant including the weight.
Exam tips
- Two forces in, one resultant out; show the subtraction in your working.
- Keep this force separate from the weight, because the question usually asks only about the fluid forces.
Try the lesson
The start of the Pressure in a fluid 2 (HT only): This creates a resultant force upwards. lesson, made from checked teaching notes for this specification point.
Have a go first
A submerged object has a greater water pressure on its bottom surface than on its top surface. What is the direction of the resultant force caused by this pressure difference?
Listen
The downward push on the top face and the upward push on the bottom face do not cancel, because the bottom lies deeper and the pressure there is greater. Subtracting one from the other leaves a resultant that points straight up. Its size is the pressure difference multiplied by the area of the face, and that works out to be the weight of the fluid the object has pushed out of the way. This is why a stone feels lighter under water and why a boat with a large submerged volume can support a heavy load. The upward resultant does not depend on what the object is made of, only on the volume submerged and the density of the fluid, so a steel block and a wooden block of the same size receive exactly the same upward push. A strong answer earns marks for these points. the upward force on the base exceeding the downward force on the top. the resultant of those two forces acting upwards. its size found from the pressure difference multiplied by the area. linking that resultant to the weight of the fluid displaced. Exam tip: Two forces in, one resultant out; show the subtraction in your working.
See it
Key words
Say what each word means before you turn the card.
pressure difference
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displaced fluid
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See every definition
- pressure difference
- The difference between the greater pressure at the bottom and the pressure at the top, which multiplied by the area of the face gives the size of the upward resultant.
- displaced fluid
- The fluid an object has pushed out of the way, whose weight equals the size of the upward resultant.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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