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# Question on pressure

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1. Suppose you have a balloon resting on the ceiling.

Force a; force of ceiling on balloon = 5
Weight = 5
Upthurst = 10

Area of balloon I contact with ceiling = 2

Calculate the Pressure?
Would you have three values of pressure depending on which force u use?
2. (Original post by ali malikzade)
Suppose you have a balloon resting on the ceiling.

Force a; force of ceiling on balloon = 5
Weight = 5
Upthurst = 10

Area of balloon I contact with ceiling = 2

Calculate the Pressure?
Would you have three values of pressure depending on which force u use?
The force of the ceiling on balloon (5) is the resultant force, because the resultant force = Upthrust-Weight. So use that Force, then to work out the pressure use the area (Pressure = Force/ Area)
3. (Original post by TheBride)
The force of the ceiling on balloon (5) is the resultant force, because the resultant force = Upthrust-Weight. So use that Force, then to work out the pressure use the area (Pressure = Force/ Area)
There is no resultant force. The two forces in the downward direction weight and force of ceiling on balloon (normal reaction) is equal to the upward force of upthurst. Hence the forces on the balloon are balanced. Upthurst=weight+normal reaction.
4. You use the normal reaction, since it is the Newton's 3rd law pair (with an equal and opposite force acting on the ceiling due to the balloon)
5. (Original post by ali malikzade)
There is no resultant force. The two forces in the downward direction weight and force of ceiling on balloon (normal reaction) is equal to the upward force of upthurst. Hence the forces on the balloon are balanced. Upthurst=weight+normal reaction.
The "force of the ceiling on the balloon" is the resultant force. Unless the ceiling is not supported on a wall (I assumed the ceiling was stable) then where would that force come from? It is the resultant from the upthrust-weight.
6. (Original post by TheBride)
The "force of the ceiling on the balloon" is the resultant force. Unless the ceiling is not supported on a wall (I assumed the ceiling was stable) then where would that force come from? It is the resultant from the upthrust-weight.
This is not the precise physics definition of resultant force. Resultant force is what you get when you add all of the real forces up. In this case, there are three real forces: weight, reaction, upthrust (or more precisely buoyancy) which, when added, cancel to produce no resultant force.
7. (Original post by mik1a)
This is not the precise physics definition of resultant force. Resultant force is what you get when you add all of the real forces up. In this case, there are three real forces: weight, reaction, upthrust (or more precisely buoyancy) which, when added, cancel to produce no resultant force.
Ok, thank you for clearing up my sloppiness XD, I wont be so vague in future!

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