How does the energy loss change when blobbing on concrete instead of water?

[ChessEnthusiast]

Homework Statement

Blobbing is one of the extreme attractions in aquaparks. A person is lying on a big raft positioned on the surface of water and filled with low-pressured air.
Another person jumps down from a given height onto the opposite end of that raft, throwing the lying person in the air.

It is estimated, that the loss of energy is approximately 35%.

How would the energy loss change if we were to move that raft from water onto concrete?
Justify your answer.

2. The attempt at a solution
In the presented position, there is an upthrust, pushing the raft out of water. What follows is that when you jump onto that raft, the energy you add to the system will be used for the attempt to push the raft deeper in the water, but this will be balanced, because the volume inside water will increase.

Thus, if we were to move this raft to concrete, when the jumper would hit the surface, more energy would be used into throwing the person in air.

However, I am not sure of my explanations and I would be glad if you could advise me.

 

[berkeman]

Homework Statement

Blobbing is one of the extreme attractions in aquaparks. A person is lying on a big raft positioned on the surface of water and filled with low-pressured air.
Another person jumps down from a given height onto the opposite end of that raft, throwing the lying person in the air.

It is estimated, that the loss of energy is approximately 35%.

How would the energy loss change if we were to move that raft from water onto concrete?
Justify your answer.

2. The attempt at a solution
In the presented position, there is an upthrust, pushing the raft out of water. What follows is that when you jump onto that raft, the energy you add to the system will be used for the attempt to push the raft deeper in the water, but this will be balanced, because the volume inside water will increase.

Thus, if we were to move this raft to concrete, when the jumper would hit the surface, more energy would be used into throwing the person in air.

However, I am not sure of my explanations and I would be glad if you could advise me.

Looks to be correct in the prediction, and close on the mechanisms. Can you say a bit more about what the difference is in the raft's behavior on the end where the person jumps down onto it?

 

[ChessEnthusiast]

On water:
The end will "collapse" but that process of collapsing will be disturbed by the water's upthrust force.

On concrete:
The end will collapse further, decreasing the pressure in the given area if the raft, increasing the pressure of air on the other end - way more dangerous for the jumper

 

[berkeman]

On water:
The end will "collapse" but that process of collapsing will be disturbed by the water's upthrust force.

I don't really understand what that means. What is "upthrust", and what causes it?

On concrete:
The end will collapse further, decreasing the pressure in the given area if the raft, increasing the pressure of air on the other end

The pressure waves in the raft travel at the speed of sound, so the pressure is uniform all over the inside of the raft at any given time (for practical purposes in this problem). Squeezing the one end of the raft (by the jumper landing) causes an increase in pressure that causes the other parts of the raft to expand out. What can you do to maximize the squeezing of the one end of the raft to maximize the expanding at other ends of the raft?

 

[ChessEnthusiast]

What can you do to maximize the squeezing of the one end of the raft to maximize the expanding at other ends of the raft?

1) We can increase the mass of the jumper
2) We can decrease the pressure of the air in the raft

 

[berkeman]

1) We can increase the mass of the jumper
2) We can decrease the pressure of the air in the raft

True. But what is it about being on water or concrete that makes a difference in the temporary pressure increase inside the raft when the jumper hits it...?

 

[ChessEnthusiast]

On concrete, no energy will be wasted for "vertical movement" - since it will not be possible, wheareas in water, a part of this raft will sink when the jumper hits it

 

[berkeman]

On concrete, no energy will be wasted for "vertical movement" - since it will not be possible, wheareas in water, a part of this raft will sink when the jumper hits it

Bingo!