Physics question...propulsion in a swimming pool

As to the highlighted text, boats don't produce more power exhausting the engine's exhaust above the water line. They DO however allow the engine to perform where it should have it's power. But this is NOT truly affecting real output. The reason you do not exhaust engine exhaust below the water line because the water pressure restricts the output of the exhaust fumes coming out. Imagine how your engine runs when your exhaust pipe is below the water line by 6 inches to a foot. That resistance means your engine cannot breath as well as it could when OUT of the water. It doesn't increase the engine power, it simply allows it to breath properly to get the power it will normally have.

The problem is you are not making the dual comparison of air/water release.
How fast would that "jet pack" be going if instead of floating in the air, he were to remain submerged under water? If he were to stay under water, he would accelerate much faster due to the resistance of the water that is surrounding him.
I never said that the water coming out of the squirter would not accelerate ANY, I simply stated that the actual propulsion value would be greater if released below the water line than above, as the OP was asking.

 

Yes, if you shoot water out of the super-soaker you will move in the opposite direction.

Holding it underwater or having the water stream hit the side wall will make NO difference at all. This one always seems to confuse people. Way back people used to think that rockets would not work in a vacuum because there is nothing to "push off" of. This is simply not accurate. For every action there is an equal and opposite reaction, period.

 

Holding the nozzle underwater WILL make a difference in the physical world we live in because you're pushing against something more 'solid' if you will. The water will resist motion moreso than air so when you pump a jet of water under water, more of the energy is put toward moving you rather than shooting the jet in air.

The 'equal and opposite reaction' principle still applies, it just comes down to how the energy that you've created is dispersed and put to work.

 

That is not accurate. What would happen if the nozzle was in space in a vacuum? Think it would somehow have "less to push off of" and this would reduce performance?

 

I think it is. the friction and displacement under water by the exhaust provides more thrust than the drag on the raft above the surface.

 

Kind of why it's hard to swim in the air.

 

Think about the whole idea of equal and opposite reaction. If you shoot a nozzle into the air, the action is propelling the water through the air. Water moves through the air fairly easily so you don't get much force acting on the cannon to produce movement. Now put the nozzle in the water and pull the trigger. You're now pushing water into more water so more of your reaction is felt as movement because it's harder to push a stream of water through more water.

In short, it is accurate

 

This is not correct. What you are shooting the water into means nothing. You could shoot the water into a vacuum or into a vat of ice cold molasses, it would make ZERO difference in the amount of thrust you would create. This is same reason people used to think that rockets would never work in space, because there is nothing to "push off of". It simply is not accurate. The thrust comes from throwing the water away from you, nothing more. It does not come from the water pushing off of anything...

 

OK, one more time, you do not NEED something to push off of, I never said that or implied it, but when you have something to push off from, more of the energy is put into movement rather than lost or dispersed by the atmosphere. Shooting the water gun in the air, the thrust is lost by pushing the water through the air, any movement will be very minimal. If you thrust water into water, you're pushing into a mass, it resists so you end up propelling yourself forward.

In space, the energy is the fuel in the rockets. It's a different atmosphere. The laws of physics still apply but you can't always directly compare what happens in space to what happens in your bath tub.

Once again, yes, it is accurate

 

Okay, I will try again:
If you shoot a shotgun into the air it kicks with X amount of force. Now shoot it again but this time only three feet from a brick wall. Does the amount of force you feel in the shotgun change? Does the buckshot hitting the wall somehow change the amount of force felt in the shotgun? It does not.

Once the water has left the very tip of the gun it no longer has the ability to affect the gun. What the water encounters after leaving the gun does not affect it. You could even have the water enter some kind of magical acceleration tube and accelerate it to near the speed of light, it would have ZERO affect on the person holding the water gun...

 

The answer is potato.

Your arguement is invalid.

 

OK, I'll try one more time... A water cannon is not like a shot gun. When you fire a projectile into a target, you're right, it doesn't matter if the target is 10' away or 100' away, your recoil will be the same.

Now, back to the water cannon. Water is NOT a single projectile. Spray the water into the air and the ONLY reaction you get is whatever minor recoil you'll get from a water cannon. Now fire a water cannon with the tip under water. You have 2 reactions going on, the recoil which was the same as it was out of water, except now the water can't freely leave the cannon. It has to push the other water out of the way. In order for it to push the other water out of the way so it can leave the barrel, it will create another force back on the gun.

See where I'm going with this?

 

fail.

 

I am going to PlastiDip my entire truck and bet it in a game of cash poker, perhaps against a pony...

 

Annnd... we've lost him

 

again,.....the friction and displacement under water by the exhaust provides more thrust than the drag on the raft above the surface. The friction and displacement of just air produces less thrust .
and what is plastidip?

 

It sounds like you are referring to some pressure being created on the tip of the gun due to water resistance, which might add some thrust (the second reaction you speak of). However, you must realize that even if this was the case the amount of energy is the same either way. This "second reaction" you speak of would have to rob energy from the "first reaction" and the net affect would be exactly the same. No additional thrust is created by shooting the gun under water.

If you have 10 watts of energy to work with and you use this to make thrust, there is no way possible to make this 10 watts do more work by shooting it under water. In fact, shooting the gun under water would reduce the amount of energy you have to do useful work, there would be frictional loses (heat).

 

Again, I understand and you're just not getting it. Say you create some unit of energy with the water cannon. It will be the same energy whether it's above the water or below the water. The difference is how you put that energy to use. It will just shoot the water through the air above the water meaning the work done was propel the water. Below the water it can't just expel the water freely, it is pushing against the other water so the work will be done on the raft. You will get more movement as a result. Same amount of energy spent, it's just a matter of what you do with it...

If you don't get it now, I give up and you'll have to find a physics book or have someone explain it to you in person.

 

Last chance,.it is easier to push air out of the way than water, so the thrust generated is now pushing harder in the forward direction. Because it's harder to push in the exhaust direction.

 

In your scenario above you would make LESS thrust since you would be making heat instead of doing useful work!