[chris (OP)]

I was asked a question on the radio this morning that was quite insightful and I thought I'd pitch it here for discussion.

The caller said words to the effect: "When I hold a hose with water flowing out, I can feel it pushing me back. But when I fire the hose at a nearby wall, the force back on me seems to be greater. Why? And does the same apply to a space rocket blasting off?"

I won't bias the conversation from the get-go by saying what I suggested. I'd be interested in everyone else's thoughts first...

[RD]

... does the same apply to a space rocket ...

The "ground effect" is only good for a distance comparable with the diameter of the rocket , (or width of wing).

[evan_au]

the force back on me seems to be greater

The occasional droplets bouncing off the wall and hitting you in the eye might force you back?
...more a biological reflex than momentum transfer...

[Bored chemist]

Why is it harder to hold a hose pointing against a wall?

[jeffreyH]

The only way to tell for sure is to seat the hose in a device that can measure the momentum transfer and then point it first away from a wall and then into a wall. You cannot assume there will not be an effect.

[chris (OP)]

The suggestion I offered goes as follows - let me know what you think.

I proposed that, at very close range from the wall, water issuing from the hose forms a continuous column between the hose end and the wall surface. This means that "new" water exiting the hose sees a line of water in its path, which will provide some back-pressure and therefore exert forces back on the hose end in different, likely rapidly altering, directions, making the hose harder to hold.

Thoughts?

[Colin2B]

Why is it harder to hold a hose pointing against a wall?

This is indeed the correct question.
As RD and Evan_au point out, there will be backsplash from the wall but unless this hits a significant surface area there is no equivalent of ground effect pushing the nozzle away. If the nozzle is placed extremely close to the wall there will be a nearfield effect as some water bounces back into the nozzle causing pressure waves, but it needs to be almost touching.
With jet and rocket engines there is no increase in lift but the back blast as air is reflected from a lower surface can cause vibration (as it interacts with the down blast and oscillates) and also foreign object damage due to debris being thrown up, so rockets eg shuttle are usually fired over an open pit and vented out to the side.

One point worth noting is that with a garden hose much of the reaction force you feel is due to the curve of the hose - or the nozzle if this is the angled type - water going round the curve results in a change of momentum. You can try this at home with a hose and a straight nozzle, put the hose in a straight line and it will stay fairly still but as soon as a curve is introduced near the end it will whip around.

[Petrochemicals]

Thought being that someone must have a pressure washer. From my memory it does give ex-ray resistance, suppose that's where the pressure bit comes from .

That man  Ustinov have a powerful hose though