# Homework Help: How do you calculate the enrgy required to maintain a given thrust?

1. Jul 5, 2011

### kitna paisa

1. The problem statement, all variables and given/known data
I would like to be able to calculate the energy use per second, to maintain a rocket of given mass, at zero velocity, i.e. hovering, at, say, sea level.

2. Relevant equations
I have Thrust = Force times velocity, but can't work out the energy due to the velocity of the rocket equaling zero, leading to zero work, and apparently zero energy, whilst obviously there is considerable use of energy, and thus power.

3. The attempt at a solution

2. Jul 5, 2011

### Pengwuino

You would need to know more details about the engine to figure it out.

3. Jul 5, 2011

### kitna paisa

Thanks. What I intended was to make a craft like a rocket, but which could hover. The motor could be any power plant, capable of powering a jet of water.
The water would be supplied to the craft on a continuous basis, so the craft would remain at constant mass. The power would also be provided to the craft on a continuous basis, and would most likely be electrical.
How would I work out what the energy consumption would be be in kW.Hrs for any given mass, at (craft) zero velocity, with en energy-efficiency of x%? I suspect that the result would depend on the m(dot).a configuration.
Thanks. kitna

4. Jul 5, 2011

### srivi

There are two forces acting on the rocket , by engine and by gravity.
The energy given by gravity is potential energy.

So W=dKE+dPE.

If you calculate work done by non conservative forces which does not include gravity then you must include the PE term.

5. Jul 5, 2011

### kitna paisa

Thanks I can imagine the forces involved- no problem there. To get to the question though, what is the energy equation? There is a fine example of what I am talking about in this video. It shows just such a craft as envisaged, producing thrust to raise a mass, but how was the energy calculation made?
see http://www.break.com/index/amazing-new-water-powered-jet-pack.html
Cheers
kitna

6. Jul 6, 2011

### kitna paisa

OK, the link that I gave in the previous post wasn't very good, so here is the original link for this crazy vehicle: http://jetlev.com/ !!!

Last edited by a moderator: Apr 26, 2017