Physics question (Mike, if you would..)

kojikun · Post by **kojikun** » 2002-10-22 09:16pm

Ok I'm doing to calcs for my storyverse and I'm trying to figure out two things:

how much power is expended by a jet pushing 5500 kilonewtons (500kg accelerating at 1 gee)

and how much power is absorbed if a 1cm^2 "energy collecter" collects enough power to produce 17.5 kilonewtons

I can't figure this out for the life of me. Mikes science section says P = N*m/s however I don't know how this fits into the equations.

`P = 5,500,000 * m/s'?? I don't understand. How many watts would be required to produce 5,500 kilonewtons of thrust??!

::breaks down into sobbing::

Wicked Pilot · Post by **Wicked Pilot** » 2002-10-22 10:11pm

kojikun wrote:how much power is expended by a jet pushing 5500 kilonewtons (500kg accelerating at 1 gee)

Hint: Ignore the kilonewtons, use P=E/s, E=.5mv^2

and how much power is absorbed if a 1cm^2 "energy collecter" collects enough power to produce 17.5 kilonewtons

I don't think you have enough factors to do that calc

I can't figure this out for the life of me. Mikes science section says P = N*m/s however I don't know how this fits into the equations.

There are many ways to compute power. Just remember what power is, rate of doing work. Go from there.

Wicked Pilot · Post by **Wicked Pilot** » 2002-10-22 10:16pm

By the way, it only takes 4900 Newtons to accelerate a 500kg object at 1g.

Newtonian Fury · Post by **Newtonian Fury** » 2002-10-22 11:10pm

First of all, how old are you? Have you ever taken physics in high school or college? Because if you have, you would realize that you have not provided enough information to compute your last two questions.

Newtonian Fury · Post by **Newtonian Fury** » 2002-10-22 11:20pm

Here are some pointers:

1] Know what force is

2] Know what velocity and acceleration are

3] Some equations:

F = ma , F= force, a = acceleration, m = mass

Kinetic Energy = 1/2 * mv^2 ,v = instantaneous velocity

Power = Energy / Time

Power = Fv , probably relevant here

Hope this helps

kojikun · Post by **kojikun** » 2002-10-22 11:23pm

Thank you all for being so informative. Your help and suggestions have aided me in - wait no nevermind.

Ok, situation:

A 500kilo helicopter is hovering here on earth. Assuming constant mass and 100% of the power is lift/thrust, how much power must it produce to hover.

This is indeed enough information to figure it out how much power is required. 500kg hovering in 1gee = 500kg * 11m/s^2 or 5,500kN.

Now, considering battery operated model helis use a specific amount of power (from the batteries, duh), they must also produce as given amount of thrust.

So yes, what i have provided IS enough information to figure this out.

***

Fury, like i said in the first post, I know the shit, i just cant convert it.

according to the science page here, P = Nm/s. P = 5,500,000m/s

Now, is m mass? if so then {P=E/s}:

E/s = 500(5,500,000)/s
E/s = 2,750,000,000/s
P = 2.75e9 watts, however I do not believe this is true so something im doing is wrong.

Newtonian Fury · Post by **Newtonian Fury** » 2002-10-22 11:45pm

A 500kg helicopter hovering would need to exert a 500*9.8, or 4900 Newtons. How much power needed to produce that kind of force depends on the motor. Suppose an electric motor can exert this kind of force, calculating the power is quite simple. Simply use the equation P = iV, where i is the current through the motor while V is the voltage drop accross it. And that is the power expended by the motor.

Now, considering battery operated model helis use a specific amount of power (from the batteries, duh), they must also produce as given amount of thrust.

Batteries don't produce thrust. You're either confusing the units here, or you're not understanding the concept.

The reason I am questioning your understanding is because if you know this stuff, you wouldn't have to look up this stuff at a website. And what is there to convert? It's all in metric. And obviously, one unit type can't be converted to another.

Durandal · Post by **Durandal** » 2002-10-23 01:04am

A 500kilo helicopter is hovering here on earth. Assuming constant mass and 100% of the power is lift/thrust, how much power must it produce to hover.

How high? It has to get up there first, and that requires work.

This is indeed enough information to figure it out how much power is required. 500kg hovering in 1gee = 500kg * 11m/s^2 or 5,500kN.

Wrong. 1g is 9.8m/s^2. 1 Newton is 9.8m/s^2 * 1kg.

So yes, what i have provided IS enough information to figure this out.

If you could have figured it out all along, why did you ask for help?

Fury, like i said in the first post, I know the shit, i just cant convert it.

You screwed up the most basic figure high school physics students are taught: Earth's acceleration due to gravity.

according to the science page here, P = Nm/s. P = 5,500,000m/s

Power is not in m/s. It is in Watts, or J/s.

I'll clear a few things up.

Power is a rate, just like velocity. It is the rate at which energy is being expended, so it is measured at P = E/t.

Your entire approach is fucked up. You can't determine Watts without a timeframe. You can determine the amount of work required, but you can't go beyond that. How long does the helicopter have to float? How long does it take to rise to whatever height you didn't give?

kojikun · Post by **kojikun** » 2002-10-23 09:46am

nevermind. I figured it out.

Durandal, you can figure out how much power is produced for a given amount of thrust.

Thrust is amount of acceleration forward for a constant input of energy (aka power). Pilot was right when he said to go with ke and shit.

F = ma
E = (1/2)*mv*2

Ok so 2kg * 1m/s = 1J kinetic energy. Constant application of one joule per per second produces 1 Watt of power; so constant acceleration of 2kg at 1m/s^2 requires 1 watt of power.

So P = (1/2)*ma. I think.

Damnit, wheres Mike when I need him.

kojikun · Post by **kojikun** » 2002-10-23 09:57am

youre correct tho durandal, i fucked up with accel. can you blame me? im american, i barely know how to convert feet to meters. XP

1 meter = 3.28 feet

so
32f/s ~= 9.75m/s

victorhadin · Post by **victorhadin** » 2002-10-23 11:06am

Power is not the same as force. Power, if I remember correctly, if the force in Newtons multiplied by the speed, and so you would need the airspeed of the aircraft to calculate power output, and that brings drag into the equation as well.

Rarely is anything simple, alas.

Newtonian Fury · Post by **Newtonian Fury** » 2002-10-23 11:16am

kojikun wrote:Durandal, you can figure out how much power is produced for a given amount of thrust.

Wrong. You need a time frame since power is a rate.

Thrust is amount of acceleration forward for a constant input of energy (aka power).

Wrong again. Force(which you're referring as thrust here) is mass(not energy) of the object times its acceleration.

E = (1/2)*mv*2

Wrong yet again. Kinetic energy = (1/2)*m*(v^2)

so constant acceleration of 2kg at 1m/s^2 requires 1 watt of power.

You got this right.

So P = (1/2)*ma. I think.

Wrong. To find the power expended by a particular force, P = F*v, where F is the force applied on the object and v is velocity of the object. Note that since velocity is relative to which direction the force is applied, you might need to find a component of the force that is in the direction of the velocity, and the power computed may be negative.

Lord Edam · Post by **Lord Edam** » 2002-10-23 11:24am

For a helicopter in stationary hover

Power = [(thrust)^3/2] / [sqrt(2*airdensity*rotor area)]

Wicked Pilot · Post by **Wicked Pilot** » 2002-10-23 11:46am

Let me step in real quick.

Mass (m)
You should all know what this is.
SI unit is the Kilogram (kg)

Velocity (v)
Change of displacement over change of time
Derived unit is meters per second (m/s)

Acceleration (a)
Change of velocity over change of time
Derived unit is meters per second per second (m/s^2)

Force: (F)
You all know what this is.
SI unit is the Newton (N)
Helpful equation: Force=mass*acceleration (F=ma)

Work (W)
Transfer of energy via force
SI unit is the Joule (J)
Helpful equation: Work=Force*displacement (W=Fd)

Kinetic Energy (Ek)
Energy associated with state of motion of an object
SI unit is the Joule (J)
Helpful equation: Ek=0.5mv^2

Work Energy Theorm
Change in the kinetic energy of an object equals the net work done on the object

Power (P)
The rate of doing work
SI unit is the Watt (W)
Helpful equations: P=W/t P=Ek/t

Post by **Darth Wong** » 2002-10-23 12:32pm

kojikun wrote:Ok I'm doing to calcs for my storyverse and I'm trying to figure out two things:

how much power is expended by a jet pushing 5500 kilonewtons (500kg accelerating at 1 gee)

Depends on whether you include waste or not. If it's a jet turbine anchored to the ground, it's not doing any work at all, so its useful power output is zero. However, its total power expenditure (including waste heat) would be the chemical potential energy of any reacted fuel.

Alternatively, you might be talking about the power applied to its exhaust gas, for which you would need to know the outlet gas velocity and flowrate in kg/s.

I'm afraid there simply isn't enough information in your question to answer it. Units cannot simply be converted from one to another willy-nilly.

and how much power is absorbed if a 1cm^2 "energy collecter" collects enough power to produce 17.5 kilonewtons

Lower limit: zero. Force does not necessarily translate to power. If your mass is 60kg, your chair is exerting roughly 600 N of force upwards against your butt. It doesn't require a power source in order to do so.

I can't figure this out for the life of me. Mikes science section says P = N*m/s however I don't know how this fits into the equations.

The units of power are Nm/s (Newton-metres per second). Without displacement, there is no power regardless of force.

How many watts would be required to produce 5,500 kilonewtons of thrust??!

::breaks down into sobbing::

Relax. You shouldn't try to run before you learn to walk, and you obviously haven't learned to walk yet, young padawan.

Zoink · Post by **Zoink** » 2002-10-23 02:12pm

If you don't have enough info, make reasonable assumptions.

kojikun wrote: how much power is expended by a jet pushing 5500 kilonewtons (500kg accelerating at 1 gee)

Does the jet weigh 500kg? Or is that how much thrust there is?

A really basic way of doing it:

M=500kg
g=10m/s^2 (for simplicity)
Vo=200m/s (assumption)
L/D=10 (assumption)

For a jet, Power = Thrust x Velocity

Velocity is determined by you. What speed is the aircraft going at the time? I'll assume 200 m/s.

Thrust. To accelerate 500kg at 1g, you'll need 5000N. You'll also need the lift/drag ratio (this changes with angle of attack, so you'll have to just take a "typical value"). A 500 kg aircraft needs 500kg (5kN) of lift, so if the drag ratio is 10 (ie 0.1 D/L, assume) it will have 500N of drag. Thats 5500N total thrust needed.

If your jet is going 200 m/s, then thats 5500N * 200m/s = 1,100,000W

Wicked Pilot · Post by **Wicked Pilot** » 2002-10-23 02:30pm

kojikun wrote: how much power is expended by a jet pushing 5500 kilonewtons (500kg accelerating at 1 gee)

You do have enough info.

Useful power
Change in Kinetic Energy after one second of full thrust from full stop is 0.5(500kg)(9.8m/s)^2 = 24010J
Power = 24010 Watts

Power exerted by engine
Distance covered after one second of full thrust from full stop is .5(9.8m/s^2)(1s)^2 = 4.9m
Work done during that second is (5500000N)(4.9m) = 26950000J
Power = 26950000 Watts

Analysis of craft in question

Engine is exerting at least 2695000 Watts, of which only 24010 is useful. Your engine is messed up big time. If you were to say 5500N instead of 5500kN, then your ship would make sense. Remember, one Newton is the force required to accelerate one Kilogram at 9.8m/s^2.

kojikun · Post by **kojikun** » 2002-10-23 04:55pm

Wong has spoken. I'll just crawl around now. ;p

StarDestroyer.Net BBS

Physics question (Mike, if you would..)

Physics question (Mike, if you would..)

Re: Physics question (Mike, if you would..)

Re: Physics question (Mike, if you would..)