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# Physics Question - How does thrust and mph work?

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### #1boolean  Members   -  Reputation: 1822

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Posted 01 April 2012 - 02:18 PM

I'm trying to wrap my head around how thrust converts into miles per hour (It's for some numbers to be used in my game)

According to HowStuffWorks.com, 1 pound of thrust equals 32 feet per second, which works out at 21 mph. I'm assuming this means that 2 pounds of thrust on a 1 pound object would mean it would be:

(2 pounds of thrust - 1 pound of the object ) = 1 pound of thrust * 21mph = 21 mph.

If it was 3 pounds of thrust, it would be:

(3 pounds of thrust - 1 pound of the object ) = 2 pounds of thrust * 21 = 42 mph.

This seemed to make sense as a general rule, so I tried applying it to an F-15 which apparently generates around between 25,000 and 29,000 pounds of thrust per engine and the plane itself weighs around 45,000 pounds. So I take around (55,000 - 45,000) = 10,000 pounds of thrust * 21mph = 210000 mph!

Since we know the plane travels at 600mph and we know the weight is 45,000 pounds, does it not make sense that at 45,001 pounds of thrust the plane is moving forwards at 21mph? The original formula seems to suggest so. Would it not only then need 45,028 pounds of thrust to move at 600mph?

Where has my logic broken down? Is the formula I'm using broken? If I know the weight and pounds of thrust, is it not as straight forward to convert it to mph as I think it is? Perhaps there is just something special about twin jet engines that makes it a bad example to use?

disclaimer - I'm bad at maths and possibly as smart as a shoe

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### #2Net Gnome  Members   -  Reputation: 773

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Posted 01 April 2012 - 02:35 PM

What you want are Newton's First and Second laws of motion (velocity and acceleration) as well as his laws of gravitation. They are not too complex as long as you are comfortable with algebra and a little bit of calculus (only if you want to understand how the laws and equations came about, otherwise algebra will work fine).

http://en.wikipedia.org/wiki/Newton's_laws_of_motion
http://en.wikipedia.org/wiki/Newton's_law_of_universal_gravitation

and relate those to thrust: http://en.wikipedia.org/wiki/Thrust

Tie all those together into the equations of motion, and you can move stuff http://en.wikipedia....tions_of_motion

With all that said, i think your issue is you're multiplying a velocity by a force, which isnt giving you what you want. In your case you have a Thrust (which is a force), and a force is a mass (airplane) by an acceleration. if you know the mass of the airplane, and its current thrust, you can find an acceleration, you can then use that acceleration to figure out how fast the aircraft is going after a certain amount of time has passed.

### #3Brother Bob  Moderators   -  Reputation: 9684

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Posted 01 April 2012 - 02:50 PM

If this is the page you've been reading, you have the units wrong. It doesn't say a pound of thrust is 32 feet per second, but 32 feet per second per second. It is an acceleration and not a speed, which makes sense since it is a force acting on a mass and thus accelerating it.

The page also describes thrust fairly well: A pound of thrust is the amount of thrust it would take to keep a 1-pound object stationary against the force of gravity on Earth.

### #4Luckless  Crossbones+   -  Reputation: 2413

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Posted 01 April 2012 - 03:58 PM

If you are still having issues getting your head around everything for it, then try making the switch to working in metric. I know many people who grew up working in imperial units, and had real issues doing physics, but then found it easier to understand when they were presented stuff in metric units.
If your signature on a web forum takes up more space than your average post, then you are doing things wrong.

### #5boolean  Members   -  Reputation: 1822

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Posted 01 April 2012 - 07:22 PM

If this is the page you've been reading, you have the units wrong. It doesn't say a pound of thrust is 32 feet per second, but 32 feet per second per second. It is an acceleration and not a speed, which makes sense since it is a force acting on a mass and thus accelerating it. The page also describes thrust fairly well: A pound of thrust is the amount of thrust it would take to keep a 1-pound object stationary against the force of gravity on Earth.

Ahhh, that might be the cause of the issue. I actually thought it was a typo at first

I just noticed Net Gnome updated his post which explains this really well. I think I'm getting confused with the acceleration and the final speed something is travelling at. I'm guessing then something could have a million pounds of thrust behind it, but without actually knowing when your taking the measurement you might as well be saying "This tree is as tall as a long piece of string"

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### #6jbadams  Senior Staff   -  Reputation: 23916

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Posted 24 March 2013 - 06:13 AM

I'm just going to punt this over to our Math & Physics forum.

### #7Bacterius  Crossbones+   -  Reputation: 12119

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Posted 24 March 2013 - 07:07 AM

necrooooo- ah, never mind.

“If I understand the standard right it is legal and safe to do this but the resulting value could be anything.”

### #8Khatharr  Crossbones+   -  Reputation: 4497

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Posted 24 March 2013 - 09:35 PM

Almost a year, man. You should have waited for the first.

void hurrrrrrrr() {__asm sub [ebp+4],5;}

There are ten kinds of people in this world: those who understand binary and those who don't.

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