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- Thread starter ttgont
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Hope this helps,

Bruce S. Levison, NAR #69055

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Originally posted by teflonrocketry1

Hope this helps,

Bruce S. Levison, NAR #69055

So what you are saying is that it in fact will move the 4.5 "object" but only for one second. Then I suppose, speed comes in to play as to how fast, thus how far the object moved...?

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Bruce S. Levison NAR #69055

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How much an object is moved by a force depends on a number of things, but the basic equation is F = m * a or force equals mass multiplied by the acceleration.

Weight is a force. It's a mass x the acceleration of gravity which is 32 ft/sec/sec.

So consider your 4.5 pound-sec engine with a burn time of one second, try to move a 4.5 pound object. What happens will depend both on direction and friction.

Consider the simplest case where the weight is on a frictionless surface (like a skating rink or in space), and let's consider the acceleration that might occur to the object.

With a little rearrangement we get Thurst(#)/Weight(#) = a in units of g so 4.5/4.5 = 1 g

We previously said that 1 g was 32 ft/s/s so again assuming no friction we can use the equations relating acceleration, velocity, distance and time to figure out what's going on.

v = a * t = 32 ft/s/s * 1 s = 32 ft/s so that how fast it's moving at the end of the burn.

d = 1/2 * a * t * t = 0.5 * 32 ft/s/s * 1 s * 1 s = 16 ft so it's move 16 ft in 1 second. Once the motor stops and if there is no other losses such as friction or gravity, then the position as a function of time after the burn is d = do + v * t = 16 ft + (32 ft/s * t s)

1 second after engine burnout it's 48 ft from the starting position, 2 seconds after burn 80 ft. etc.

In the real world we have gravity, so you have to look at the direction of the forces and account for gravity and friction. Forces add vectorally so if you try to lift a 4.5 pound object (like a rocket) with a 4.5 pound thrust motor (a 4.5 pound force) the object goes nowhere because the forces are opposite and exactly cancel. If you took a motor with the same total impulse but with a 0.5 second burn time, the motor's thrust our be 4.5/0.5 or 9 pounds for 1/2 second, and the object would accelerate upward for 1/2 second at 1 g net acceleration.

Rockets should have a 5 to 1 thrust to weight ratio for stable flight, 10:1 is better.

I hope I answered your questions. You might find the following webreferences useful.

Bob Krech

https://www.lerc.nasa.gov/WWW/K-12/airplane/bgmr.html

https://www.lerc.nasa.gov/WWW/K-12/aerores.htm

https://www.apogeerockets.com/education/index.asp

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