First of all, please ignore me if you already know this and correct me if I'm wrong... I'm just trying to be of use.
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Originally Posted by Guyute
what do you think the up force would be needed to lift a 130lb robot in the air 12' using a 4.5' propeller
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You need the same amount of force to lift your robot no matter what you lift it with. A 5-foot propellor, a 15-foot propellor, a rope, your arms... it doesn't make any difference.
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Originally Posted by Brant Bowen
I'm not very good at physics, but the force would have to be greater than 130 lbs.
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Yes!
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Originally Posted by Brant Bowen
i think, maybe something like 130 times 9.8 to counteract gravity???
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No.
Talking about mass and force in the classical system can be misleading, because the units for pounds of mass and pounds of force act really weird. Let's do it in metric first.
Imagine dropping your robot out of an airplane. Neglecting air resistance, it will accelerate at a rate of around -9.8 m/s/s (notice the acceleration is negative, because the robot is gaining downward speed!). If the mass of your robot is 60 kg (about 130 pounds), the total force on it will be
F = ma
F = 60 * -9.8
F = -588
-588 Newtons. The force is negative because it's pulling your robot down.
Now, add a propellor to your robot that provides 588 Newtons of up force (lift). The total (net) force is
F_total = F_gravity + F_propellor
F_total = -588 + 588 =
F_total = 0
zero! So is your robot flying upwards?
F = ma
a = F/m
a = 0/60
a = 0
No! The total acceleration is zero, so your robot's speed will not change. If it started out falling, it will keep falling. If it started out rising, it will keep rising. If it started out stationary, it will stay still.
So bring your robot back down to earth. If you put it on the ground and start the propellor, the net force on it will again be zero, as will its acceleration. Since it's stationary to start with, it won't rise off the ground. If you put it on a scale, however, the scale will read zero, because the force of the propellor is exactly balancing the weight of the robot.
So if you made the propellor put out even a
tiny bit more lift, the robot would accelerate upwards!
In the classical system, as I understand it, a pound of force is the force gravity makes on an object whose mass is one pound of mass. So putting 130 pound of force on a 130-pound-mass robot would just make its acceleration zero -- it would not lift off. Any more than 130 pounds of force, and the robot *would* lift off! So (9.8 * 130) pounds of force like Brant suggests would definitely be enough, but less force would work as well, and (9.8 * 130) pounds of force would probably be overkill.
If you have to talk about force in the classical system, I suggest you use the unit of force called the
slug.
Hope that was helpful!
~Aaron