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Physics/Force calculation


QUESTION: Hi Steve: Thanks for offering your assistance to those of us who are math challenged.  My question is simple I think...see attached sketch describing 2 scenarios.  In A, a load (a man) is on a vehicle (skateboard) with a small engine that can propel the man forward.  In B, the small engine is separated from the man and skateboard and instead pulls him forward at an angle that can vary.  If the loads (the man) and the engines (1/4 HP) are equal, is there a difference in the demand on the small engine to move the load forward?  I am not a physicist or mathematician, but suspect that if the load is directly over the engine, that it might require more force to move the load forward and to maintain it in moving forward, in part because of the torque required to initiate the movement as well as to maintain it since the load is exerting itself directly on the wheels of the skateboard versus at an angle when it is pulling it forward. If there is a difference, can it be described as a proportion based on the 45 degree angle?  If there is an efficiency (say in miles per gallon), what might that be?  Thank-you again for any help you can provide.  [I tried attaching the sketch I described, but could not, so please send me an email response and I will send it to you outside this website]

ANSWER: There is no attached sketch.  Are you saying if it's at a horizontal or vertical angle?  The question is potentially asking about 4 different questions, a sketch really would help...

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Trying to attach the sketch I referred to.

The way you draw it, they both provide the same thrust in the horizontal direction provided that there is no slippage of the wheels.  The tension in the rope will be higher than the thrust, that must be compensated by the weight of the motor.  The most powerful design is the first, since the weight can be put on the wheels and the rolling friction will be highest.  The most stable would probably be the second, where the center of gravity was shifted forward for steering and the person on the board had an upper-body hold on the source of thrust.


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Dr. Stephen O. Nelson


I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.


I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.

Ph. D. from Duke University in physics, research in nuclear astrophysics reactions, gamma-ray astronomy technology, and advanced nuclear reactors.

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