Test Your Knowledge...

[OttoNP]

I've never measured anything, but here's something interesting...

My boat has about 190 HP and it will top out at around 58 MPH, from that I can figure out how much drag it's facing. Now some of that 190 HP is wasted running pumps, alternator, prop slipping, etc...but we'll assume it all goes into moving my boat. 58 MPH is about 26 m/sec and 190 HP is about 141683 Watts, so my boat is facing about 141683 / 26 = 5464 N or about 1228 lbs of force. In reality the drag force is less than this becuase of all the ineffiencies of transmitting the power. The RPM of my engine is around 5000 RPM or 83 RPS or 521 radians per second at this point, so the torque it's generating is about 141683/521=272 N-M or 200 ft-lbs. We can also see what the torque is at the prop, my boat has a ratio of 1.94, so my prop is spinning at 2577 RPM when my engine is at 5000 RPM. but the HP is the same so it has a torque of 528 N-M or 388 ft-lbs.

How close are these? The HP and torque at the engine are very close, the prop torque will also be somewhat close, but it will be less in real life due to losses through the tranny, power used for the alternator, pumps, etc... The drag force would be more accurately calculated from timing how long it took the boat to go from 58-50 MPH, I'd guess about a second or so, but that's just a guess. If it took 1 seconds, then the accereation rate would be 3.6 m/sec^2 and the force would be 4867 N which is 1094 lbs, that's pretty close to the estimate, but I think maybe it took longer to slow down, not sure.

The total friction a car faces when driving, not skidding or braking, can be fit to a equation like this:

F=a+b*V+c*V^2

where F is the force, V is the velocity, and the rest are constants. You pretty much have to experiment to determine these, time how long it takes you to slow down in five mile increments, I.E. go like 70 MPH and then coast (no brakes, no engine drag) and see how long it to get to 65 MPH, then see how long it takes to go from 65-60, then 60-55, etc...

If you do that you could figure out that equation as well as determine the min HP needed for each speed, how efficient things are, etc...

Nick
________
MEDICAL MARIJUANA GROW

[criminally_minded]

Why doesn't someone get a couple of tyres (one on the hub and one without) and roll them along a flat surface? That should provide some clarification....

[ohsix]

What would that clarify? That just confused me even more. I'm guessing the one with the hub would roll further because it has more mass and would take more energy to stop it. I think we have all agreed that if two objects are rolling at the same speed the one with more mass will go further without any means other than the friction of the surface stopping them.

[criminally_minded]

Thats sort of what I was getting at Wes. If you're talking about the finding which object would roll further (more mass or less mass), it seems like a fairly simple test.