the units don't work.

Matt

 

Thanks for the link, and the excellent interview.... Too bad about some of the answers too.....

Matt

 

Who's units? Which ones?

 

The formula is actually

P = TN/5250

where:
P = Power, hp
N = Rotational shaft speed, rpm
T = Torque, lb-ft

This is a rotational horsepower breakdown of the 'classic' definition of one horsepower equalling 33,000 pounds lifted one foot in one minute.

Since 'horsepower' is not a directly measured unit per se (it comes from a measurement of torque over a period of time) this formula and it's results are often quite predictable. Slow revving diesel engines will always produce a higher torque number than horsepower numbers. Screaming, tiny engines will tend to have the opposite effect. The kicker is, no matter the type of engine, if it is able to spin at 5250 rpm and make power, the horsepower will always equal the torque.

 

Acceleration is in units of meters per second squared (in metric).

Power is not kg meters per second squared (in metric).

Power is kg meters squared per second squared (in metric)

This is called dimensional analysis.

Power isn't equal to acceleration. One is an apple, the other is an orange.

Matt

 

Ugh, dimensional analysis...brings back memories (of last semester ).

 

If you carry the units, and they don't work out (like your time is in units of meters), you know you have a problem! Dimensional Analysis is just fractions... If you think of it that way, it's not as intimidating.

FWIW, there's a unit set called natural units, where c=1 along with a lot of other constants that I don't remember any more (theoretical physicists use it a lot), and then they just add all the constants at the end to make the units work! Makes the algebra easier.... I was never good at it.

Matt

 

Oh, I know.

When I said "Power is acceleration," what I really meant was "power determines the rate of acceleration (because gearing is indirectly involved)."

Sorry for the confusion.

 

looking to keep a little clarity on subjects that are abused by almost all. Fact is, power and acceleration are realted, but torque at the wheels is what detetmins acceleration. You can have a lot of power at higher speeds, with little acceleration. That's just the way it is.

Matt

 

You're right, torque at the wheels determines acceleration.

You're also right that power is one variable in the formula to see how a car will perform.

I think the best thing to do is find the total area underneath the torque curve on a graph.