OT: Physics

[Julian Barnes]

What is the difference between foot-pounds (or whatever the fancy, metric equivalent is nowadays) and foot-pounds/second?

[The Natural Philosopher]

one is energy, the other is power?

[Tim Streater]

The first is an amount of work (assuming you're moving the pounds up some feet against gravity), the second is the rate of doing that work.

If you move 10 pounds up 10 feet that's a fixed amount of work. But you work a lot harder if you do it in 10 seconds rather than an hour.

[Cursitor Doom]

That's the key difference AFAICS. Say if I have a tractor that can pull a thousand pound weight through one foot, then that requires a thousand foot pounds of energy. If the tractor does that work in say 10 seconds, then it's half as POWERFUL as another that can do it in 5 seconds. Power is all about the RATE at which energy is produced or consumed.

[Syd Rumpo]

The metric version is the newton-metre or joule, and in this case is a measurement of energy, 1 Nm or 1 J being roughly the energy required to raise an apple upwards against gravity[1] by one metre. It doesn't matter how long this takes, it's still the same amount of work or energy.

Adding the /s turns this into a unit of power. In fact, 1 Nm/s (or 1 J/s) is a watt. If you raised your apple by 1 metre in one second, the power needed over that time would be 1 W. If you took 10 seconds, the power used for those 10 seconds would be 100 mW and if you did it in

1/1000 of a second you'd use 1 kW, albeit only for 1 ms. [1] Apple mass is ~= 100 g, gravity is ~= 10 m/s/s, so apple weighs 1 kg m/s/s = 1 N.

Cheers

[Albert Zweistein]

Yes foot-pounds is energy, pounds-feet is torque and pounds on your feet is 'Oh f*ck that hurts'.

[newshound]

Agree with all the other postings, but just to be pedantic you should be saying pounds weight (or lbf), for clarity that it is referring to force, not to mass.

[Dave Liquorice]

Something not right with the numbers and units there but can't put my finger on it.

A mass of 1 kg at the surface of earth (gravity approx 10 m/s/s) has an weight of 10 Newtons or 1 kgf.

Mass = 0.1 kg, weight = 1 Newton or 0.1 kgf.

[Syd Rumpo]

I think it's the line break wot messes it up. Underscore is hard space here...

Apple mass is 100_g or 0.1_kg g is 10_m/s/s so apple weighs 1_kg.m/s/s = 1_N

Cheers

[Bill Wright]

Foot pounds is stamping on the floor. Foot pounds per second is doing it fast.

Bill

[Brian-Gaff]

The definition of the time over which its measured I guess. Ive seen Instantaneous written as well, which I interpreted as peak measurement. Myfavourite measurements of the past was music power. So you would see an amp with 50 watts a channel music power, but when measuresd both channels running the peak before clipping was about 10 watts, and continuous about 6 watts if the psu did not go belly up in the meantime. grin. Brian

[Brian-Gaff]

Yes see my Watts analogy. Brian

[The Natural Philosopher]

IIRC its 9.8m/s^2

[Syd Rumpo]

Well, if you want to be picky, it's defined as 9.80665 m/s/s

Cheers

[Cursitor Doom]

m/s/s is the same thing as m/s^2 (acceleration due to gravity)

[Tim Streater]

Although of course the actual value varies depending where you are on the planet.

[Dave Liquorice]

And how far you are from it. Top of a tower block has less gravity than the bottom so a clock(*) at the top gains on a clock at the bottom.

[The Natural Philosopher]

well only if it uses gravity to modulate a pendulum.

[Syd Rumpo]

Looks like you hooked one, Dave.

Cheers

[Huge]

Quite a big one, too.