You lift a 10. lb physics book up in the air a distance of 1 ft. at a constant velocity of 0.5 ft/s. the work done by gravity is?

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- Jul 6th 2006, 02:30 PMbabygirlHelp!
You lift a 10. lb physics book up in the air a distance of 1 ft. at a constant velocity of 0.5 ft/s. the work done by gravity is?

- Jul 7th 2006, 04:21 AMtopsquarkQuote:

Originally Posted by**babygirl**

The angle between the weight and displacement is 180 degrees.

$\displaystyle W_G = \vec w \cdot \vec s = (mg) \Delta h \, cos(180) = -mg \Delta \, h$

Thus the work done by gravity is = -10*1 ft-lbs = -10 ft-lbs.

-Dan

(And what Physics book is still using English units??) - Jul 7th 2006, 05:06 AMCaptainBlackQuote:

Originally Posted by**topsquark**

unit system, and that using $\displaystyle g=32 ft/s^2$ would give an answer

in $\displaystyle ft-poundals$ (or what ever the units are - its >30 years since

I used them).

Quote:

-Dan

(And what Physics book is still using English units??)

killer-slugs (kilo-slugs) I expect).

RonL - Jul 7th 2006, 06:26 AMThePerfectHackerQuote:

Originally Posted by**CaptainBlack**

- Jul 7th 2006, 11:08 AMtopsquarkQuote:

Originally Posted by**CaptainBlack**

I've honestly forgotten how a foot-poundal is defined. I'll have to look that up sometime.

-Dan - Jul 7th 2006, 01:24 PMCaptainBlackQuote:

Originally Posted by**topsquark**

commonly used for weight, it is in that system's the unit of Mass. The unit of

force is the poundal - the force that would accelerate a MASS of 1 pound

at 1 ft/s^2.

RonL - Jul 7th 2006, 01:30 PMCaptainBlackQuote:

Originally Posted by**ThePerfectHacker**

RonL - Jul 8th 2006, 05:40 AMtopsquarkQuote:

Originally Posted by**CaptainBlack**

-Dan

(I*knew*there was a reason we were switching over to metric!) - Jul 8th 2006, 05:51 AMQuickQuote:

Originally Posted by**topsquark**

**HORRIBLE**for math, it is quite usefull in approximating distance in everyday life without using a ruler...

1 inch=the length of 1 digit in your finger (I think your thumb)

1 foot=the length of your foot

1 yard=the length of a long stride

1 mile=I have no idea (probably the distance from two ancient cities)