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?
First note that we don't need to know that the book is moving at a constant velocity.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??)
I think that an explanation of why $\displaystyle g$ is $\displaystyle 1$ in thisOriginally 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).
Quite - Mars probes have been lost over this sort of thing (revenge of the-Dan
(And what Physics book is still using English units??)
killer-slugs (kilo-slugs) I expect).
RonL
Actually, g IS 32 ft/s^2. Note that the book's weight is given, not its mass. (ie. lb is a unit of weight, not mass). Thus mg = 10 lb, so the value of g is not directly used here. The 1 in my equation was the "1 ft." I suppose I should have labelled the units as I put them in the equation to make that clear.Originally Posted by CaptainBlack
I've honestly forgotten how a foot-poundal is defined. I'll have to look that up sometime.
-Dan
Its a dreadful feature of the Customary/Imperial Unit system that the pound isOriginally 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
You know, even though the customary system is HORRIBLE for math, it is quite usefull in approximating distance in everyday life without using a ruler...Originally Posted by topsquark
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)