Thread: Work

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?

Originally Posted by babygirl

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.

The angle between the weight and displacement is 180 degrees.


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

-Dan

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

Originally Posted by topsquark

First note that we don't need to know that the book is moving at a constant velocity.

The angle between the weight and displacement is 180 degrees.


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

I think that an explanation of why is in this
unit system, and that using would give an answer
in (or what ever the units are - its >30 years since
I used them).

-Dan

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

Quite - Mars probes have been lost over this sort of thing (revenge of the
killer-slugs (kilo-slugs) I expect).

RonL

Originally Posted by CaptainBlack

Quite - Mars probes have been lost over this sort of thing (revenge of the
killer-slugs (kilo-slugs) I expect).

Heard that story a long time ago from my father. So funny, how embarrasing is that

Originally Posted by CaptainBlack

I think that an explanation of why is in this
unit system, and that using would give an answer
in (or what ever the units are - its >30 years since
I used them).

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.

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

-Dan

Originally Posted by topsquark

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.

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 is
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

Originally Posted by ThePerfectHacker

Heard that story a long time ago from my father. So funny, how embarrasing is that

Good grief - now I feel old

RonL

Originally Posted by CaptainBlack

Its a dreadful feature of the Customary/Imperial Unit system that the pound is
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

Oh dear, I didn't know that. Okay, so we're both right!

-Dan

(I knew there was a reason we were switching over to metric!)

Originally Posted by topsquark

(I knew there was a reason we were switching over to metric!)

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...

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)