A person lifts a 4.9 kg cement block a vertical distance of 1.50 m and then carries the block horizontally a distance of 12.40 m.
How much work is done by the person in the process?
___J
How much work is done by gravity in the process?
___J
how do you do it?
2006-10-31 13:01:40 · 4 answers · asked by tingerpoo 2 in Science & Mathematics ➔ Physics
W = F*h = mgh = (4.9 kg)(9.8 m/s²)(1.5 m) = 72.03 J
The block is lifted AGAINST gravity, so the work done by gravity is -72.03 J. If the block had fallen this same distance, the work done by gravity would have been +72.03 J.
Notice that the distance the person walked is irrelevant in this problem.
2006-10-31 13:07:50 · answer #1 · answered by Anonymous · 0⤊ 0⤋
I agree with Blue. The other answers so far have the right idea, but have minor errors.
To understand the idea of gravity doing negative work, keep in mind that gravity is exerting a force downward and the cement block is moving upward, so it is moving a NEGATIVE distance (relative to the force). Thus gravity is doing negative work.
One other observation: I object to problems like this where you're supposed to decide that no work is done in moving an object horizontally. OF COURSE there is work done when you move the object horizontally. You had to exert a force against the object in order to move it horizontally, so you have a force acting through a distance. The force accelerates the object and it overcomes friction. Of course you also have to exert a force to decelerate the object, and that force is doing negative work (like gravity), so that balances out. But it's misleading to pretend that there are no forces acting through distances when an object is moved horizontally (except in space, maybe ... if, that is, there is "horizontal" in space). There, I've had my say.
2006-10-31 14:00:25 · answer #2 · answered by actuator 5 · 0⤊ 0⤋
WORK DONE BY PERSON:
W=FD=[(4.9kg)(9.8m/s/s)] (12.40m)=595.4J
WORK DONE BY GRAVITY:
W=FD=[(4.9kg)(9.8m/s/s)] (1.50m)=72.03J
work is the product of the FORCE (this could be the weight=mass x accceleration due to gravity) and the COMPONENT OF THE DISTANCE IN THE SAME DIRECTION AS THE MOTION (horizontal or vertical component)..... if there is an angle that bothers you, just use the formula
W=(Fd)(the cosine of the angle)
2006-10-31 13:09:41 · answer #3 · answered by dumb-sel in distress 3 · 0⤊ 0⤋
Work = Force * Distance
W = Fd
W = 4.9(9.80) * 12.4
W = 595.45 j
No work done in moving horizontally (no resistive force).
No work done by gravity (it didn't move the object).
2006-10-31 13:07:07 · answer #4 · answered by teh_popezorz 3 · 0⤊ 0⤋