Bob lifts a 5kg box 2 m off the ground, walks 7 m to the left and then gently places the box back on the ground. Assume that during each segment the box moves at constant speed.
a calculate the work done in each part of moving box
b. what is net work.
c. what is another path that requires the same amount of work

Question

Bob lifts a 5kg box 2 m off the ground, walks 7 m to the left and then gently places the box back on the ground. Assume that during each segment the box moves at constant speed.
a calculate the work done  in each part of moving box
b. what is net work.
c. what is another path that requires the same amount of work

anonymous · Answer

So I know that W=Fd and F is equal to Ma
I know lifting distance is 2m but the force I am assuming is mass * gravity but unsure

anonymous · Answer

I got 98 for work for lifting and -98 dropping

anonymous · Answer

However I cannot figure out from walking across because acceleration is not gravity

anonymous · Answer

I figured out the path needed to do the same amount of work and that was walking back to the orgin

fifciol · Answer

Work = dot product of F and d in other words W=F*d*cos theta where theta is angle between force and displacement
when you walk across with constant speed there is only one force acting on you - gravity what is cos of theta? assuming force and displacement being perpendicular?

anonymous · Answer

cos(pi)

fifciol · Answer

no look at the picture below:
|dw:1375786268882:dw|

fifciol · Answer

what is now angle?

anonymous · Answer

pi/2

fifciol · Answer

yup

anonymous · Answer

however still a little confused

anonymous · Answer

what is acceleration

fifciol · Answer

so cos of pi/2 is ?

fifciol · Answer

there is no acc when you move with constant speed

anonymous · Answer

So is the force zero

fifciol · Answer

no this situation really should have looked like this:
|dw:1375786472394:dw|

fifciol · Answer

because there's no acc in up down direction

fifciol · Answer

but what matters in your question is the angle between force and direction of motion

anonymous · Answer

gravity

anonymous · Answer

cos(pi/2) is the angle that the box is lifted and dropped

fifciol · Answer

it's 0 and pi

fifciol · Answer

when you lift object gravity does negative work( force is 180 degrees out of phase with displacement, cos 0f pi is -1) when you drop it gravity does positive work ( cos 0 =1)

fifciol · Answer

when you do negative work gravity does positive work and vice versa

fifciol · Answer

when force is perpendicular to the direction of motion this force never does work ( cos pi/2 =0)

anonymous · Answer

so no work is done is basically what you told me

fifciol · Answer

http://ocw.mit.edu/courses/physics/8-01-physics-i-classical-mechanics-fall-1999/video-lectures/lecture-11/ the best teacher and explanation I've ever seen :)