Question

As a builder lifts a $3.0kg$ brick at a steady speed from the ground to a platform $2.0$ meters high.
How much work is done on the brick by the builder and by the earth while it is being lifted, and what is the net work done on the brick by all forces while it is being lifted?

• A. Work Done by Builder $=60Joules$, Work Done by Earth $=-60Joules$, Net work $=0Joules$
• B. Work Done by Builder $=-60Joules$, Work Done by Earth $=60Joules$, Net work $=0Joules$
• C. Work Done by Builder $=60Joules$, Work Done by Earth $=0Joules$, Net work $=60Joules$
• D. Work Done by Builder $=-60Joules$, Work Done by Earth $=0Joules$, Net work $=-60Joules$
• E. Work Done by Builder $=60Joules$, Work Done by Earth $=60Joules$, Net work $=120Joules$

Answer 1

The correct option is A Work Done by Builder $=60Joules$, Work Done by Earth $=-60Joules$, Net work $=0Joules$

We know , work done is given by ,

$W=Fd\cos \theta$ ,

where $\theta$ is the angle between force $F$ and displacement $d$ ,

given $m=3.0kg,d=2.0m,$

when builder lifts the brick , $\theta =0$

therefore , work done by builder , $W=Fd\cos 0=mgd\cos 0$

or $W=3.0\times 10\times 2.0\times 1=60J$

for the force by earth $\theta =180$

work done by earth , $W^{\prime }=Fd\cos 180=mgd\cos 180$

or $W^{\prime }=3.0\times 10\times 2.0\times -1=-60J$

now net work done $=W+W^{\prime }=60-60=0J$