Question

As a builder lifts a 3.0 kg 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. 60 N, -60 N, 0
• B. -60 N, 60 N, 0
• C. 60 N, 60 N, 0
• D. -60 N, -60 N, 0
• E. 60 N, 60 N, -60 N

Answer 1

The correct option is A 60 N, -60 N, 0

Let, $F$ be the force applied by the builder
Thus, $F=mgF=3\times 10N=30N$
Work done is given by
$W=\overrightarrow{F}\cdot \overrightarrow{\Delta x}W=F\Delta xcos\theta$
Since the force applied by the builder is in the direction of displacement of the brick ($\theta =0^{\circ },cos\theta =1$), work done by the builder is given by
$W_b=Fh=30\times 2=60N-m$
Also, the force applied by the earth is opposite to the direction of displacement of the brick ($\theta ={180}^{\circ },cos\theta =-1$), work done by the builder is given by
$W_e=-Fh=-30\times 2=-60N-m$
Net work done on the brick,
$W_n=W_b+W_e=0$