Question

Two blocks are connected by a very light string passing over a massless and frictionless pulley (the figure ). Traveling at constant speed, the 20.0-N block moves 72.0cm to the right and the 12.0-N block moves 72.0cm downward.

A. During this process, how much work is done on the 12.0-N block by gravity?

B. During this process, how much work is done on the 12.0-N block by the tension in the string?

C. During this process, how much work is done on the 20.0-N block by gravity?

D. During this process, how much work is done on the 20.0-N block by the tension in the string?

E. During this process, how much work is done on the 20.0-N block by friction?

F. During this process, how much work is done on the 20.0-N block by the normal force?

Answer 1

A. During this process, how much work is done on the 12.0- block by gravity?

work = force * distance = 12N*0.72m = 8.64 J

B. During this process, how much work is done on the 12.0- block by the tension in the string?

work = -12N*0.72 = -8.64 J

C. During this process, how much work is done on the 20.0- block by gravity?

zero, because the displacement of the block is perpendicular to the force

D. During this process, how much work is done on the 20.0- block by the tension in the string?

Work = 12N*0.72 = 8.64 J

E. During this process, how much work is done on the 20.0- block by friction?

forces are balanced, so the friction force must be -12N (because the tension is +12N). So

work done by friction = -12*0.72 = -8.64 J

F. During this process, how much work is done on the 20.0- block by the normal force?

zero, because normal force is perpendicular to displacement

(NOTE: total work on each block is zero, because they move at constant speed).