Question

For the following scenarios identify the work done on the objects that are in parenthesis ( ), label them as either positive(+), negative (-) or zero

A) carrying a (brick)horizontally with decreasing speed

B) Rolling a (barrel) up a sloped hill at constant speed

C) spinning a (ball)on a string overhead in a horizontal cirlce at a constant speed

D) leaning against a (wall)

E) (you) taking a walk in the park

f) A (rock) that is on the side walk, no movement

Answer 1

A) work done = change in mechanical energy.

Here speed of brick is decreasing, $\therefore$ kinetic energy of brick is decreasing. Hence work done on the brick is negative.

B)

Speed or kinetic energy of barrel is constant. But height of barrel is increasing. $\therefore$ potential energy of barrel is increasing. Hence work done on barrel is positive.

C)

Here speed and height of ball is constant. Hence work done is zero.

D)

work done =force * displacement.

Here displacement of the point on which force is acting is zero. Hence work done is zero.

E)

Speed and height of my centre of mass is constant. Net mechanical energy is constant. Hence work done is zero.

F)

Displacement is zero. Hence work done is zero.