In which of the following scenarios is work done?

Question

a. A weightlifter holds a barbell overhead for 2.5 s.

b. A construction worker carries a heavy beam while walking at constant speed along a flat surface.

c. A car decelerates while traveling on a flat stretch of road.

d. A student holds a spring in a compressed position.

Answer 1

(A) No work, no change in overall elevation of the weight.

(B) If there is friction between the feet of the construction worker and the floor, the friction force creates negative work. Since friction opposes line of motion.

(C) The friction force on the tires create negative work. In order for tires to move they need to have traction, traction is generated via the friction between the ground and the rubber tire.

(D) There is work in the spring since we are compressing it. The only time we do not have work on a spring is when we hold the spring at natural unstretched state.

Answer 2

In which of the following scenarios is work done?

Lets consider each of the scenarios in light of the definition of work.

Work is said to be done if a net force acts on a body as the body displaces [in the direction of force].

W_ork = F_net•dcosθ (as long as F and d are in exactly the same or opposite direction, cosθ=1)

a. A weightlifter holds a barbell overhead for 2.5 s.

As long as there is force but no displacement, work = 0

b. A construction worker carries a heavy beam while walking at constant speed along a flat surface.

While the weight of the beam and friction are forces, they are balanced by other forces (constant velocity) so there is no net force even though there is displacement so W=0

c. A car decelerates while traveling on a flat stretch of road.

Deceleration only occurs when there is net force and when objects are decelerating, there is displacement. Some work is done in this case.

d. A student holds a spring in a compressed position.

Once again there are forces but no displacement so no work is done. W=0

Mr. A