Question

Select all that apply.
No work is accomplished in which of the following situations?

a boy carried a 4.0-newton box 3.0 meters across the room at a constant velocity.
a boy pushes on a box weighing 4.0 newtons and it doesn't move.
a boy lifts a 4.0-newton box to a height of 1.0 meter.

Answer 1

In the second one. Because to have work, needs to exist displacement. As he the boy pushes the box but it doesn't move, there is not work done.

Answer 2

Answer:

a boy pushes on a box weighing 4.0 newtons and it doesn't move.

Explanation:

The definition of work is the following:

$W=Fd cos \theta$

where

F is the force applied

d is the displacement

$\theta$ is the angle between the direction of the force and of the displacement

From the formula, we notice that if there is no displacement (d=0), then the work done is zero (W=0).

The choices given are:

a boy carried a 4.0-newton box 3.0 meters across the room at a constant velocity.  --> in this case, work is done (displacement is non-zero)

a boy pushes on a box weighing 4.0 newtons and it doesn't move.  --> in this case, NO work is done (because the displacement is zero)

a boy lifts a 4.0-newton box to a height of 1.0 meter. --> in this case, work is done (displacement is non-zero)

So, the correct choice is

a boy pushes on a box weighing 4.0 newtons and it doesn't move.