In which circumstances is work done?

[mehmet]

Homework Statement

In which of the following circumstances is work done? If work is being done, what is the force acting on the object in each case?

a. A rocket accelerating from the launch pad
b. A ski-tow hauling a skier up a ski slope
c. A skier going down the slope at a steady speed
d. A book resting on a table
e. A rocket traveling through space at a steady velocity remote from any planet or stars.

Relevant Equations

Work done = F x d (force x distance)

So I have honestly no idea what kind of force is acting on the objects in each case, nor if work is being done. I attempted to identify the force for ‘d’ and I wrote the following: “work is being done and forces acting on the book are balanced, the weight of the book acts downwards and the normal reaction acts upwards which causes the book to rest on the table, this is gravitational force as it pulls the mass together and doesn’t push them apart. However, this makes no sense to me.

 

[Doc Al]

forces acting on the book are balanced, the weight of the book acts downwards and the normal reaction acts upwards which causes the book to rest on the table,

For "d", this part is correct. There are two forces acting on the book: Its weight (gravity) acting down and the normal force acting up. And they balance, so there's no net force.

But is work being done? For work to be done, there must be a displacement.

 

[CWatters]

So I have honestly no idea what kind of force is acting on the objects in each case,

For a) ..

There are two forces that must be overcome. One is present even when the rocket is stationary on the pad. The other force is because the rocket has mass and the problem says it's accelerating. What force does the rocket generate to overcome these two?

 

[haruspex]

Problem Statement: In which of the following circumstances is work done?

a. A rocket accelerating from the launch pad

It happens not to matter for answering the question, but the wording is rather loose. It should be clearer about whether we are discussing work done by the rocket engine or work done on the rocket mass (as opposed to its exhaust). If the engine were only firing strongly enough to keep the rocket moving at constant velocity, the engine would still be doing a great deal of work, but none on the rocket.