No Work Done on a Textbook: Explanation of Concept

[Hockey101]

1. Hey everyone! I am currently studying for my final exam and I am trying to review work to start out. When I was reading, it gave me an example of where a person is holding a textbook and the person moves the textbook in the horizontal direction. Under the picture they gave an explanation which says: No work is done on a textbook when carried in the horizontal direction at a constant velocity.



2. None



3. Considering Work depends on displacement, I just thought that if you move a textbook up or down and that is displacement, then why can't there be displacement if you move the textbook (or anything) horizontally?

 

[Fredrik]

Do you know the definition of "work"? I mean, do you know how to calculate it? What do you get if you use that formula to calculate the work performed moving a book horizontally?

 

[Hockey101]

From the site that I am reading from...Work is the "force-displacement product". The formula shows to be: W = FΔs cos θ

 

[Hockey101]

Ok...so is it because since there is a vertical force and the displacement is horizontal, then that's why work is not being done? So with that in mind, if there was a horizontal force and a horizontal displacement, then there would be work being done on the textbook?

 

[Fredrik]

Right. The $\theta$ in that formula is the angle between the force vector and the displacement vector. So when the force and the displacement are perpendicular, we have $\cos\theta=\cos\frac\pi 2=0$. When they are parallel, we have $\cos\theta=\cos 0=1$.

 

[Hockey101]

Thank you so much!