Applying Force on a Cylinder at an Angle

[Heisenberg7]

A few months ago, our teacher gave us a problem regarding cylinder motion. He asked us to calculate the acceleration of a cylinder that's acted upon by a force F at an angle theta due to the x axis. Now, for some reason, he told us that the cylinder would rotate. Now, I'm not sure if he supposed that this would happen or is that what would actually happen? How could the cylinder rotate if there is no force that creates torque? (Assuming there is no friction)

 

[Baluncore]

How could the cylinder rotate if there is no force that creates torque? (Assuming there is no friction)

Is the cylinder resting on a flat surface?
Why do you assume there is no friction?

 

[Heisenberg7]

Is the cylinder resting on a flat surface?
Why do you assume there is no friction?

Yes, it is resting on a flat surface.
Because our teacher told us to assume that there is no friction.

 

[Baluncore]

Are you certain the force is applied to the axis of the cylinder?

 

[Heisenberg7]

Are you certain the force is applied to the axis of the cylinder?

Definitely. I have drawn the picture. You can see it there.

 

[kuruman]

If there is no friction and the force is applied on the axis, there is no torque about the axis of the cylinder. The cylinder will slide along the surface as is it were a rectangular block. Perhaps you can ask your teacher to clarify this point.

 

[berkeman]

Because our teacher told us to assume that there is no friction.

Can you post the exact wording of the question? Is that the only figure that was supplied with the question?

 

[Heisenberg7]

If there is no friction and the force is applied on the axis, there is no torque about the axis of the cylinder. The cylinder will slide along the surface as is it were a rectangular block. Perhaps you can ask your teacher to clarify this point.

The thing is, I got this problem on a competition. And I assumed that this would happen too (no rotation). But, in the solution they assumed that the cylinder would rotate and obviously since I did not assume that, I lost a ton of points. But when I got home and took a look at the solution, it was pretty obvious to me that it would be impossible to have torque in this case. What's even weirder is that they called it F_rot (the force that causes rotation) without any context at all. I even asked the guy who ran the competition if there is friction during the solving part (it wasn't mentioned) and he told me that there is no friction. So, it's fair to say that they made a mistake here.

 

[kuruman]

Is there a web reference for this competition? Competitions are not necessarily flawless. One of our very own, @haruspex, has provided a compilation of flawed questions in the Australian Physics Olympiad appearing from 2009 to 2019 here. I don't know why the compilation stops at 2019, but I like to think that, thanks to @haruspex's efforts, the organisers of the competition mended their ways and implemented better quality control. Even better, they might have entrusted @haruspex with the proofreading.