# Q & A: force and torque

Q:
This one could be a little complicated but the fate of my physics understanding depends on the answer. So I understand most of the principles of torque and how it works however what confuses me is how forces perpendicular to the radius act on the linear movement of an object. For instance, if you have a puck traveling on a frictionless surface and you apply a force at the edge of the puck perpendicular to the radius. Compare this to a similar puck but you apply the force at the radius. Does the first force cause a torque as well as a linear acceleration on the puck? If it does how does the linear acceleration of 1st puck compare with the acceleration of the 2nd puck? I hope you can answer this because I posed the question in physics class and my teacher didn't know and we tried to set up an experiment with an air hockey table but the mass of puck was not large enough to see the results clearly.thanks
- Erik (age 16)
Michigan
A:
"Does the first force cause a torque as well as a linear acceleration on the puck?"
Yes, exactly.
"If it does how does the linear acceleration of 1st puck compare with the acceleration of the 2nd puck? "
Same F, same m, therefore same a.

Mike W.

(published on 11/22/2010)

## Follow-Up #1: torque

Q:
Hello Mike, May I ask how did YOU know that the force will produce both linear acceleration AND moment/torque(do you know an experiment that proved it or you read it somewhere etc.)..because frankly I tried thinking about it and, to me, it didnt make any sense! May you please convince me. Also, I'm a bit confused on when to use the term "moment" and "torque". I tried wikipedia but unfortunately it didnt do me any good: http://en.wikipedia.org/wiki/Torque Why are the terms different in Physics and Mechanical Engineering?shouldnt the technical stuff in both these fields be the same?! or Im missing something? Thanks Alot.
- Anonymous (age 16)
A:
The magnitude of the torque is simply the product of the force times the distance from the chosen "center", multiplied by the sine of the angle between the force direction and the direction to the center. Here I assumed you were referring to total torque about the center of the disk, so that direction is just the radial direction. You thus specified that the angle is 90°.

This is an demonstration that we routinely do in our beginning mechanics course, with pucks sliding on dry ice, with one end of a string attached  to the middle of one puck and the other end wound around the outside of the other puck. So it's very vivid in my memory.

Usually in this area physicists use "moment" in the phrase "moment of inertia", where it does not mean torque at all.  I was unaware that the word is sometimes used in the phrase "moment of force", where it does mean the same as torque.

Mike W.

(published on 12/31/2010)