Simple question (help?) -- Forces to spin up a wheel

[lc99]

Homework Statement

A wheel is spun up from rest by application of a force applied to the rim of the wheel. Consider the wheel to be in the plane of the paper, rotating clockwise. This force must point in what direction.
1) at the bottom of the wheel, pointing right
2)at the top, pointing left
3) towards the center of wheel
4)into page
5) impossible to tell with the information

Homework Equations

T = Fxr

The Attempt at a Solution

I eliminated the first 4, but at the same time, i don't think the answer is impossible...
1) I don't think bottom of the wheel would be correct because if it points to the left at the bottom then we also have centripetal force inward to the center. The resultant torque would be turning the wheel counterclockwise.
2) this is the same as my answer for 1), it is still counterclockwise.
3) It doesn't have to be towards the center for it to rotate clockwise
4) i don't think this will affect the rotation
5) ANS?

 

[haruspex]

I don't think bottom of the wheel would be correct because if it points to the left

It says to the right.

centripetal force inward to the center

Forces internal to the wheel will provide the centripetal forces needed for its parts, but the wheel as a whole (i.e. its mass centre) has no centripetal acceleration.

The resultant torque would be turning the wheel counterclockwise.

This is the relevant point.

 

[lc99]

It says to the right.

Forces internal to the wheel will provide the centripetal forces needed for its parts, but the wheel as a whole (i.e. its mass centre) has no centripetal acceleration.

This is the relevant point.

Ohhh. The answer is 4)into the page. I forget the torque includes the direction of force. So, since the object is clockwise, I use the right hand rule to figure out that the torque is into the page... Which means force is into the page, right?

 

[haruspex]

the torque is into the page.

Yes.

Which means force is into the page, right?

No. Torque is cross product of force and lever arm, so perpendicular to the force.

 

[lc99]

Yes.

No. Torque is cross product of force and lever arm, so perpendicular to the force.

I'm not sure . I'm convinced that the answer is E) . Unless the answer is towards the center of the wheel, which it doesn't have to be..

 

[haruspex]

I'm convinced that the answer is E)

It is, but there were flaws in some of your reasons for eliminating (1). The last sentence, that it would turn the wrong way, was enough.

 

[NascentOxygen]

I'm convinced that the answer is E)

lc99, precisely why are you agreeing that the required direction of the force is impossible to determine here?

 

[haruspex]

Precisely why are you agreeing that the required direction of the force is impossible to determine here?

Was that intended for me?

 

[NascentOxygen]

Was that intended for me?

No. ☺ I am asking lc99.

 

[lc99]

lc99, precisely why are you agreeing that the required direction of the force is impossible to determine here?

Through elimination, I thought the answer would be E). But, if i were to try to explain why, would it be because the direction of the force could any of many options? It would need to point upper left (or I am not sure how to explain), but i can draw a picture?

 

[haruspex]

would it be because the direction of the force could any of many options?

yes, but identifying two would do.

 

[NascentOxygen]

if i were to try to explain why, would it be because the direction of the force could any of many options?

The requirement of "a force applied to the rim of the wheel" could be satisfied by a force from any of an infinite number of directions and applied at an appropriate point on the rim.

It would need to point upper left (or I am not sure how to explain), but i can draw a picture?

I don't follow, but this doesn't sound right.