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

In summary: 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.
  • #1
lc99
161
3

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?
 
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  • #2
lc99 said:
I don't think bottom of the wheel would be correct because if it points to the left
It says to the right.
lc99 said:
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.
lc99 said:
The resultant torque would be turning the wheel counterclockwise.
This is the relevant point.
 
  • #3
haruspex said:
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?
 
  • #4
lc99 said:
the torque is into the page.
Yes.
lc99 said:
Which means force is into the page, right?
No. Torque is cross product of force and lever arm, so perpendicular to the force.
 
  • #5
haruspex said:
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..
 
  • #6
lc99 said:
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.
 
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  • #7
lc99 said:
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?
 
Last edited:
  • #8
NascentOxygen said:
Precisely why are you agreeing that the required direction of the force is impossible to determine here?
Was that intended for me?
 
  • #9
haruspex said:
Was that intended for me?
No. ☺ I am asking lc99.
 
  • #10
NascentOxygen said:
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?
 
  • #11
lc99 said:
would it be because the direction of the force could any of many options?
yes, but identifying two would do.
 
  • #12
lc99 said:
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.
 

1. What is the purpose of spinning up a wheel?

The purpose of spinning up a wheel is to create rotational motion or torque, which can then be used for various applications, such as powering a machine or vehicle.

2. What forces are involved in spinning up a wheel?

The primary force involved in spinning up a wheel is the applied force, which can be exerted by a person or a machine. Other forces that may come into play include friction, air resistance, and gravitational force.

3. How can I calculate the necessary force to spin up a wheel?

The necessary force to spin up a wheel can be calculated using the formula F = Iα, where F is the applied force, I is the moment of inertia of the wheel, and α is the angular acceleration. This formula is based on Newton's Second Law of Motion.

4. Can the direction of the applied force affect the speed of the spinning wheel?

Yes, the direction of the applied force can affect the speed of the spinning wheel. If the force is applied in the same direction as the rotational motion, it will increase the speed of the wheel. If the force is applied in the opposite direction, it will decrease the speed of the wheel.

5. Are there any safety precautions to consider when spinning up a wheel?

Yes, it is important to be cautious when spinning up a wheel, especially if using a machine or tool to apply the force. Make sure to follow proper safety guidelines and wear appropriate protective gear. Also, be mindful of the potential dangers of objects flying off the wheel due to the high speed and force involved.

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