Which Way Will the Spool Roll? Q&A on Newton's Law

AI Thread Summary
When the thread is pulled as indicated, the spool will roll to the right due to the torque generated by the force applied. Newton's Third Law explains that the forces exerted by the thread and the spool are equal and opposite, but understanding the motion requires analyzing both translational and rotational dynamics. Newton's Second Law is crucial for determining how the net forces and torques affect the spool's movement. Participants emphasize the importance of identifying all forces acting on the spool and setting up equations for both translation and rotation to deduce the resulting acceleration. A clear diagram of the forces can aid in visualizing the problem and finding the solution.
ksle82
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Q: if the thread(see attach) is pulled in the direction shown, which way will the spool roll?

i know that the spool will roll to the right according to Newton's law (for every action there's an equal and opposite direction). but there's also a moment apply to the spool (the moment arm is from center to the bottom of the thread). could you say that there's is an equal and opposite moment(with moment arm from center to the top of the thread)? What law states this?
-what else is this question trying to get you to understand, if any?
 

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ksle82 said:
i know that the spool will roll to the right according to Newton's law (for every action there's an equal and opposite direction).
Why do you think Newton's 3rd law is relevant here? All that tells you is that the force that the thread exerts on the spool is equal and opposite to the force that the spool exerts on the thread. So what?

If you want to understand how the spool moves, first identify all the forces acting on the spool. Then apply Newton's 2nd law to both translational and rotational motion and see what you can deduce.
 
Sorry Doc Al for my poor understanding, but I don't quite understand what you said. The force applied on the roll will produce a torque (which cause rotational motion to the right) and traslational motion to the left. But i don't know how to deduce from Newton's Second Law that the roll must rotate to the right instead of left.
 
Harmony said:
Sorry Doc Al for my poor understanding, but I don't quite understand what you said. The force applied on the roll will produce a torque (which cause rotational motion to the right) and traslational motion to the left. But i don't know how to deduce from Newton's Second Law that the roll must rotate to the right instead of left.
thank you for clarifying for my bad word choices(moment and moment arm). Torque is the correct term for it.

I have to agree with Harmony. I don't see how 2nd law fits into this. According to Newton's 2nd Law:
The rate of change of the momentum of a body is directly proportional to the net force acting on it, and the direction of the change in momentum takes place in the direction of the net force.
-http://en.wikipedia.org/wiki/Newton%27s_law
 
As Doc Al says, you need to consider only the forces acting on the spool.
 
ksle82 said:
I have to agree with Harmony. I don't see how 2nd law fits into this.
If ever you want to determine how forces affect the motion of a body, Newton's 2nd law is key.

Start by identifying the forces acting on the spool. Hint: There are two horizontal forces acting on it. In additional to their translational effects, these force also exert torques on the spool.

Once you have properly identified the forces (please draw yourself a clear diagram indicating where the forces act), then set up Newton's 2nd law for (1) Translation and (2) Rotation. You'll get two equations. Solve for the resulting acceleration.

(Another hint: Assume that the spool rolls without slipping.)
 
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