David Morin's Rope around a Pole example

• BeyondBelief96
In summary, the rope can exert a maximum force of T0N on the boat if the coefficient of static friction is µ.

BeyondBelief96

1. A rope wraps an angle θ around a pole. You grab one end and pull with a tension T0. The other end is attached to a large object, say, a boat. If the coefficient of static friction between the rope and the pole is µ, what is the largest force the rope can exert on the boat, if the rope is to not slip around the pole?The question I have is really just about the diagram. Here it shows the two tension forces at both ends, my question is how did he get the angle there to be sin(d(theta)/2)? Well, more importantly the d(theta)/2, I understand why its sin. I was unable to derive that myself, so I looked at the solution to see what angle I needed to use, and from there I was able to do the problem, but I still need to know how he got that angle.

My thoughts are that the angles on both sides of those tension forces contribute an angle of theta/2 and so that add up to being a total of theta. but I guess I still don't know why geometrically.

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Hello @BrandonBerisford,

Welcome to PF!

Try to think in terms of "similar triangles."

You haven't, as of yet, found what the other similar triangle actually is.

Suppose the pole has a radius r. Now imagine a right triangle with hypotenuse r (feel free to look at your diagram while doing so). How do the other legs and the angle θ fit in?

collinsmark said:
Hello @BrandonBerisford,

Welcome to PF!

Try to think in terms of "similar triangles."

You haven't, as of yet, found what the other similar triangle actually is.

Suppose the pole has a radius r. Now imagine a right triangle with hypotenuse r (feel free to look at your diagram while doing so). How do the other legs and the angle θ fit in?
Ahh! so i cut the big arc in half with a radius of r, making two right triangles with each having a small angle of d(theta)/2 and drawing out the triangles from the sides with the tensions, it's a similar triangle with those and comparing angles we can say its the same?

collinsmark
BrandonBerisford said:
Ahh! so i cut the big arc in half with a radius of r, making two right triangles with each having a small angle of d(theta)/2 and drawing out the triangles from the sides with the tensions, it's a similar triangle with those and comparing angles we can say its the same?

I think that should put you on the right track!

But to be sure, it wouldn't hurt to show the rest of your work for the remainder of the problem. There's some integration left, and perhaps a small-angle approximation that might fit in there somewhere.

BrandonBerisford said:
Ahh! so i cut the big arc in half with a radius of r, making two right triangles with each having a small angle of d(theta)/2 and drawing out the triangles from the sides with the tensions, it's a similar triangle with those and comparing angles we can say its the same?

I can't quite picture what you are saying because I seem to be drawing it wrong and not getting the same result. Can anyone supplement this with a picture, please?

Electrowonder said:
I can't quite picture what you are saying because I seem to be drawing it wrong and not getting the same result. Can anyone supplement this with a picture, please?

Note that the post is from last year.

PeroK said:
Note that the post is from last year.

Was I supposed to make a new topic about the drawing portion?

Electrowonder said:
I can't quite picture what you are saying because I seem to be drawing it wrong and not getting the same result. Can anyone supplement this with a picture, please?
Draw the bisector of the angle dθ and project the two arrows backwards so that these three lines meet. Do you see some similar triangles?

Electrowonder
Oh I get it now, thank you!

1. What is the purpose of David Morin's Rope around a Pole example?

The purpose of this example is to illustrate the concept of rotational motion and how it relates to the conservation of energy.

2. How does the rope around a pole affect the motion of the pole?

The rope provides a force that causes the pole to rotate around its axis. This force also creates tension in the rope, which must be balanced by the weight of the pole to maintain equilibrium.

3. What is the relationship between the rope's tension and the pole's weight in this example?

The tension in the rope is equal to the weight of the pole, as this is necessary for the pole to remain in rotational equilibrium.

4. How does the rope's length affect the motion of the pole?

The shorter the rope, the faster the pole will rotate around its axis. This is because a shorter rope creates a smaller lever arm, resulting in a larger torque and angular acceleration.

5. What is the significance of the pole's moment of inertia in this example?

The pole's moment of inertia, which is a measure of its resistance to rotational motion, affects how quickly the pole will rotate in response to the force from the rope. A larger moment of inertia means the pole will rotate more slowly, while a smaller moment of inertia results in faster rotation.