I have no idea how to start this help

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Homework Help Overview

The problem involves a stuntman with a mass of 70 kg swinging from a 4.0-meter-long rope in a vertical circular motion. The discussion focuses on determining the tension in the rope at various points in the swing, specifically at the start, at a height of 1.5 meters, and at the bottom of the arc.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning, Assumption checking

Approaches and Questions Raised

  • Participants discuss using free-body diagrams and the conservation of energy to analyze the motion. There are questions about the angles involved in the calculations and how they relate to the forces acting on the stuntman. Some participants express confusion about deriving velocity without time and the relevance of calculated angles.

Discussion Status

Some participants have offered guidance on using conservation of energy and force summation principles. There is an ongoing exploration of the calculations needed to find tension at different points, with varying interpretations of the angles and forces involved. No consensus has been reached regarding the final answers or the correctness of the calculations.

Contextual Notes

Participants are working under the assumption that the stuntman starts from rest and are navigating the complexities of circular motion and energy conservation without explicit numerical solutions being provided. There is uncertainty about the angles and their implications in the calculations.

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A stuntman whose mass is 70 kg swings from the end of a 4.0-m-long rope along the arc of a vertical circle. Assuming he starts from rest when the rope is horizontal, find the tensions on the rope that are required to make him follow his circular path,(a) at the beginning of his motion, (b) at a height of 1.5 m above the bottom of the circular arc, and (c) at the bottom of his arc.

I think on the first part you take the sum F=0 I think
 
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Use free-body diagrams (there will be angles involved at the 1.5m above bottom point). Use the principle of conservation of energy to find the energy at various heights. Assume he starts at v=0.

The force summation in the direction of the length of the rope must be equal to F = v^2/r as he is undergoing circular motion. There are only two forces involved: rope tension and gravity.
 
so will the angles made on the triangle be 45degrees? and I use them in my calculations?
 
Where are you getting the 45 degrees from? Are you using trigonometry?
 
Well the angle at 1.5m from the bottom of the circle would be arccos(2.5/4) = 0.89 rad --> pi/2 - 089 rad = 0.68 rad is the angle at that moment. Using the equation F = ma, you get F = m*V^2/r. However, I don't see how you can get velocity if you don't have time... Can someone help it out? I'm not getting it either. And I'm not sure when the angle I calculated comes into play
 
mezarashi said:
Use free-body diagrams (there will be angles involved at the 1.5m above bottom point). Use the principle of conservation of energy to find the energy at various heights. Assume he starts at v=0.

The force summation in the direction of the length of the rope must be equal to F = v^2/r as he is undergoing circular motion. There are only two forces involved: rope tension and gravity.

b)ohhh, I see what u mean by using the conservation of energy. So, it is mgh = 0.5mv^2 + mgh,
9.8 * 4 = 0.5 * v^2 + 9.8 * 1.5
v = 7m/s.

Plugging in the equation F = ma,
F = m * v^2 / r

F = 70kg * 7^2 / 4
F = 857N
So the answer is 857 N? I think this number is too large...

For a, the answer should be 0, because when you plug in F = m * v^2/r, since v is 0, F would also be 0
 

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