Calculating the Mass of a Cycloid Wire with Constant Density | Integration Help

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In summary, the problem involves finding the mass of a wire shaped like a cycloid with constant density k. The integration of k*sqrt(2-2cost) can be simplified using the half angle formulas. The final answer is m=8k. The next step is finding the centroid by integrating x*k*ds and multiplying with 1/m. Another trick involving sin(t)*sin(t/2) can be used to simplify the integration.
  • #1
kasse
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Homework Statement



What is the mass of a wire shaped like the arch x=t-sint, y=1-cost (t from 0 to 2*pi) of a cycloid C that has constant density D=k?

The Attempt at a Solution



I must integrate D*ds. I find that ds=sqrt(2-2cost)dt. Is this wrong? If it's right, I don't know how to integrate k*sqrt(2-2cost).
 
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  • #2
There is a trick. sqrt(1-cos(t)) can be written neatly in terms of sin(t/2). Check out half angle formulas.
 
  • #3
m=8k is my answer then.
 
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  • #4
Dick said:
There is a trick. sqrt(1-cos(t)) can be written neatly in terms of sin(t/2). Check out half angle formulas.

I think it's supposed to be sqrt((1-cos(t))/2) that can be written in terms of sin(t/2), right?
 
  • #5
Right. I didn't mean that they were equal - just that they were closely related.
 
  • #6
Then I'll try to find the centroid, first the x:

Then I must integrate x*k*ds and multiply with 1/m=1/8k.

If I've done right, I'll have to integrate (t-sin(t)*sin(t/2). Is it time for another trick?
 
  • #7
Not as tricky as the first one. t*sin(t/2) is a routine integration by parts. I would write sin(t)*sin(t/2) as 2*sin(t/2)*cos(t/2)*sin(t/2) using double angle formula and do a substitution. Nothing unusual here.
 

1. How do I calculate the mass of a cycloid wire with constant density?

To calculate the mass of a cycloid wire with constant density, you will need to use the formula: M = ρL, where M is the mass, ρ is the density, and L is the length of the wire. In this case, the length of the wire can be found by integrating the arc length of the cycloid curve.

2. What is a cycloid curve?

A cycloid curve is a curve formed by the path of a point on the circumference of a circle as it rolls along a straight line. It is a parametric curve and has applications in physics, engineering, and mathematics.

3. How do I find the arc length of a cycloid curve?

The arc length of a cycloid curve can be found by using the formula: L = 8a, where a is the radius of the generating circle. This formula holds true for all cycloid curves, regardless of the size of the generating circle or the starting point of the curve.

4. What is integration and why is it useful in calculating the mass of a cycloid wire?

Integration is a mathematical technique used to find the area under a curve. In this case, we use integration to find the arc length of the cycloid curve, which is then used to calculate the length of the wire and ultimately, its mass. Integration is useful in this calculation because it allows us to find the length of a curve that cannot be easily calculated using traditional geometry methods.

5. Can the mass of a cycloid wire with constant density be calculated using other methods besides integration?

Yes, it is possible to calculate the mass of a cycloid wire with constant density using other methods such as calculus of variations or the principle of virtual work. However, integration is the most commonly used method and is often the most straightforward and efficient way to calculate the mass of a cycloid wire.

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