- #1
bollocks748
- 10
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Radial force of a rotating stick!
1. The Problem
In this problem we want to learn a little bit about what is sometimes called dynamical loading. Our simple system consists of a uniform stick of length L and mass M hinged at one end. We would like to calculate the forces on the (frictionless) hinge when the stick is released from rest at an angle theta_0 with respect to the vertical. You may find it useful to combine work and energy equations with torque (N II) equations.
3.1 Show that the radial force exerted on the stick by the hinge is F_r = Mg/2*(5 cos(theta) - 3 cos(theta_0)), where theta is the angle of the stick with respect to the vertical after it is released.
3.2 Show that the tangential (tangent to the direction of motion, perpendicular to the stick) force exerted on the stick by the hinge is F_t = Mg/4*sin(theta).
2. The attempt at a solution
Alright, I've spent several hours trying to figure this one out, and it's really killing me. I didn't use torque in the solution, but I did set it up initially like this:
t= L/2 * mg sin(theta) = I * alpha
I then set up the energy as the height of the center of mass tilted to theta_0 equal to the height relative to the tilting of theta plus both forms of kinetic energy.
mgcos(theta_0)*(L/2)= mgcos(theta)*(L/2) + 1/2 m v^2 + 1/2 I w^2
substituting V/(L/2) for w, I got 2/3 m v^2 for the total kinetic energy. Using that, I set it up so it was equal to mv^2/(L/2) (the radial force), and I was left with a function that has cos(theta_0) - cos(theta), which is the opposite of what I need.
After trying to figure out something else, I tried making an equation for work and substituting that in for KE instead.
I figured the work to be F_r((L/2)cos(theta)-(L/2)cos(theta_0)), the difference between the heights of the initial positions times the radial force. When substituting that in for 1/2 m v^2 + 1/2 I w^2, I ended up with:
mg(.5cos(theta_0)-.5cos(theta)) = F_r ( 1/2 cos(theta)-1/2cos(theta_0)), which can't be simplified to the equation I need. So I am officially stuck!
Any help would be much appreciated. :-)
1. The Problem
In this problem we want to learn a little bit about what is sometimes called dynamical loading. Our simple system consists of a uniform stick of length L and mass M hinged at one end. We would like to calculate the forces on the (frictionless) hinge when the stick is released from rest at an angle theta_0 with respect to the vertical. You may find it useful to combine work and energy equations with torque (N II) equations.
3.1 Show that the radial force exerted on the stick by the hinge is F_r = Mg/2*(5 cos(theta) - 3 cos(theta_0)), where theta is the angle of the stick with respect to the vertical after it is released.
3.2 Show that the tangential (tangent to the direction of motion, perpendicular to the stick) force exerted on the stick by the hinge is F_t = Mg/4*sin(theta).
2. The attempt at a solution
Alright, I've spent several hours trying to figure this one out, and it's really killing me. I didn't use torque in the solution, but I did set it up initially like this:
t= L/2 * mg sin(theta) = I * alpha
I then set up the energy as the height of the center of mass tilted to theta_0 equal to the height relative to the tilting of theta plus both forms of kinetic energy.
mgcos(theta_0)*(L/2)= mgcos(theta)*(L/2) + 1/2 m v^2 + 1/2 I w^2
substituting V/(L/2) for w, I got 2/3 m v^2 for the total kinetic energy. Using that, I set it up so it was equal to mv^2/(L/2) (the radial force), and I was left with a function that has cos(theta_0) - cos(theta), which is the opposite of what I need.
After trying to figure out something else, I tried making an equation for work and substituting that in for KE instead.
I figured the work to be F_r((L/2)cos(theta)-(L/2)cos(theta_0)), the difference between the heights of the initial positions times the radial force. When substituting that in for 1/2 m v^2 + 1/2 I w^2, I ended up with:
mg(.5cos(theta_0)-.5cos(theta)) = F_r ( 1/2 cos(theta)-1/2cos(theta_0)), which can't be simplified to the equation I need. So I am officially stuck!
Any help would be much appreciated. :-)