How Fast Does the Center of Mass of a Falling Stick Move?

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Homework Help Overview

The problem involves a stick of mass M and length L that is initially held vertically on a frictionless surface and then pushed at the top, causing it to fall. The objective is to find the speed of its center of mass when it makes an angle θ with the vertical, using both force and energy methods.

Discussion Character

  • Mixed

Approaches and Questions Raised

  • Participants discuss using Newton's laws of translation and rotation, torque equations, and energy methods. Some suggest writing acceleration in terms of angular variables and derivatives, while others question the inclusion of translational motion of the center of mass.

Discussion Status

There are multiple lines of reasoning being explored, including the application of energy methods and the relationship between angular and translational velocities. Participants are actively questioning assumptions about the motion of the stick and the implications of frictionless conditions.

Contextual Notes

Some participants express confusion regarding the horizontal motion of the center of mass and the effects of forces acting on the stick. There is also mention of previous discussions on similar problems, indicating a shared context among participants.

  • #31
Tanya Sharma said:
So, $$ ω^2 = \frac{12g(1-cosθ)}{L(3sin^2θ+1)} $$ looks alright to you ?

I like it :smile:

Tanya Sharma said:
And then

$$ v_{cm} = -\frac{L}{2}(\dotθsinθ) $$

gives $$ ω = -[\frac{3gL(1-cosθ)(sinθ)}{(3sin^2θ+1)}]^\frac{1}{2} $$

Is this fine ?
why minus? And you miss a factor of 2. $$ ω =2\sqrt{ \frac{3g(1-cosθ)}{L(3sin^2θ+1)} }$$

and Vcm=-ωsinθ L/2.
 
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  • #32
Oh ! that was a typo..I meant

$$ v_{cm} = -[\frac{3gL(1-cosθ)(sinθ)}{(3sin^2θ+1)}]^\frac{1}{2} $$

Minus makes perfect sense...
 
  • #33
Tanya Sharma said:
Oh ! that was a typo..I meant

$$ v_{cm} = -[\frac{3gL(1-cosθ)(sinθ)}{(3sin^2θ+1)}]^\frac{1}{2} $$

Minus makes perfect sense...

$$ v_{cm} = -[\frac{3gL(1-cosθ)(sin^2θ)}{(3sin^2θ+1)}]^\frac{1}{2} $$
sinθ has to be squared if you pull it under the square root. But otherwise it is perfect:thumbs: Have a good rest, you deserve it. :smile:

ehild
 
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  • #34
ehild...I am falling short of words to express my gratitude...All I will say is "THANK YOU ehild" .

You rock! :smile:
 
  • #35
Tanya Sharma said:
ehild...I am falling short of words to express my gratitude...All I will say is "THANK YOU ehild" .

You rock! :smile:

You are welcome. That was a challenging problem. I am also exhausted :smile:

ehild
 

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