The pendulum is released from rest with θ = 30deg

In summary, the conversation is about finding the equation of motion and the speed of a pendulum released from rest with an initial angle of 30 degrees. After discussing various solutions and concepts, it is concluded that the equation of motion is given by ##\ddot θ = -4.905sinθ rad/s^2## and the speed is determined to be ##6.26\sqrt {cosθ - 0.866} m/s##. There is also a mention of using torque and energy conservation in the solution process.
  • #1
Alexanddros81
177
4

Homework Statement


13.54 The pentulum is released from rest with θ = 30deg. (a) Derive the equation of motion
using θ as the independent variable. (b) Determine the speed of the bob as a function of θ.
Fig P13_53.jpg


The solutions given in the textbok are a) ##\ddot θ = -4.905sinθ rad/s^2##
b) ##6.26\sqrt {cosθ - 0.866} m/s##

Homework Equations

The Attempt at a Solution



Pytels_Dynamics080.jpg
[/B]
I understand something is not correct.
I guess the positive direction for the bob now is to the left.
 
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  • #2
Where did the 7.5 come from? You have not been given the tension, you have been given that the bob is released from rest.
 
  • #3
part (a) think torque = I times dω/dt
part (b) think energy conservation.
 
  • #4
Hi! After doing some research on the web I came up with the following solution (by an engineer)
I don't understand though why he took ##a_T## to be ##-rα##. Does it has to do with the
the tangential and radial axes sense?
He mentions ##F=ma_T## as an inertial force.

Pytels_Dynamics081.jpg


Pytels_Dynamics083.jpg
 
  • #5
Alexanddros81 said:
took ##a_T## to be −rα
If you start with arc length = radius x arc angle then differentiate twice wrt time you will get this. The sign depends on your conventions. In vectors, ##\vec {a_T}=\vec r\times\ddot{\vec\theta}##.
You can get the second result (velocity) by integrating or by work conservation.
 
  • #6
Alexanddros81 said:
Hi! After doing some research on the web I came up with the following solution (by an engineer)
I don't understand though why he took ##a_T## to be ##-rα##. Does it has to do with the
the tangential and radial axes sense?
He mentions ##F=ma_T## as an inertial force.

View attachment 212931

View attachment 212937
Your last result v = 6.26√(cosθ - cos30deg) is correct.
 

1. What is the initial angle of the pendulum?

The initial angle of the pendulum is 30 degrees.

2. How is the pendulum's motion affected by the initial angle?

The initial angle of the pendulum affects the amplitude and period of its motion. A larger initial angle will result in a larger amplitude and longer period, while a smaller initial angle will result in a smaller amplitude and shorter period.

3. What factors determine the speed of the pendulum?

The speed of the pendulum is determined by the length of the pendulum, the acceleration due to gravity, and the initial angle. A longer pendulum will have a slower speed, while a shorter pendulum will have a faster speed. The acceleration due to gravity, which is typically 9.8 m/s^2, also affects the speed of the pendulum. Finally, the initial angle also plays a role in determining the speed of the pendulum.

4. How does the pendulum's motion change over time?

The pendulum's motion will gradually decrease in amplitude due to energy loss from friction. This will result in a decrease in the pendulum's speed and a shorter period. Eventually, the pendulum will come to rest at its equilibrium position.

5. Can the initial angle of the pendulum be changed after it is released?

Yes, the initial angle of the pendulum can be changed after it is released. This will result in a different amplitude and period of motion. However, the energy loss due to friction will still affect the pendulum's motion over time.

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