Cylindrical coordinates: unit vectors and time derivatives

In summary, the conversation discusses finding the unit vectors for cylindrical coordinates in terms of unit vectors in rectangular coordinates and the time derivatives of these unit vectors. The individual asking for clarification is provided with a solution by Chestermiller, showing how the unit vectors simplify and how the solution relates to the original problem.
  • #1
Mason Smith
21
2

Homework Statement


upload_2019-1-28_14-39-18.png


Homework Equations

The Attempt at a Solution


I have found expressions for the unit vectors for cylindrical coordinates in terms of unit vectors in rectangular coordinates.
upload_2019-1-28_14-40-20.png

I have also found the time derivatives of the unit vectors in cylindrical coordinates. However, I am having trouble seeing how it simplifies. For instance, I do not understand how to arrive at the following for the rho hat unit vector.
upload_2019-1-28_14-46-30.png

Can someone enlighten me, please?
 

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  • #2
You say you have the time derivatives of the unit vectors. But, if the derivative of ##\hat{\rho}## is not that given, then you must have made a mistake.
 
  • #3
Mason Smith said:

Homework Statement


View attachment 237940

Homework Equations

The Attempt at a Solution


I have found expressions for the unit vectors for cylindrical coordinates in terms of unit vectors in rectangular coordinates.
View attachment 237941
I have also found the time derivatives of the unit vectors in cylindrical coordinates. However, I am having trouble seeing how it simplifies. For instance, I do not understand how to arrive at the following for the rho hat unit vector.
View attachment 237943
Can someone enlighten me, please?
You have $$\dot{\hat{\rho}}=(-\sin{\phi}\hat{x}+\cos{\phi}\hat{y})\dot{\phi}$$But you already showed that $$\hat{\phi}=(-\sin{\phi}\hat{x}+\cos{\phi}\hat{y})$$
Do you see how it works out now?
 
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Likes Mason Smith
  • #4
Chestermiller said:
You have $$\dot{\hat{\rho}}=(-\sin{\phi}\hat{x}+\cos{\phi}\hat{y})\dot{\phi}$$But you already showed that $$\hat{\phi}=(-\sin{\phi}\hat{x}+\cos{\phi}\hat{y})$$
Do you see how it works out now?
That makes perfect sense. Thank you so much for the insight, Chestermiller! :smile:
 

1. What are cylindrical coordinates?

Cylindrical coordinates are a coordinate system used to describe the position of a point in three-dimensional space. They consist of a radial distance from a fixed point, an angle from a fixed axis, and a height or distance from a fixed plane.

2. What are unit vectors in cylindrical coordinates?

Unit vectors in cylindrical coordinates are vectors that have a magnitude of 1 and are used to specify the direction of a point in the coordinate system. In cylindrical coordinates, the unit vectors are denoted as er, , and ez, representing the radial, angular, and height directions respectively.

3. How are cylindrical coordinates converted to Cartesian coordinates?

To convert from cylindrical coordinates to Cartesian coordinates, we use the following equations:

x = r cos(θ)

y = r sin(θ)

z = z

where r is the radial distance, θ is the angle, and z is the height or distance from the fixed plane.

4. How do you find the time derivatives in cylindrical coordinates?

To find the time derivatives in cylindrical coordinates, we use the chain rule. The time derivatives of the position coordinates are:

𝑟̇ = 𝑟̇0 cos(θ) - 𝑟0𝜃̇ sin(θ)

𝜃̇ = 𝑟0𝜃̇ cos(θ) + 𝑟̇0 sin(θ)

𝑧̇ = 𝑧̇0

where 𝑟̇0, 𝜃̇, and 𝑧̇0 are the time derivatives of the radial, angular, and height coordinates, respectively.

5. Why are cylindrical coordinates useful?

Cylindrical coordinates are useful because they can simplify the mathematical equations needed to describe certain physical phenomena. They are particularly useful for problems involving cylindrical objects or systems with cylindrical symmetry. They also allow for a more intuitive understanding of complex three-dimensional phenomena.

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