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FAQ (Frequently Asked Questions)
Category:
CDS 101/110 Fall 2004
Identifiers: H0 H1 H2 H3 H4 H5 H6 H7 H8 L0.0 L1.1 L1.2 L2.1 L2.2 L2.3 L3.1 L3.2 L4.1 L4.2 L5.1 L5.2 L6.1 L7.1 L9.1
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Is there a better way to change inputs to a system in Matlab (gains) and run the examples in an automated fashion?
Submitted by: jianghao
Submitted on: October 4, 2004
Identifier:
L2.1
Sure you can. By using the Bode plot, you can get the result of system response under different input frequency automatically(here assume the inputs can be decomposed into sine functions with different frequencies). To get Bode plot, you only need to tell Matlab the Transfer Function of the system(will learn very soon), then everythins is left for Matlab to run. Matlab can also generate output for other kinds of input pattern, eg. step function.
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For the model on slide 13, which line is the foxes, and which is the rabbits?
Submitted by: asa
Submitted on: October 4, 2004
Identifier:
L2.1
The lower-amplitude, blue line is the rabbits; the higher-amplitude green line is the foxes. You can tell because the rabbit line peaks first in each cycle, which makes sense since it's the prey.
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Why isn't there a term for the rabbit death rate besides being killed by the foxes?
Submitted by: asa
Submitted on: October 4, 2004
Identifier:
L2.1
The rabbit birth rate is really the "net" birth rate -- it incorporates the birth rate as well as the death rate if there were no foxes. Similarly for the foxes; the death rate is the "net" death rate.
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In the predator-prey example, where is the fox birth rate term?
Submitted by: waydo
Submitted on: October 4, 2004
Identifier:
L2.1
In the model from class it is assumed that the fox birth rate depends only on the supply of rabbits for food, so it is included as the interaction term.
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I don't understand why you don't use the rabbit death rate or the fox birth rate
Submitted by: aotang
Submitted on: October 4, 2004
Identifier:
L2.1
For this question, one can refer to the answer provided by asa for last question. Another possible understanding, which is adopted in the book, is to assume rabbits never die other than being eaten. This is ok reasonable especially for the strong interaction case. Also note the the last term of the second equation can be viewed as "somewhat" model for birth rate for foxes, which is affected by the number of prey they can have.
As admitted in the book,"This simple model makes many simplifying assumptions....--but it often is sufficient to answer simple questions about the system." It is definitely good to think carefully what are the underlying assumptions models make. However, it does not mean the most complete model is the best because they may be too complicate to give out any useful insights. Hence one should target at the right level of simplification which suits his/her needs best.
"All models are wrong(inaccurate), some are useful".
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Example 1: The point moving back and forward, what does it map to on the car, and what is the dampening?
Submitted by: haomiao
Submitted on: October 4, 2004
Identifier:
L2.1
The point moving back and forth corresponds to the bottom of the tire. The input forcing is the ground, so when the ground goes up and down it also forces the tire up and down. Obviously this is a very simplified model which wouldn't take into account gravity. The damping would represent the fluid in the suspension shock absorber.
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In the mass-spring system modelling the car, one of the springs is fixed to a wall. How does that model the car when that "spring" on the car is connected to the chassis?
Submitted by: waydo
Submitted on: October 4, 2004
Identifier:
L2.1
If the weight of the car is very large compared to the effects of the driving force on the suspension, then modelling the car as a fixed wall with springs is not a bad approximation. However if it is not the case, the boundary conditions you would have in order to model a suspension system in a car may be diffrent. You may leave out the wall and have the weight of the car supported by a spring.
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