diffrent plots if I add the command assume (0<=t) why ?
1 view (last 30 days)
Show older comments
so I want to get laplace transform of f(t)=cos(12t)*exp(-0.4t). the strange this is that if add the command : assume (0<=t) I get the right laplace transform but the plot of f(t) is not starting from zero ,but insted start from -20 . hot to fix this problem ?
clc;clear all;
syms t
assume(0<=t);
f_t3 = piecewise(0<=t,cos(12*t)*exp(-0.4*t));
f_s3=laplace(f_t3);
pretty(f_s3)
fplot(f_t3,[-20 20]);
but if I write the same code but with out assume(0<=t) I get the right plot but not the right laplce transform of f(t) .
clc;clear all;
syms t
f_t3 = piecewise(t<0,0,0<=t,cos(12*t)*exp(-0.4*t));
f_s3=laplace(f_t3);
pretty(f_s3)
fplot(f_t3,[-20 20]);
0 Comments
Answers (2)
Walter Roberson
on 23 Jul 2022
assume(0<=t);
You solemnly swear that t will never be complex or negative
fplot(f_t3,[-20 20]);
but you make t negative down to -20 anyhow.
2 Comments
Paul
on 23 Jul 2022
Let me see if I understand this correctly
syms t
assume(0<=t);
f_t3 = piecewise(0<=t,cos(12*t)*exp(-0.4*t))
So the piecewise is simplified away due to the assumption. laplace() works fine on f_t3. But then fplot() comes along and basically does a subs type of thing over [ -20 20 ] w/o caring about the assumption.
In the second case, fplot has no reason not to work, but laplace() and piecewise() just don't play nice together. I'm not quite sure why that has to be the case, but I'm really not suprised.
Is that about right?
Paul
on 23 Jul 2022
syms t
f_t3 = cos(12*t)*exp(-0.4*t)*heaviside(t);
f_s3=laplace(f_t3)
fplot(f_t3,[-20 20]);
See Also
Community Treasure Hunt
Find the treasures in MATLAB Central and discover how the community can help you!
Start Hunting!