Plot always appears as a straight line

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Hieu Nguyen
Hieu Nguyen 2017-12-4
评论： Roger Stafford 2017-12-5
v = [0:100];
m = 4.65*10.^-26;
y = ((m*v)/(4.14*10.^-21)).*exp((-m*v.^2)/2*4.14*10.^-21);
plot(v,y)
title('Density fucntion at t1 and t2')
Hi, I don't know how to fix this but my graph appears to be a straight line intead of an exponential curve. Thanks a lot!
  1 个评论
Roger Stafford
Roger Stafford 2017-12-5
@Hieu. Here's a demonstration of why you should be careful with how you use parentheses:
x = 60/5*12
y = 60/(5*12)
z = 60/5/12
Notice that in 'y' and 'z' there is division by 12, whereas in 'x' it is multiplied. That is apparently the source of your difficulty with the quantity 4.14*10.^-21. It was multiplied rather than divided because you left out the all-important parentheses.

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采纳的回答

Roger Stafford
Roger Stafford 2017-12-4
Are you sure the second "4.14*10.^-21" doesn't belong in the denominator:
y = ((m*v)/(4.14*10.^-21)).*exp((-m*v.^2)/(2*4.14*10.^-21));
If so, and if you change the range of v to:
v = [0:1000];
you will get a nicely curved plot.
  2 个评论
Hieu Nguyen
Hieu Nguyen 2017-12-4
Let's define that quantity as A. I am trying to plot this function: f(x) = ((mx)/A) * exp((-mv^2)/2A). I still get a straight line.
Greg
Greg 2017-12-4
编辑：Greg 2017-12-4
Did you put the parenthesis around 2.*A?

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更多回答（1 个）

Greg
Greg 2017-12-4
First, I suspect you missed parenthesis is your exp denominator. The magic of using constants. Second, most functions look like straight lines if you stay near the origin. Try plotting further out in v.
v = 0:1000;
m = 4.65*10.^-26;
denom = (4.14*10.^-21);
y = ((m*v)/denom).*exp((-m*v.^2)/(2*denom));
a = axes(figure);
plot(a,v,y);
title('Density function at t1 and t2')
  2 个评论
Hieu Nguyen
Hieu Nguyen 2017-12-4
I see. Thanks a lot! One of the reasons I am not fond of matlab sometimes.
Greg
Greg 2017-12-4
编辑：Greg 2017-12-4
That has nothing to do with MATLAB. That's a mathematical given.
Also, it was unfair to accept my answer. It was identical, and 10 minutes later than, Roger Stafford's answer.
Please unaccept mine and accept his.

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