# I am trying to do this problem, but I'm having some issues with steps 2 3 and 4. I thought I did it right, but the values stored in the "velocites" vector all share the same value and they shouldn't. Any help would be appreciated

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Caleb Wilson on 31 Aug 2017
Edited: John BG on 6 Sep 2017
InitialVelocity = 2 * rand(1) - 1;
F = 1; tau = 1; N = T/dt; T = 10; dt = 0.01; n = 0;
velocities = zeros(1,N); for i = 1:N n = n + 1; Pc = dt/tau; X = rand; if X < Pc vel = ((n-1)*dt) + F * dt; %Collision occurs else vel = F * dt; %No collision occurs end velocities(i) = vel; end
velocities
Jan on 31 Aug 2017
Edited: Jan on 31 Aug 2017
Please use the "{} code" button for posting readable code. Increase the size of the screenshot, it is a pain to read the small characters.

Jan on 31 Aug 2017
Edited: Jan on 31 Aug 2017
Your assumption is wrong: the values stored in the "velocites" vector do not share the same value. About 1 of 100 is different. Simply try it:
all(velocities == velocities(1))
You get FALSE in the vast majority of cases.
But at least most of the elements of velocities contain the same value , because this is the expected result of the code. In 99% of the cases the elements are set to:
vel = F * dt;
Velocity is force times time. Hm. Perhaps you mean:
a = F / m; % With m is the mass and a the acceleration
vel = vel + a * dt

John BG on 5 Sep 2017
Edited: John BG on 6 Sep 2017
Hi Caleb
I have reproduced the steps listed in your question, not to optimise code or speed, but to precisely reproduce the experiment.
1.
start variables
clear all;clc;close all
dt=.01;tau=1;m=1;N=1e3;T=N*dt
F=1; % N = kg*m/s^2
pcol=dt/tau % collision probability
n=[1:1:N]; % we can use it but k comes handy
t=[0:dt:T-dt]; % not used
2.
Single run
v=zeros(1,N);
v(1)=randi([-100 100],1,1)/100; % step 1)
for k=2:1:N
X=randi([0 1],1,1);
if X v(k)=F*dt; end
if ~X v(k)=v(k-1)+F*dt; end
end
3.
100 runs
N2=100;
Vn=zeros(N2,N);
for s=1:1:N2
v=zeros(1,N);
v(1)=randi([-100 100],1,1)/100;
for k=2:1:N
X=randi([0 1],1,1);
if X v(k)=F*dt; end
if ~X v(k)=v(k-1)+F*dt; end
end
Vn(s,:)=v;
end
.
zoomed image, full runs are N=1e3 samples long.
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John BG
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