# Need some help w/matlab

1. Feb 9, 2009

### bcjochim07

1. The problem statement, all variables and given/known data
A 75.0 g ball with radius 10cm is dropped. I am supposed to write a program that plots y as a function of time, including acceleration due to gravity and drag force of D=0.25*area*velocity.

2. Relevant equations

3. The attempt at a solution
Here's what I tried:

>> n=21;
>> t=zeros(1,n);
>> v=zeros(1,n);
>> y=zeros(1,n);
>> h=0.5;
>> g=-9.80;
>> m=0.075;
>> A=pi*0.10*0.10;
>> for i = 2:n
t(i)=(i-1)*h;
fdrag=0.25*A*v*v;
v(i)=v(i-1)+a*h;
y(i)=y(i-1)+v(i-1)*h+0.5*a*h*h;
end;
??? Error using ==> mtimes
Inner matrix dimensions must agree.
>>

t is time, h is time increment, m is mass, a is total acceleration, A is area

2. Feb 9, 2009

### cjl

Well, the cause for that error is that the default multiplication operation in Matlab is the matrix multiply. For something like this, you do not want the matrix multiplication - you want to extract just the item you are currently working on and use that value. There are some other problems too, but we can start by fixing that.

Also, it is always best to have a thorough understanding of exactly what you are trying to get the code to do in each section before coding - do you have all of that worked out?

3. Feb 10, 2009

### CEL

Complementing what cjl said, you should make:
fdrag=0.25*A*v(i-1)*v(i-1);