Matlab gives me a blank plot?

Hi! I am trying to write a script that plots every X and Y position of a ball after it is thrown horizontally at a wall that is slanted 60 degrees from the x axis. The ball then bounces of the wall and the plot is basically like a projectile motion plot. There is a picture and more information the the vector mechanics for engineers: dynamics 11 edition book, problem 171 in chapter 13. I do not know why I keep getting a blank plot. I looked at the results and the Y axis is what it should be at the end (0 because it lands), so I do not understand why it is returning a blank plot. I also tried putting hold on and off before and after the for loop respectively, as well as at the beginning and end of the for loop. I have also tried plot(X,Y) instead of line command and I have tried doing plot(X,t) and (Y,t) but I am not getting anything for the plot. The weirdest thing is that it works on my friend's computer and not mine. I decided to copy and paste examples of random plots online and those work, but this script gives me a blank plot for some reason.

Here is my script: Yo=3; %Initial Height

Xo=0; %Initial Distance in the X direction

Vox=25; %Initial Horizontal Velocity

r=0.9; %Coefficient of Restitution

Vw=0; %Velocity of wall before being hit by ball

Vw2=0; %Velocity of Wall after being hit by ball

g=32.2; %Gravity % Wall angled at 60% from the X-axis

% Find the distance, d, from the foot of the wall to the point B where the % ball will hit the ground after bouncing off the wall.

Vt=Vox*sind(30); %Initial Velocity Tangential to Wall

Vn=-Vox*cosd(30); %Initial Velocity Normal to Wall % Vt2=Vo2*sind(30-alpha)=12.5 which is the tangential velocity after % hitting the wall.

% r=(Vw2-Vn2)/(Vn-Vw)is the coefficient of restitution along normal to wall. % Simplified, it is: Vo2cosd(30-alpha)=19.48=Vn2

%Vt2/Vn2= tand(alpha-30)=12.5/19.48. Therefore:

alpha=atand(12.5/19.48)+30;

%Plug into Vn2 to get: Vo2=19.48/cosd(alpha-30); %Vo2 is the initial Velocity of the ball after it hits the wall

Voy2=Vo2*sind(alpha); %Voy2 is the initial Velocity in the Y direction after hitting the wall

Vox2=Vo2*cosd(alpha); %Vox2 is the initial Velocity in the X direction after hitting the wall

hmax=(Voy2)^2/(2*g); %Maximum Height the ball reaches

T1=Voy2/g; %Time from A to Maximum Height

T2=sqrt((2*(hmax+Yo))/g); %Time from Maximum Height to B (when it lands)

Ttotal=T1+T2; %Total time from when the ball hits the wall until it lands

for t=0:0.01:Ttotal Y=Yo+Voy2*t-0.5*g*t^2 %Y position of ball X=Xo+Vox2*t %X position of ball line(X,Y) end title('X Position vs. Y Position of Ball')

xlabel('X Position of Ball') %label for X-axis

ylabel('Y Position of Ball') %label for Y-axis

Thank you in advance!

Answer 1

[Chad Greene]

You're overwriting Y each time through the loop. If you want to keep the loop, try this:

t=0:0.01:Ttotal ; 
for k = 1:length(t)
   Y(k)=Yo+Voy2*t(k)-0.5*g*t(k)^2 ;%Y position of ball X=Xo+Vox2*t %X position of ball line(X,Y) 
end

However, loops are slow. It's fine for this simple case, but it's a good idea to break free of reliance on loops. A faster, simpler, easier-to-read option would be:

t = 0:0.01:Ttotal;
Y = Yo+Voy2.*t-0.5*g*t.^2 ;

[Kevin Bui]

Thank you! Your solution worked perfectly! The alternative was even better (much simpler). I personally do not like to use loops but could not figure out how to solve this problem without one. I do have one more question, though. Why is Y being overwritten each time through the loop? If I have the plot in the loop and the hold command on, then shouldn't it keep plotting each Y value on the same plot (since the line command is being looped for each new value of Y)? Wouldn't it define Y based on a value of t, plot that, define a new y based on another t, plot that, etc? Thank you for your help!