My graph is blank

I am trying to calculate and graph the variations of g with height for the different planets I have in my variables, but once i hit run i get the graph, but no line being plotted. This is the code

clc;
load project_1.mat
g= @(x,M)((G*M)./(x.^2));
dist = vpa(linspace(1,10^8,1000));
figure
hold on

g_Earth = g(dist, Mass_Earth);

Unrecognized function or variable 'Mass_Earth'.

g_Jupiter = g(dist, Mass_Jupiter);
g_Mars = g(dist, Mass_Mars);
g_Moon = g(dist, Mass_Moon);
plot(dist, g_Earth)
plot(dist, g_Jupiter)
plot(dist, g_Mars)
plot(dist, g_Moon)
xlabel('Height from surface of planet');
ylabel('Acceleration due to gravity');
title('g vs Height from surface of planets')
legend({'Earth','Jupiter','Mars','Moon'})
savefig('task3_graph.fig')

[Torsten]

G is undefined.

[Walter Roberson]

hold on freezes the axes limits. After you plot

xlim auto; ylim auto

[Bryan]

how would you go to define it then? I placed G as G = 6.6743*10^-11; %(m^3)(kg^-1)(s^-2)in the workspace but I still get the same issue.

Answer 1

[Walter Roberson]

Tested.

Your .mat file does not contain anywhere near what you thought it does. Instead there is a 3 x 1 string array for each object, with the strings being the character representation of the measurements. You need to convert the string to numeric form to do calculations.

load project_1.mat

Mass_Earth = double(EarthArray(1));
%r_Earth = double(EarthArray(2));

Mass_Jupiter = double(JupiterArray(1));
%r_Jupiter = double(JupiterArray(2));

Mass_Mars = double(MarsArray(1));
%r_Mars = double(MarsArray(2));

Mass_Moon = double(MoonArray(1));
%r_Moon = double(MoonArray(2));

G = 6.6743*10^-11; %(m^3)(kg^-1)(s^-2)
dist = linspace(1,10^8,1000);
g= @(x,M)((G*M)./(x.^2));

figure
hold on 

g_Earth = g(dist, Mass_Earth);
g_Jupiter = g(dist, Mass_Jupiter);
g_Mars = g(dist, Mass_Mars);
g_Moon = g(dist, Mass_Moon);

plot(dist, g_Earth)
plot(dist, g_Jupiter)
plot(dist, g_Mars)
plot(dist, g_Moon)

set(gca, 'YScale', 'log')

xlabel('Height from surface of planet');
ylabel('Acceleration due to gravity');
title('g vs Height from surface of planets')
legend({'Earth','Jupiter','Mars','Moon'})
savefig('task3_graph.fig')

[Bryan]

it worked ! thank you very much.

[Walter Roberson]

xlabel('Height from surface of planet');

No, the values are distance from the center of the planet, not from the surface of the planet. Have a look

g= @(x,M)((G*M)./(x.^2));

The surface of the planet is distance 0 above the surface of the planet, so if we examine the value x = 0, if the label is correct, we should get the gravity at the surface of the planet. But we have a division by 0.

If you want x to be height above surface then you need different code.

[Walter Roberson]

load project_1.mat

Mass_Earth = double(EarthArray(1));
r_Earth = double(EarthArray(2));

Mass_Jupiter = double(JupiterArray(1));
r_Jupiter = double(JupiterArray(2));

Mass_Mars = double(MarsArray(1));
r_Mars = double(MarsArray(2));

Mass_Moon = double(MoonArray(1));
r_Moon = double(MoonArray(2));

G = 6.6743*10^-11; %(m^3)(kg^-1)(s^-2)
dist = linspace(1,10^8,1000);
g= @(x,M,r)((G*M)./((x+r).^2));

figure
hold on 

g_Earth = g(dist, Mass_Earth, r_Earth);
g_Jupiter = g(dist, Mass_Jupiter, r_Jupiter);
g_Mars = g(dist, Mass_Mars, r_Mars);
g_Moon = g(dist, Mass_Moon, r_Moon);

plot(dist, g_Earth)
plot(dist, g_Jupiter)
plot(dist, g_Mars)
plot(dist, g_Moon)

set(gca, 'YScale', 'log')

xlabel('Height from surface of planet');
ylabel('Acceleration due to gravity');
title('g vs Height from surface of planets')
legend({'Earth','Jupiter','Mars','Moon'})
savefig('task3_graph.fig')