Error in kinematics (line 22) disp(['Initial velocity: ', num2str(solu), ' m/s'])

Question

Nurul 2024-1-12

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2069121-error-in-kinematics-line-22-disp-initial-velocity-num2str-solu-m-s

编辑： Dyuman Joshi 2024-1-12

% Prompt the user for input values
disp('Please enter the following values:')
disp('Enter 0 for the unknown variable')
prompt1 = 'Initial velocity (m/s): ';
u = input(prompt1); % u is the initial velocity
prompt2 = 'Final velocity (m/s): ';
v = input(prompt2); % v is the final velocity
prompt3 = 'Displacement (m): ';
s = input(prompt3); % s is the displacement
prompt4 = 'Acceleration (m/s^2): ';
a = input(prompt4); % a is the acceleration
% Check which variable is unknown
if u == 0 % u is unknown
    % Define the symbolic variable
    syms u
    % Create the equation with the unknown variable
    eqn = v^2 == u^2 + 2*a*s;
    % Solve the equation for the given values
    solu = solve(eqn,u);
    % Display the result
    disp(['Initial velocity: ', num2str(solu), ' m/s'])
elseif v == 0 % v is unknown
    % Define the symbolic variable
    syms v
    % Create the equation with the unknown variable
    eqn = v^2 == u^2 + 2*a*s;
    % Solve the equation for the given values
    solv = solve(eqn,v);
    % Display the result
    disp(['Final velocity: ', num2str(solv), ' m/s'])
elseif s == 0 % s is unknown
    % Define the symbolic variable
    syms s
    % Create the equation with the unknown variable
    eqn = v^2 == u^2 + 2*a*s;
    % Solve the equation for the given values
    sols = solve(eqn,s);
    % Display the result
    disp(['Displacement: ', num2str(sols), ' m'])
elseif a == 0 % a is unknown
    % Define the symbolic variable
    syms a
    % Create the equation with the unknown variable
    eqn = v^2 == u^2 + 2*a*s;
    % Solve the equation for the given values
    sola = solve(eqn,a);
    % Display the result
    disp(['Acceleration: ', num2str(sola), ' m/s^2'])
else
    % Display an error message
    disp('Invalid input. Please enter 0 for the unknown variable.')
end

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Answer 1

Dyuman Joshi 2024-1-12

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2069121-error-in-kinematics-line-22-disp-initial-velocity-num2str-solu-m-s#answer_1388356

编辑：Dyuman Joshi 2024-1-12

在 MATLAB Online 中打开

The output from solve() will be a symbolic number. And symbolic numbers are not accepted as inputs to num2str, see at the end.

Note the assumption I have included to keep only the real and positive solution.

syms u real positive

solu = solve(29 == u^2 + 2*9.8*1)

solu =

You can replace the disp() call with fprintf -

fprintf('Initial velocity: %f m/s', solu)
Initial velocity: 3.065942 m/s

%Showing the eror at the end of the post
num2str(solu)
Error using num2str
Input to num2str must be numeric.

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Answer 2

Hassaan 2024-1-12

0
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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2069121-error-in-kinematics-line-22-disp-initial-velocity-num2str-solu-m-s#answer_1388426

编辑：Hassaan 2024-1-12

在 MATLAB Online 中打开

% Prompt the user for input values
disp('Please enter the following values:')
disp('Enter 0 for the unknown variable')
prompt1 = 'Initial velocity (m/s): ';
u = input(prompt1); % u is the initial velocity
prompt2 = 'Final velocity (m/s): ';
v = input(prompt2); % v is the final velocity
prompt3 = 'Displacement (m): ';
s = input(prompt3); % s is the displacement
prompt4 = 'Acceleration (m/s^2): ';
a = input(prompt4); % a is the acceleration
% Check which variable is unknown
if u == 0 % u is unknown
    % Define the symbolic variable
    syms u
    % Create the equation with the unknown variable
    eqn = v^2 == u^2 + 2*a*s;
    % Solve the equation for the given values
    solu = solve(eqn, u);
    % Display the result
    disp(['Initial velocity: ', mat2str(double(solu)), ' m/s'])
elseif v == 0 % v is unknown
    % Define the symbolic variable
    syms v
    % Create the equation with the unknown variable
    eqn = v^2 == u^2 + 2*a*s;
    % Solve the equation for the given values
    solv = solve(eqn, v);
    % Display the result
    disp(['Final velocity: ', num2str(double(solv)), ' m/s'])
elseif s == 0 % s is unknown
    % Define the symbolic variable
    syms s
    % Create the equation with the unknown variable
    eqn = v^2 == u^2 + 2*a*s;
    % Solve the equation for the given values
    sols = solve(eqn, s);
    % Display the result
    disp(['Displacement: ', num2str(double(sols)), ' m'])
elseif a == 0 % a is unknown
    % Define the symbolic variable
    syms a
    % Create the equation with the unknown variable
    eqn = v^2 == u^2 + 2*a*s;
    % Solve the equation for the given values
    sola = solve(eqn, a);
    % Display the result
    disp(['Acceleration: ', num2str(double(sola)), ' m/s^2'])
else
    % Display an error message if no variable is set to 0
    disp('Invalid input. Please enter 0 for the unknown variable.')
end

I replaced num2str with mat2str, which converts the entire array of solutions to a string format that can be concatenated with other strings. If you still get multiple solutions and you need to choose the physically meaningful one, you might need to add a check to filter out the solutions based on your specific requirements (like selecting only positive solutions).

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