Here are some modifications that should get you closer to what you wan't. Note the comments I've added in the code to point out the changes. Note that the animation won't show when run here in Answers, but it does work on the desktop.
The main issue was that you were overwriting the animated line plot with a scatter plot (unless I misunderstood what you were trying to do).
px = 10;
py = 150;
j = 10;
% c = 2; % don't need to index if you are adding one point at a time
dt = 0.01;
figure
ruta = animatedline('Color', 'r', 'LineWidth', 3, 'LineStyle', ':'); % <<< moved outside the loop
xlim([10 15]) % <<< specify axis limits so it doesn't keep resizing
ylim([0 1000])
grid on
while j < 15 % <<< reduced so this would run faster for testing
Vel = 150 + j;
V = min(50, Vel);
alpha = atand(150);
Vx = V * cosd(alpha);
Vy = V * sind(alpha);
px = px + Vx*dt; % you don't need to index if you are adding one point at a time
py = py + Vy*dt; % you don't need to index if you are adding one point at a time
j = px;
% c = c + 1; % you don't need to index if you are adding one point at a time
addpoints(ruta, px, py);
% auto = scatter(px, py, 50, 'filled'); % don't overwrite with the scatter plot
drawnow;
% delete(auto);
end
