Solving the differential equation gives unexpected results. How to fix that?

When I solve the differential equation using Matlab, the result is as shown. It's correct, but entering input parameters such as m0 (initial mass), m0 > 0 but the resulting equation has log(- m0) leads to wrong results. Does anyone know how to fix this? Thank you very much everyone.

[Paul]

Hi Ly,

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What's supposed to happen in this system when t is large and m0 - k*t < 0? (asuming k > 0)

Is the sign on the k*v0 term correct (assuming k > 0)?

Is the equation for ysol correct? As written, the second term on the right is integrating backwards in time. Is that the intent?

Answer 1

[Torsten]

Better ?

v(t) = v(0) - g*t - v0*log(m0/(m0-k*t))

[Lý Khánh]

Thank you. But is there any way to display the output using such an expression?

[Torsten]

I think you have to do it "by hand".

[Lý Khánh]

Thank you.