A particle of mass m is projected vertically upwards with speed V in air that produces
a resistance to motion kv when the particle's speed is v. By using the chain rule and writing
z(double dot)= vv'(z), find an expression for H and deduce that m(V-gT) = kH.

How can you deduce that, with explanation please? I really struggle with the chain rule so any help would be appreciated.

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