Question

now what is the derivation of this?

Answer 1

$p \: = \frac{d( \frac{ -1kx {}^{2} }{2} )}{dt} \\ p \: = \: - k \times \frac{1}{2} \times 2 \times (x \times \: \frac{dx}{dt} ) \\ p \: = \: - kx \frac{dx}{dt} \\ p \: = - kxv \\ because \: velocity \: = v \: = \frac{dx}{dt}$