- #1
WWCY
- 479
- 12
Homework Statement
Hi everyone, I'd appreciate it if someone could help look through my working and check if it makes sense!
I have the following integral:
$$P(t) = \int_{-\infty}^{a} \int_{-\infty}^{-\infty} f_p(p) \ f_{x} (x - pt/m) \ dp \ dx$$
I want to find an expression for ##\frac{dP(t)}{dt}##, so I first "bring in" the integral over ##x##.
$$P(t) = \int_{-\infty}^{-\infty} f_p(p) \Big [ \int_{-\infty}^{a} f_x (x - pt/m) \ dx \Big ] \ dp$$
Making the substitution of ##u = x - pt/m##
$$\frac{d}{dt} \int_{-\infty}^{a} f_x (x - pt/m) \ dx = \frac{d}{dt}\int_{-\infty}^{a - pt/m} f_x (u) \ du = -\frac{p}{m} f_x(a - pt/m)$$
and finally,
$$\frac{dP}{dt} = -\frac{1}{m} \int_{-\infty}^{\infty} f_p (p) . p . f_x (a - pt/m) \ dp$$
Is this right? Many thanks in advance.