Question

What is the mass of a falling rock if it produces a force of 50 N?

Answer 1

××$F=m \times a$×

50N is your force and the acceleration is -9.8m/s^2 due to gravity. 

So, you just plug that in. 

$50 N=m\times-9.8m/s^2\\ \frac{50}{-9.8}=m\\ m=-5.102$

BUT you know that mass cannot be negative, so you just disregard the negative sign and the mass of the rock is 5.102 grams.