Hello there!
The formula for Force is F = MA, or
Force = Mass x Acceleration
Well, since we have a rock falling in free fall, we can use the magical number 9.8 m/s^2 (also known as gravity). We have our value for force, 147N (Newtons).
So now, we have the value of our acceleration, A, and our value of the force, F, in our formula.
147 = 9.8M
To solve for M, we need to divide 147 by 9.8.
147/9.8 = 15
This means our Mass is 15 kg (remember your units!)
I hope this helps!
