The minimum input force she'll need to lift the ball is 35 N.
Step-by-step explanation:
Mechanical advantage of a single pulley is 1. As, she applies 70 N of force to lift the bowling ball, so the output force(weight of the ball) is also 70 N.
Now, adding another pulley gives a mechanical advantage of 2. We have,
M.A = (Output Force)/(Input Force)
Substituting the values we get,
![2=(70)/(F_(i))](https://img.qammunity.org/2019/formulas/physics/high-school/umj1c5lhchvtvxozf30bnpl13aenl1hena.png)
= 35 N
Input force equals to 35 N needs to be applied.