The ball can't reach the speed of 20 m/s in two seconds, unless you THROW it down from the window with a little bit of initial speed. If you just drop it, then the highest speed it can have after two seconds is 19.6 m/s .
If an object starts from rest and its speed after 2 seconds is 20 m/s, then its acceleration is 20/2 = 10 m/s^2 .
(Gravity on Earth is only 9.8 m/s^2.)
Concave lens. These are used in making the objectives of reflection telescopes
Explanation:
Given
radius


mass disc 
mass of person 
velocity of Person 
moment of inertia 

Initial angular momentum



Final Moment inertia


final angular momentum

Conserving angular momentum



The answer is 34 you have to add the numbers and divide them by how many numbers there are