Answer:
medium
Examples:
A medium is a thing that the sound wave would travel through, like sound waves traveling through air. A mechanical wave cannot travel without a medium, like if the wave were trying to travel through a vacuum.
Acceleration of the ball is 
Explanation:
The acceleration of the ball can be found by using Newton's second law of motion, which states that the net force acting on an object is equal to the product between the mass of the object and its acceleration:

where
F is the net force
m is the mass
a is the acceleration
For the ball in this problem, we have
m = 0.50 kg (mass)
F = 25 N (force)
thereofre, the acceleration of the ball is

Learn more about Newton's second law:
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Answer:
-2.79 m/s²
Explanation:
Given:
v₀ = 20 m/s
v = 11 m/s
Δx = 50 m
Find: a
v² = v₀² + 2aΔx
(11 m/s)² = (20 m/s)² + 2a (50 m)
a = -2.79 m/s²
Round as needed.