Answer:
0.82 m
Explanation:
The ball is in free fall - uniform accelerated motion with constant acceleration downward,
(acceleration of gravity). So we can use the following suvat equation to solve the problem:

where
v is the final velocity
u = 4 m/s is the initial velocity
a is the acceleration
s is the displacement
At the maximum displacement, v = 0 (the velocity becomes zero). Substituting and solving for s, we find:

The echo is heard 2.80 s later, this means this is the time the sound takes to travel to the reflecting object and then back to us. So, during this time, the sound wave has covered the distance L between us and the object twice:

The speed of the sound wave is:

, and since it is moving by uniform motion, we can find the distance covered by the wave using

And we said this corresponds to twice the distance between us and the reflecting object, so:

so, the object is 480 meters away.
Answer:
the liquid has less height than the mercury
h_{ liquid} = 
Explanation:
The pressure as a function of the height is given by
P = ρ g h
where ρ is the density of the liquid, g the acceleration of gravity and h the height reached by the column of the liquid
In that case they say that the pressure is the standard one that is P = 1.01 10⁵ Pa = 760 mmHg
The first way to give the pressure is in SI units and the second way is the height that the mercury column reaches
In the case of building a barometer with a liquid that has a density greater than that of mercury
ρ_liquid > ρ_Hg
the pressure
P =ρ_lquid g h_liquid
if we have the same pressure
ρ_{Hg} g h_{Hg} = ρ_{liquid} g h_{liquid}
h_{ liquid} =
therefore the liquid has less height than the mercury
Many ecosystems and plants are damaged or destroyed when a volcano erupts.