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:
200000 J
Explanation:
From the question given above, the following data were obtained:
Mass (m) of roller coaster = 1000 Kg
Velocity (v) of roller coaster = 20 m/s
Kinetic energy (KE) =?
Kinetic energy is simply defined as the energy possess by an object in motion. Mathematically, it can be expressed as:
KE = ½mv²
Where
KE => is the kinetic energy.
m =>is the mass of the object
V => it the velocity of the object.
With the above formula, we can obtain the kinetic energy of the roller coaster as follow:
Mass (m) of roller coaster = 1000 Kg
Velocity (v) of roller coaster = 20 m/s
Kinetic energy (KE) =?
KE = ½mv²
KE = ½ × 1000 × 20²
KE = 500 × 400
KE = 200000 J
Therefore, the kinetic energy of the roller coaster is 200000 J.