We will make the comparison between each of the sizes against the known wavelengths.
In the case of the <em>hydrogen atom</em>, we know that this is equivalent to 
m on average, which corresponds to the wavelength corresponding to X-rays.
In the case of the <em>Virus</em> we know that it is oscillating in a size of 30nm to 200 nm, so the size of the virus is equivalent to the range of the wavelength of an ultraviolet ray.
In the case of <em>height</em>, it fluctuates in a person around 
to 
m, which falls to the wavelength of a radio wave.
Answer:
Explanation:
Conservation of momentum is used to solve
Unfortunately we have a missing piece of information such as the initial velocity of the unknown mass train.
If we ASSUME that the second train is at rest
5000(100) + m(0) = 5000(50) + m(50)
which means m = 5000 kg
However, I'll show you the importance of knowing that initial velocity by finding it assuming the other answers are valid
if m = 15000 kg
5000(100) + 15000(v₀) = (5000 + 15000)(50)
v₀ = 33 ⅓ m/s
if m = 10000 kg
5000(100) + 10000(v₀) = (5000 + 10000)(50)
v₀ = 25 m/s
if m = 8000 kg
5000(100) + 8000(v₀) = (5000 + 8000)(50)
v₀ = 18.75 m/s
So you can see why I had to assume an initial velocity. Any of the masses could work if the initial velocity is chosen correctly.
