Magnitude of displacement means the total displacement. Basically taking your final distance and subtracting it from your initial distance so it would be 14m
==> Take an Earth weight. Don't forget the unit.
==> Multiply it by 0.166 .
==> The product is its Moon weight. Don't forget the unit.
Example:
If I weigh 1,000 lbs on Earth.
Multiply 1,000 lbs by 0.166 .
I weigh 166 lbs on the Moon
Using q=1.60217662 E-19 C we get:

Then using 9.10938356E-31 kg as the mass, we get
Mass of yellow train, my = 100 kg
Initial Velocity of yellow train, = 8 m/s
mass of orange train = 200 kg
Initial Velocity of orange train = -1 m/s (since it moves opposite direction to the yellow train, we will put negative to show the opposite direction)
To calculate the initial momentum of both trains, we will use the principle of conservation of momentum which
The sum of initial momentum = the sum of final momentum
Since the question only wants the sum of initial momentum,
(100)(8) + (200)(-1) = 600 m/s
Radioactive "decay" means particles and stuff shoot OUT of a nucleus.
After that happens, there's less stuff in the nucleus than there was before.
So the new mass number is always less than the original mass number.