D) Kilograms is the standard unit of mass
I would say friction because it requires to surfaces in order for the force to take place
correct me if I'm wrong.
Answer:
2.7 J
Explanation:
The energy of one photon is given by

where
h is the Planck constant
f is the frequency
For the photons in this problem,

So the energy of one photon is

The number of photons contained in 1.0 mol is
(Avogadro number)
So the total energy of
photons contained in 1.0 mol is

Let the mass of 2500 kg car be
and it's velocity be
and the mass of 1500 kg car be
and it's velocity be
.
After the bumping the mass be M and it's velocity be V.
By law of conservation of momentum we have

2500 * 5 + 1500 * 1=4000 * V
V = 14000/4000 = 7/2 = 3.5 m/s
So the velocity of the two-car train = 3.5 m/s
The letter "B" is used for magnetic field