Anyone really good at physics?

Newton's 3rd law says that there's an equal and opposite reaction for the 3 N push. The 2nd law says that F = ma.

Now, we want the acceleration of the 75 kg person. If the applied force is 3 N (reaction force to the push), then

3N = 75kg x a

a = 3N/75 kg = 1/25 m/s^2 = .04 m/s^2

And for the derivation of a newton:

it's just a unit/name we made up for the quantity

1 kg x m/s^2. In conceptual terms, it is the force required to give a 1 kg object an acceleration of 1 m/s^2. The unit of 1 Newton is derived completely from the definition of force in Newton's 2nd Law.

Newton's third law of motion says that forces come in pairs. For every force, an equal force is exerted in the opposite direction. So, when the 75-kg person pushes the 12-kg person with a force of 3.0 N, the 12-kg person also exerts a 3.0-newton force back against the 75-kg person. If we don't have to contend with friction, then the 3.0-N force constitutes the net force, and we can use the mathematical expression of Newton's second law to solve this:

a = Fnet / m

a = 3.0 N / 75 kg = 0.040 m/s²

A newton is the basic metric unit of force in the mks (meter-kilogram-second) branch of the metric system. Since force = mass x acceleration, the newton is equal to the base unit of mass (the kilogram) multiplied by the unit of acceleration (meters per second per second). Therefore, a newton is equal to a kg*m/s/s, or a kg*m/s².

I hope that helps. Good luck!