Newton's Second Law
Newton's Second Law is used in physics constantly and there are numerous equations derived from this concept. This concept is:
Force = Mass * Acceleration
Force is in Newton units, where mass is in kilograms and acceleration is in meters/seconds^2. Force is something that is necessary in understanding different systems in physics problems. This explains the constant force that a person feels, because the constant acceleration of gravity is equal to 9.81 m/sec^2. Therefore if one were able to multiply their mass (in kilograms) by gravity, that would be the force acting down on them at any given time.
It is possible to visualize this concept. Imagine you have a constant force, say 10 Newtons, now imagine the mass you are moving across a table is extremely large, this means that by using simple algebra, it is easy to say that the acceleration is going to be small. The same holds if the acceleration is high, then the mass will have to be small, in order for it to also equal 10 Newtons.
Say you have a block that weighs 10 kg. You have a hand pushing against it with a force that is equal to 10 Newtons. Because F=ma
10 Newtons = 10 kg * a, therefore acceleration is equal to 1 meter/sec^2
Now imagine a block that weighs 10 kg. You still have the same hand pushing against it with the same 10 Newtons.
10 Newtons = 1 kg * a, therefore acceleration is equal to 10 meter/sec^2