You mean Newton's 2nd Law ah?
Constant velocity = no resultant force. Why?
Recall F = ma.
A particle at constant velocity (say 15m/s) stays at 15m/s, and as
such has no acceleration (or retardation)
Since a = 0, substituting a = 0 into (F = ma),
you'll have a resultant force of zero, regardless of the mass of
the bus.
However, when you've acceleration (or retardation), there is a
force exerted.
Take for example a shopping basket having a mass of 1.5kg and a
push of 12N by a shopper. What is its initial acceleration?
Applying F = ma,
12N = 1.5kg x a.
a = 8m/s^2.
You'll be able to see as it picks up velocity in a particular
direction.
But the acceleration isn't the factor why you fall.
Bring in Newton's First Law, regarding inertia.
An object at rest tends to stay at rest; an object in motion tends
to continue its motion with a constant velocity (magnitude and
direction) except when acted upon by an unbalanced force.
As good as saying, when your bus accelerates, you tend to stay "at
rest". Then suddenly, you also start to experience a force. You'll
be able to experience it when the bus undergoes retardation. (or
otherwise deceleration, or otherwise negative acceleration)
"DO NOTE: All calculations for F = ma: F and a are vectors. Please
explicitly know the direction of force and acceleration you're
applying, as this formula can potentially give rise to a negative
value, especially when calculating retardation.
