The acceleration of the car is solved by subtracting the initial speed from the final speed then dividing the result by the elapsed time.
initial speed = 72 km/hr = 20 m/s
final speed = 0 m/s
elapsed time = 5 seconds
acceleration = (0 m/s – 20 m/s) / 5 s
acceleration = - 20m/s / 5 s
acceleration = -4 m/s^2
It can never be shorter than a component - magnitude of avector is the square root of the sum of the components squared, and a square function never produces a negative number. However, it can be the same size as its component, if that component is the only one
The mass of an object determines the object's inertia.
"Multiple accelerations" is a puzzling phrase, and I'd be curious to understand it
better. Sadly however, you haven't explained it at all.
If the multiple accelerations are the accelerations of multiple objects, then
the net force on each object is the product of (its mass) x (its acceleration).
If the multiple accelerations are the acceleration of one object at different times,
then at any instant of time, the net force on the object is the product of (its mass) x
(its acceleration at that instant).
