Question

How much force is exerted on a 3kg object accelerating from 12m/s to rest in 5.5 seconds?

Answer 1

initial velocity, u = 12 m/s
final velocity, v = 0 m/s
time, t = 5.5 sec
applying first law of motion,
v= u + at
⇒0 = 12 + a ×5.5
⇒5.5 a = -12
⇒a = -12/5.5 = - 2.18 m/s²
negative means retardation
so,
force = mass × acceleration
         = 3× 2.18
         = 6.54 kg m/s² or 6.54 N

Answer 2

force=acceleration×mass
mass=3kg
acceleration=v-u/t=12-0/5.5=2.18 m/s2
force=ma=3×2.18= 6.54newton