Question

A car traveling at 36km/hr speeds up to 70km/hr in 5 seconds. What is its acceleration? If

the same car stops in 20s, what is the retardation?

Answer 1

hello friend
pls find ur answer below

first case ( when the car accelerates )
given, u= 36km/hr = 10m/s 
v = 70km/hr = 20m/s
t= 5 sec.
a =?
we know that a = v-u/t 
so a = ( 20-10) / 5 
a= 10/5
a= 2m/s²

second case ( when car retards ) 
given,
u= 20m/s 
t= 20 sec.
v= 0m/s 
a=?

using first equation of motion, we get 
= v = u + at
= 0 = 20 + 20a
= - 20 = 20a 
= -1 m/s² 

so the retardation of car is -1m/s²

hope this clears all ur doubts ..

Answer 2

 A car traveling at 36 km/hr speeds up to 70 km/hr in 5 seconds. What is its acceleration? If the same car stops in 20 s, what is the retardation? 

Initial velocity = u = 36km/hr = 36 x 5/18 = 10 m/s
Final velocity = v = 70 km/hr = 70 x 5/18 = 19.4 m/s
Time = t = 5 sec

acceleration = a = v - u /t
= 19.4-10/5
= 1.88 m/s²

=============================
t = 20 sec

Initial velocity = u = 19.4 m/s
Final velocity = v = 0 m/s

acceleration = 0-19.4/20
= -0.97 m/s²

Hence ,
the retardation = -0.97 m/s²