Question

18. Two cars A and B travelling towards each other on
a straight road with speed 20 ms-1 and 10 ms-
respectively. When they are 400 m apart both drivers
apply brakes simultaneously and each car comes at
rest in the same time. The ratio of their magnitude
of retardations is
(1) 2:1
(2) 1:2
(3) 1:1
(4) 4:1
​

Answer 1

Answer:

a 2:1

Explanation:

it is because.......

Answer 2

Answer:

If we use{ V=U + at} and input the values we will obtain 2:1 ratio but on the other hand if we use {V^2=U^2+2as}we will obtain 4:1 ratio.

Explanation:

By using;

V=U + at

0=20+at;   -20/t = a. for u=20m/s where v is "0"----------(1)

0=10+at;   -10/t = a . for u= 10m/s----------------(2)

By equating them (1) and (2)

a1/a2=-20/t/-10/t = 2/1 ; 2:1

By using

V^2-U^2=2as

0^2-20^2=2*a*400

-400/800 = a

-0.5m/s^2 ----------(1)

0^2-10^2=2*a*400

-100/800=a

-0.125 m/s^2--------(2)

By equating them (1) & (2)

a1/a2=-o.5/-0.125= 4/1;  4:1