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A body starts from rest. What is the ratio of the distance travelled by the body during the 4th and 3rd second?

A

7/5

B

5/7

C

7/3

D

3/7

Step-by-Step Solution

  1. Method 1: Galileo's Law of Odd Numbers For a particle starting from rest with uniform acceleration, the distances traversed during equal successive intervals of time are in the ratio of odd numbers: 1:3:5:7:91 : 3 : 5 : 7 : 9 \dots . Distance in 1st second 1\propto 1 Distance in 2nd second 3\propto 3 Distance in 3rd second 5\propto 5 Distance in 4th second 7\propto 7 Therefore, the ratio of the distance travelled in the 4th second to that in the 3rd second is 7:57:5 or 7/57/5.
  2. Method 2: Kinematic Formula Distance travelled in the nn-th second is given by Sn=u+a2(2n1)S_n = u + \frac{a}{2}(2n - 1). Since the body starts from rest, u=0u = 0. S4=a2(2×41)=7a2S_4 = \frac{a}{2}(2 \times 4 - 1) = \frac{7a}{2}. S3=a2(2×31)=5a2S_3 = \frac{a}{2}(2 \times 3 - 1) = \frac{5a}{2}.
  • Ratio S4S3=7a/25a/2=75\frac{S_4}{S_3} = \frac{7a/2}{5a/2} = \frac{7}{5}.
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