Question

A pendulum 1m long makes 20 vibrations in 40 s. Find the time taken to make 30 vibrations, if its length is increased to 4 m.

Answer 1

T=2π✓l/g

1 vibration in 2 seconds

T directly proportional to ✓l so T1/T2=✓l1/l2

so T2=T1 ×✓l2/l1 =2×✓4=2×2=4 s

So 1 vibration in 4 seconds

so 30 in 120seconds or 2mins