Question

It is observed during an earthquake in japan that medium sized buildings are damaged more than very short or tall buildings. This is because

• A. The natural frequency of the medium sized buildings may be very different from the frequency of the earth quake
• B. The forced frequency of the medium sized buildings may be similar to the frequency of the earth
• C. The natural frequency of the medium sized buildings may be similar to the frequency of the earth quake
• D. The resonance frequency of the medium sized buildings may be similar to the frequency of the earth
• E. The natural frequency of the medium sized buildings may be similar to the frequency of the earth

Answer 1

The correct option is C

The natural frequency of the medium sized buildings may be similar to the frequency of the earth quake

This is because of resonance. When a body is subjected to forced vibration with a frequency which equal to or an integral multiple of the natural frequency of the body, the body vibrates with large amplitude violently. Most medium sized buildings have a natural frequency equal to frequency of earth quake (frequency of forced vibration). Thus most medium sized buildings vibrate more violently than other buildings undergoing a greater damage. Whereas the frequency of for very high or very low buildings does not match the frequency of the earthquake and hence such buildings undergo forced vibration but not resonance. Thus the amplitude of the vibration of the building is not very large and thus the building is not damaged to a large extent.