11. It takes 10 seconds to send or read an SMS text message. An earthquake hits me. I
want to warn my friend who is located 60 km due west of me. The earthquake
epicentre was 25 km due south of me. How many seconds does my friend have to get
out of the building after reading my SMS? Assume SMS transmission time is
instantaneous. Assume earthquake waves go at 1 km/s.
a. 45
b. 40
c. 30
d. 20
Earthquake SMS Warning Time
Earthquake SMS warning time determines how long your friend has to evacuate after reading your message. The epicenter lies 25 km south of you, your friend is 60 km west, and seismic waves travel at 1 km/s. SMS takes 10 seconds to send plus 10 to read, totaling 20 seconds.
Distance Calculations
Position yourself at (0,0 km). Epicenter at (0,-25 km), friend at (-60,0 km).
Distance from you to epicenter: √((0-0)²+(0-(-25))²) = 25 km
Time for waves to reach you: 25/1 = 25 seconds
Distance from friend to epicenter: √((-60-0)²+(0-(-25))²) = √(3600+625) = √4225 = 65 km
Time for waves to reach friend: 65/1 = 65 seconds
Timeline Breakdown
- You feel the earthquake at 25 seconds
- Sending SMS takes 10 seconds (sent at 35 seconds)
- Friend reads after another 10 seconds (at 45 seconds total from start)
- Friend receives warning at 45 seconds but faces waves at 65 seconds
- Available time: 65-45 = 20 seconds
Option Analysis
Exam Relevance
This MCQ tests vector distances and relative timing, common in CSIR NET physics for wave propagation. Use Pythagoras theorem for hypotenuse paths of seismic waves at 1 km/s. Practice similar problems on earthquake SMS warning time for quick solves.
