Saved Bookmarks
| 1. |
A car is initially at rest, 330 m away from a stationary observer. It begins to move towards the observer with an acceleration of 1.1 m s-2 , sounding its horn continuously. 20 s later, the driver stops sounding the horn. The velocity of sound in air is 330 m s-1 . The observer will hear the sound of the horn for a duration of(a) 20 s (b) 21 s (c) 20 2/3 s (d) 19 1/3 s |
|
Answer» Correct Answer is: (a) 20 s Let the car begin to move and sound its horn at time t0. The sound reaches the observer in 1 s. ∴ the observer begins to hear the sound at time t1 = t0 + 1 s. The displacement of the car in 20 s is (1/2) (1.1 m s-2) (20 s)2 = 220 m. ∴ at time t2 = t0 + 20 s, the horn is switched off and the car is 110 m away from the observer. The sound emitted by the horn at the instant it is switched off will travel to the observer in a further time (1/3) s. ∴ the observer stops hearing the sound at time t3 = t2 + 1/3 s. ∴ the observer hears sound for a duration of t = t3 - t1 = t2 + 1/3 s - t1 = (t0 + 20 s) + 1/3 s - (t0 + 1 s) = 19 1/3 s. |
|