# Need Help With this Physics Problem (Please)

#### jpnnngtn

This is the problem, anyone who can help me, please respond.

A supersonic jet traveling at Mach 3 (means that the speed of the jet is three times faster than the speed of sound in air) at an altitude of 20,000 m is directly overhead at t(time) = 0. How long will it be before the ground observer encounters the shockwave? Where will the plane be when it is finally heard? (Assume that the speed of sound in air is uniform at 345 m/s)

This problem is listed under the Doppler Effect in my textbook

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#### HallsofIvy

Homework Helper
Looks pretty easy to me. I BELIEVE that the shock wave itself moves at the speed of sound (if that's incorrect I'm sure there will be people happy to jump on me with both feet ).

Since v= d/t, t= d/v: 20000 m/345 m/sec.= how many seconds?

In the time that the shockwave takes to move the 20000 feet down, the jet, at three times the speed, will have moved three times as far, 60000 feet, horizontally. To find the straight-line distance between the person who hears the shock wave and the jet, use the Pythagorean theorem.

(I don't see how "Doppler Effect" has anything to do with it.)

#### jpnnngtn

you're right. Thank you. You have been most helpful. I think I was just confused because the problem was listed under the group of "Doppler Effect" problems.

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