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Home >All Categories > Science & Mathematics >Physics >Open Question Pat Member since:April 11, 2011Total points:95 (Level 1)An ambulance is moving towards a doctor to deliver a sick patient. The ambulance siren has a frequency of 489 Hz and a velocity of 31 m/s. The speed of the doctor running towards the ambulance is 5 m/s. If the speed of sound is in air that day is determined to be 340 m/s, what is the frequency of the sound heard by the doctor?Answer Question

by Buffalo Bob Member since:March 14, 2011Total points:381 (Level 2)Look up "doppler effect" on wikipedia, you will find the answer there

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9. A 2 kg car moving towards the right at 4 m/s collides head on with an 8 kg car moving towards the left at 2 m/s, and they stick together. After the collision, the velocity of the combined bodies is

A. 0
B. 0.8 m/s towards the right.
C. 0.8 m/s towards the left.
D. 2.4 m/s towards the right.
E. 2.4 m/s towards the left.

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I think you are meaning 10^2 and 10 ^ 3 in the question.
If so the speed of the car is 100 Km/hr which is quite plausible.

We need to convert Km/hr to m/s so must multiply by 1000 m/km and divide by 3600 secs / hr

so speed = 100/3.6 = 27.8 m/s

Its initial momentum was MV = 755 * 27.8 = 2.1 * 10 ^4 Ns

During braking an amount of impulse = FT = 2.5 * 5.2 * 10^3 = 1.3 * 10 ^ 4 Ns has been lost.

So the final momentum is initial - lost
= 2.1 * 10 ^ 4 - 1.3 * 10 ^ 4 = 7990 Ns

Now once more momentum = MV
so 7990 = 755 * V
V = 7990 / 755 = 10.6 m/s ( about 37 Km/hr)

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