Why did the magnitude of the Doppler Effect increase as the velocity of the sound source increased, but decreased as the velocity of sound increased?

When you drop a pebble from height a it reaches the ground with speed v if there is no air resistance. from what height should you drop it so it will reach the ground with three times the speed? (a) 9m (b) 3m (c) 6m (d) 18m

How is sound energy produced a. vibrating objects b. objects being heated c. temperature differences d. stationary objects

For which pair of objects would adding the same a