Standing on a balcony, you throw your keys to a friend standing on the ground below. one second after you release the keys, they have an instantaneous velocity of 13.9m/s, directed 45 below the horizontal. what initial velocity did you give them?
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The experiments with static charges were done by:

Wo audio speakers are placed on a vertical rail. speaker a is placed at head-level while speaker b is place at some variable height, \delta yδy, above speaker a. both speakers receive simultaneous input from a sine-wave generator so the speakers each produce a pure sinusoidal sound wave with a wavelength of 0.531 meters. calculate the lowest height that speaker b can be placed above speaker a to produce a minimum of sound heard by person standing \delta x = 7.86δx=7.86 meters directly in front of speaker a.

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