Constructive and destructive interference of sound wave

Speakers A and B are viberating in phase. They are direcley facing each other, are 7.80 m apart, and are eack playing a 73.0-Hz tone. The speed of sound is 343 m/s. On the line between the speakers there are three points where constructive interference occurs. What are the distance of these three points from speaker A?

Working with conservation of mechanical energy.

A 12 kg block is released from rest on a 30 degree frictionless incline. Below is a block that can be compressed 2.0 cm by a force of 270 N. The block momentarily stops when it compresses the spring by 5.5 cm. How far does the block move down the incline from its rest position to this stopping point? What is the speed of the block just as it touches the spring?

Conservation of energy : Determining the reduction of mechanical energy because of air drag.

A 60 kg skier leaves the end of a ski jump with a velocity of 24 m/s directed 25 degrees above the horizontal. Suppose as a result of air drag the skier returns to the ground with a speed of 22 m/s landing 14 m vertically below the end of the ramp. From the launch to the return to the ground, by how much is the mechanical energy of the skier Earth system reduced because of air drag?

Conduction

In an electrically heated home, the temperature of the ground in contact with the wall is 12.8*C. The temperature at the inside surface of the wall is 20.0*C. The wall is .10m thick and has area of 9.0m^2. Assume one kiliwatt*hour of electrical energy costs $.10. How many hours are required for one dollar's worth of energy to be conducted through the wall?

Calculating the frequency of a string.

When a guitar string is sounded along with a 440Hz tuning fork, a frequency of 5 Hz is heard. When the same string is sounded along with 436 Hz tuning fork, the beat frequency is 9 Hz. What is the frequency of the string?