Which statement best describes longitudinal waves?
a. particles of the wave medium move perpendicular to the direction of propagation of the wave.
b. longitudinal waves do not require a material medium for propagation.
c. particles of the wave medium move parallel to the direction of propagation of the wave.
d. particles of the wave medium move long distances from the source of the wave to carry energy.

Aplayground slide is 8.80 ft long and makes an angle of 25.0° with the horizontal. a 63.0-kg child, initially at the top, slides all the way down to the bottom of the slide. (a) choosing the bottom of the slide as the reference configuration, what is the system's potential energy when the child is at the top and at the bottom of the slide? what is the change in potential energy as the child slides from the top to the bottom of the slide? (include the sign of the value in your answer.)