The engine of a racecar approaching at 40.0 m/s emits a frequency of 160 Hz. What frequency does a stationary spectator hear as the car approaches? b)
What is the frequency as it moves away?

Answer(a)   the matererials that are in contact, the two materials and the nature of their surfaces/// (b)/// the force pushing the two surfaces together, pushing the surfaces together causes the more of the asperities to come together and increases the surface area in contact with each other

Distance covered = speed x timethe distance traveled going against the stream = distance traveled with the stream.therefore it is the speed and the time that are the variables.as he travels the same distance both ways, speed x time is the same for each way.going upstream his speed is 4 kph; going downstream his speed is 6 kph.therefore we have two simultaneous equations: time upstream x 4 = time downstream x 6time upstream + time downstream = 60 minutestherefore we have the equation: time upstream = 6/10 hour, time downstream therefore = 4/10 hour.distance traveled at 4km/hr in 6/10 hour = 24/10 = 2.4km.check: distance traveled at 6km/hr in 4/10 hour = 24/10 = 2.4km.

Aand b antarctica is a north pole.  field lines enter earth near the north pole.the magnetic pole near antarctica is a north pole.compasses point toward a magnetic pole that is a north pole.field lines circle earth from east to west.a compass can lead a hiker to the geographic north pole.