When you stop at a railroad crossing at night, instead of correctly perceiving two red lights that flash alternately you may perceive a single red light that moves back and forth from left to right. this is an example of

Aresistor with an unknown resistance is connected in parallel to a 9 ω resistor. when both resistors are connected in parallel to an emf source of 21 v, the current through the unknown resistor is measured with an ammeter to be 4 a. what is the resistance of the unknown resistor? answer in units of ω.

What happens when the vapor pressure of a liquid is equal to the external pressure?

Acar weighs 15,000 n, and its tires are inflated to a pressure of 190kpa. how large is the area of the car's tires that are in contact with the road?