Suppose you are a roofer and you find yourself teetering on the edge of a house with a heavy hammer in your hand. As you are about to fall, you remember that throwing the hammer can propel you backward. What physics law did you remember could save you

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.