4. The speed of a d. c. motor increases with increasing current through the armature coil. Given fixed supply voltage, two ways of changing the current supplied to the motor are: (1) a variable resistor (potentiometer) or (2) pulse width modulation (PWM). Explain how each method works. Provide one advantage and one disadvantage of each method.

Arocket starts from rest and moves upward from the surface of the earth. for the first 10.0 s of its motion, the vertical acceleration of the rocket is given by ay = 12.80 m/s32t, where the +y-direction is upward. (a) what is the height of the rocket above the surface of the earth at t = 10.0 s? b) what is the speed of the rocket when it is 325 m above the surface of the earth?

You throw a small rock straight up from the edge of a highway bridge that crosses a river. the rock passes you on its way down, 7.00 s after it was thrown. what is the speed of the rock just before it reaches the water 28.0 m below the point where the rock left your hand? ignore air resistance.

What component of earth’s atmosphere exists entirely as a result of photosynthesis?