The back emf of an electric motor is greatest
A. when the motor is just starting up.
B. when the motor is halfway to operating speed.
C. when the motor is at 71% of the operating speed.
D. when the motor is at full operating speed.
E. at all times since it is a constant when the motor is running at any speed.
D. when the motor is at full operating speed.
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Conservation of Momentum: Consider two less-than-desirable options. In the first you are driving 30 mph and crash head-on into an identical car also going 30 mph. In the second option you are driving 30 mph and crash head-on into a stationary brick wall. In neither case does your car bounce back from the thing it hits, and the collision time is the same in both cases. Which of these two situations would result in the greater impact force on your car?
A. hitting the other car B. hitting the brick wall C. The force would be the same in both cases. D. None of these choices are correct.
Two tennis balls fall through the air from a tall building. One of them is filled with lead pellets. The ball to reach the ground first is the
A) regular ball. B) lead-filled ball. C) same for both
Suppose that Earth's atmosphere had no greenhouse gases. Then Earth's average surface temperature would be
A) about the same as it is now. B) slightly cooler, but still above freezing. C) well below the freezing point of water. D) slightly warmer, but still well below the boiling point of water.
Why don't low-mass stars have the CNO cycle occurring in their cores?
A) They don't have enough carbon, nitrogen, and oxygen. B) The CNO cycle makes elements heavier than carbon, nitrogen, and oxygen. C) Their core temperatures are too low.