A 2000-kg car experiences a braking force of 10,000 N and skids to a stop in 6 seconds. The speed of the car just before the brakes were applied was
A) 1.2 m/s.
B) 15 m/s.
C) 30 m/s.
D) 45 m/s.
E) none of the above
Answer: C
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The site from which an airplane takes off is the origin. The x axis points east; the y axis points straight up. The position and velocity vectors of the plane at a later time are given by
?
m and 
m/s.
?
The plane is most likely
A. just touching down. B. in level flight in the air. C. ascending. D. descending. E. taking off.
Two police cars have identical sirens that produce sound of frequency of 570 Hz. A stationary listener is standing between two cars
One car is parked and the other is approaching the listener and both have their sirens on. The listener measures 9.0 beats per second. Find the speed of the approaching police car. The speed of sound is 340 m/s.
A billiard ball collides in an elastic head-on collision with a second stationary identical ball. After the collision which of the following conditions applies to the first ball?
a. maintains the same velocity as before c. comes to rest b. has one-half its initial velocity d. moves in the opposite direction
Reducing the magnetic field to 71% of its previous value changes the magnetic energy density in the field to _______% of its original value.
A. 39 B. 50 C. 71 D. 85 E. 15