A 100-kg box is placed on a ramp. As one end of the ramp is raised, the box begins to move downward just as the angle of inclination reaches 25°. What is the coefficient of static friction between box and ramp?
a. 0.15
c. 0.47
b. 0.27
d. 0.95
C
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A scientist claims that all red main-sequence stars are 1 billion years old. Which of the following, if true, would prove that this claim is false?
A) The discovery of a red main-sequence star in a star cluster where the main sequence turnoff is about that of the Sun B) The discovery of a red main-sequence star in a star cluster where all the other stars are red giants or red supergiants C) Stellar evolution theory predicts that the main-sequence lifetime of red stars can be as long as 100 billion years. D) The minimum lifespan of a red main-sequence star is estimated to be 10 billion years.
A resistor and a capacitor are connected in series across an ideal battery having a constant voltage across its terminals. At the moment contact is made with the battery (a) the voltage across the capacitor is A
equal to the battery's terminal voltage. B) less than the battery's terminal voltage, but greater than zero. C) equal to the battery's terminal voltage. D) zero. (b) the voltage across the resistor is A. equal to the battery's terminal voltage. B) less than the battery's terminal voltage, but greater than zero. C) equal to the battery's terminal voltage. D) zero.
Why did early astronomers conclude that Earth had to be motionless?
When you quickly jerk a cart forward that has a ball resting in the middle, the
A) front of the cart hits the ball. B) back of the cart hits the ball. C) neither, for the ball rides along in the middle as the cart moves forward. D) All of the above depending on how quickly the cart is pulled.