physics exam 3 clickers

A rigid object rotates in a counterclockwise sense around a fixed axis. If the object starts from rest at the initial angular position, moves counterclockwise with constant angular acceleration & arrives at the final angular position with the same angular speed in all three cases, for which choice is the angular acceleration the highest? 1. 3 rad, 6 rad 2. -1 rad, 1 rad 3. 1 rad, 5 rad

-1 rad, 1 rad 𝝎𝟐 = 𝝎o𝟐 + 𝟐𝜶𝚫𝜽

A mass on a spring in SHM has amplitude 𝑨 & period 𝑻. What is the net displacement of the mass after a time interval 𝑻 ?

0

A mass on a spring in SHM has 𝒂𝒎𝒑𝒍𝒊𝒕𝒖𝒅𝒆 𝑨 & 𝒑𝒆𝒓𝒊𝒐𝒅 𝑻. How long does it take for the mass to travel a total distance of 𝟔𝑨 ?

1.5T

The figure shows a person's foot. In that figure, the Achilles tendon exerts a force of magnitude 𝑭 = 𝟕𝟐𝟎 𝑵. What is the magnitude of the torque that this force produces about the ankle joint? (50 degrees, 3.5 cm)

16 N*m

A 𝟏 𝒌𝐠 ball is hung at the end of a rod 𝟏 𝒎 long. If the system balances at a point on the rod 𝟎. 𝟐𝟓 𝒎 from the end holding the mass, what is the mass of the rod?

1kg

A rigid object rotates in a counterclockwise sense around a fixed axis. Each of the following pairs of quantities represents an initial angular position & a final angular position of the rigid object. Suppose the change in angular position for each of these pairs of values occurs in 1 s. Which choice represents the lowest average angular speed? 1. 3 rad, 6 rad 2. -1 rad, 1 rad 3. 1 rad, 5 rad

2. -1 rad, 1 rad 𝝎𝒂𝒗 = 𝜽 − 𝜽𝒐/deltaT

Which class of levers do belong Fork & Chopstick?

3rd class

A mass on a spring in SHM has 𝒂𝒎𝒑𝒍𝒊𝒕𝒖𝒅𝒆 𝑨 and 𝒑𝒆𝒓𝒊𝒐𝒅 𝑻. What is the total distance traveled by the mass after a time interval 𝑻?

4A

A (static) mobile hangs as shown below. The rods are massless & have lengths as indicated. The mass of the ball at the bottom right is 𝟏 𝒌𝐠. What is the total mass of the mobile?

6kg. Use torques in two steps: (1) find the big mass on the bottom left (lower rod only), and (2) use the entire lower rod assembly (with two masses) to find the mass on top right. Finally, add up all the masses

Bonnie sits on the outer rim of a merry-go-round, & Klyde sits midway between the center & the rim. The merry-go-round makes one revolution every 2 seconds. Who has the larger linear (tangential) velocity?

Bonnie. Their linear speeds 𝒗 will be different because 𝒗 = 𝝎𝒓 & Bonnie is located farther out (larger radius r ) than Klyde.

Two spheres have the same radius & equal masses. One is made of solid aluminum, & the other is made from a hollow shell of gold. Which one has the bigger moment of inertia about an axis through its center? a) solid aluminum b) hollow gold c) same

Hollow gold Moment of inertia depends on mass & distance from axis squared. It is bigger for the shell because its mass is located farther from the center.

A spaceship is traveling to the Moon. At what point is it beyond the pull of Earth's gravity?

It is never beyond the pull of Earth's gravity.

You weigh yourself on a scale inside an airplane that is flying with constant speed at an altitude of 20,000 ft. How does your measured weight in the airplane compare with your weight as measured on the surface of the Earth?

Less than. At a high altitude, you are farther away from the center of Earth. Therefore, the gravitational force in the airplane will be less than the force that you would experience on the surface of the Earth.

An object that hangs from the ceiling of a stationary elevator by an ideal spring oscillates with a period T. If the elevator accelerates upward with acceleration 2g, what will be the period of oscillation of the object?

T

You are sitting on a swing. A friend gives you a push, & you start swinging with period T1. Suppose you were standing on the swing rather than sitting. When given the same push, you start swinging with period T2. Which of the following is true?

T1>T2 Standing up raises the center of mass of the swing, making it shorter!

The Moon does not crash into Earth because:

The Moon does not crash into Earth because of its high speed. If it stopped moving, it would, of course, fall directly into Earth. With its high speed, the Moon would fly off into space if it weren't for gravity providing the centripetal force.

Bonnie sits on the outer rim of a merry-go-round, and Klyde sits midway between the center and the rim. The merry-go-round makes one complete revolution every 2 seconds. Klyde's angular velocity is: a) same as Bonnie's b) twice Bonnie's c) half of Bonnie's d) one-quarter of Bonnie's e) four times Bonnie's

The angular velocity 𝝎 of any point on a solid object rotating about a fixed axis is the same. Both Bonnie and Klyde go around one revolution every 2 seconds.

An object is rolling, so its motion involves both rotation & translation. Which one of the following statements must be true concerning this situation? a) The total mechanical energy is equal to the sum of the translational and rotational kinetic energies and the gravitational potential energy of the object. b) The translational kinetic energy may be equal to zero joules. c) The gravitational potential energy must be changing as the object rolls. d) The rotational kinetic energy must be constant as the object rolls.

The total mechanical energy is equal to the sum of the translational and rotational kinetic energies and the gravitational potential energy of the object.

A figure skater spins with her arms extended. When she pulls in her arms, she reduces her rotational inertia & spins faster so that her angular momentum is conserved. Compared to her initial rotational kinetic energy, her rotational kinetic energy after she pulls in her arms must be

larger because she's rotating faster

The force of gravity between two planets depends on their

masses and distance apart.

A grandfather clock has a weight at the bottom of the pendulum that can be moved up or down. If the clock is running slow, what should you do to adjust the time properly?

move the weight up The period of the grandfather clock is too long, so we need to decrease the period (increase the frequency). To do this, the length must be decreased, so the adjustable weight should be moved up in order to shorten the pendulum length.

Two forces produce the same torque. Does it follow that they have the same magnitude? a) yes b) no c) depends

no

A mass on a spring in SHM has 𝒂𝒎𝒑𝒍𝒊𝒕𝒖𝒅𝒆 𝑨 & 𝒑𝒆𝒓𝒊𝒐𝒅 𝑻 At what point in the motion is 𝒗 = 𝟎 & 𝒂 = 𝟎 simultaneously?

no point

If the masses of two planets are each somehow doubled, the force of gravity between them

quadruples. Note that both masses double, so double * double = quadruple.

Two rockets are blasted into space from earth to two different planets. Rocket A needs more fuel, so it is twice the mass of rocket B. How does the escape velocity of A compare to that of B?

same value

You are holding a spinning bicycle wheel while standing on a stationary turntable. If you suddenly flip the wheel over so that it is spinning in the opposite direction, the turntable will: a) remain stationary b) start to spin in the same direction as before flipping c) to spin in the same direction as after flipping

start to spin in the same direction as before flipping

For a given circle, the radian is defined as which one of the following expressions? a) the arc length divided by the radius of the circle b) (3.141592...) times twice the radius of the circle c) two times ninety degrees divided by (3.141592...) d) the arc length divided by the circumference of the circlee) the arc length divided by the diameter of the circle

the arc length divided by the radius of the circle

A hoop and a disk have the same mass & radius. The disk has half the moment of inertia of the hoop. If they start from rest at the top of a ramp, & if the reference for potential energy is at the bottom, which has the greater total energy? a) The hoop b) The disk c) The same

the same The mass of the two objects is the same & the only energy is potential, 𝑼 = 𝒎𝐠𝒉. So, the two objects have the same total energy, which is of course 𝒎𝐠𝒉.

Ethan and Joseph are riding on a merry-go- round. Ethan rides on a horse at the outer rim of the circular platform, twice as far from the center of the circular platform as Joseph, who rides on an inner horse. When the merry-go-round is rotating at a constant angular speed, what is Ethan's angular speed? a) twice Joseph's b) the same as Joseph's c) half of Joseph's d) impossible to determine

the same as Joseph's

A mass oscillates in simple harmonic motion with 𝒂𝒎𝒑𝒍𝒊𝒕𝒖𝒅𝒆 𝑨. If the mass is doubled, but the amplitude is not changed, what will happen to the total energy of the system?

total energy will not change

Astronauts in the space shuttle float because: a) they are so far from Earth that Earth's gravity doesn't act any more b) gravity's force pulling them inward is cancelled by the centripetal force pushing them outward c) while gravity is trying to pull them inward, they are trying to continue on a straight-line path d) their weight is reduced in space so the force of gravity is much weaker

while gravity is trying to pull them inward, they are trying to continue on a straight-line path Astronauts in the space shuttle float because they are in "free fall" around Earth, just like a satellite or the Moon. Again, it is gravity that provides the centripetal force that keeps them in circular motion.

A satellite encircles Mars at a distance above its surface equal to 4 times the radius of Mars. If 𝐠𝒎 is the acceleration due to gravity at the surface of Mars, what is the acceleration due to gravity at the location of the satellite?

𝐠𝐦/𝟐𝟓

Approximately how much taller you are when you lie down compared to when you stand up?

𝟎.𝟕𝟓𝒊𝒏 It happens because the fluid in your spine compresses due to the pull of gravity while standing up, & makes you shorter. However, when you're lying down, your spine decompressed over time, & you become taller.

A force of 𝟏𝟔. 𝟖𝟖𝑵 is applied tangentially to a wheel of radius 𝟎. 𝟑𝟒𝟎 𝒎 & causes an angular acceleration of 𝟏.𝟐𝟎 𝒓𝒂𝒅/𝒔^2 . What is the moment of inertia of the wheel?

𝟒.𝟕𝟖𝒌𝐠∙𝒎𝟐