How Things Work: An Introduction to Physics week 5

In this blog you will find the correct answer of the Coursera quiz How Things Work: An Introduction to Physics week 5 mixsaver always try to brings best blogs and best coupon codes

Week- 5

Seesaws

1. A toy top is a disk-shaped object with a sharp point and a thin stem projecting from its bottom and top, respectively. When you twist the stem hard, the top begins to spin rapidly. When you then set the top’s point on the ground and let go of it, it continues to spin about a vertical axis for a very long time. What keeps the top spinning?

The top has rotational inertia.
You continue to twist the top, even though you are no longer touching it.
Gravity twists the top and keeps it spinning.
The ground exerts an upward support force on the top that keeps the top spinning.

2. To start a motionless toy top spinning, you twist it. What determines the direction in which the top spins?

In the Northern Hemisphere, the top spins clockwise. In the Southern Hemisphere, the top spins counterclockwise.
In the Northern Hemisphere, the top spins counterclockwise. In the Southern Hemisphere, the top spins clockwise.
The torque you exert on the toy top has a direction and the top undergoes angular acceleration in the direction of the torque you exert on it.
How you set the top on the ground determines the direction in which the top spins.

3. You are traveling through deep space in a large spaceship and everything in the ship is weightless. The ship is experiencing zero net force and it coasts forward. However, in preparation for docking at a space station, the ship is rotating slowly. You notice that one location in the coasting ship moves at constant velocity, even as the rest of the ship rotates about that location. What is this special location in the ship?

The ship’s center of gravity.
The ship’s center of mass.
The ship’s center of balance.
The ship’s geometrical center.

4. A tall luxury hotel has a rotating restaurant at its top. The disk-shaped floor of the restaurant rotates slowly about the center of the restaurant and completes one full rotation every 30 minutes. When the restaurant opens each day, the manager turns on the motors that make the restaurant spin, but it takes several minutes for the restaurant to begin spinning at its full angular velocity. Why doesn’t the restaurant reach full speed immediately?

The restaurant’s angular velocity is proportional to the net torque exerted on it. The motors take time to warm up and the net torque they produce on the restaurant increases steadily for the first few minutes of operation.
The restaurant’s angular velocity is proportional to its rotational mass. The motors gradually increase the restaurant’s rotational mass over the first few minutes of operation.
The restaurant’s angular velocity is proportional to 1 divided by its rotational mass. The motors gradually decrease the restaurant’s rotational mass over the first few minutes of operation.
The restaurant’s angular acceleration is proportional to the net torque exerted on it. The motors produce a net torque on the restaurant and it immediately undergoes angular acceleration. But it takes time for the angular-accelerating restaurant to reach its full angular velocity.

5. A modern bicycle has two pedals mounted on a rotating device known as a crank. Pushing down on one pedal with your foot produces a torque on the crank, about its pivot, except in which situation(s)?

When the pedal is directly in front of the pivot, any motion of the pedal will cause it to travel backward—toward the rear of the bicycle. A force exerted on the pedal in that position will produce zero torque on the crank.
When the pedal is vertically above or below the pivot, your force on the pedal is directed along the lever arm from the pivot to your force. A force that is parallel to the lever arm produces zero torque.
When the pedal is directly in back of the pivot, any motion of the pedal will cause it to travel forward—toward the front of the bicycle. A force exerted on the pedal in that position will produce zero torque on the crank.
When the pedal is moving as fast as you can move your foot, a force you exert on that pedal will produce zero torque on the crank.

6. Arm-wrestling is a simple game that two people can play. The players sit across from one another at a table, place their right elbows together on the tabletop and clasp their right hands together. When the competition starts, each person tries to twist the pair of arms counterclockwise from that person’s perspective until those arms touch the table. It’s a rotational problem, with the elbows acting as the pivot and the two players trying to rotate the pair of arms in opposite directions. Suppose you are arm-wrestling with a friend and you are winning. Compare the torque you are exerting on your friend to the torque that your friend is exerting on you.

Those two torques are equal in amount but opposite in direction.
The torque you are exerting on your friend is greater in amount than the torque your friend is exerting on you.
The torque your friend is exerting on you is greater in amount than the torque you are exerting on your friend.
Those two torques are equal in amount and direction.

7. You are arm-wrestling another friend and find that you are almost perfectly matched. Your pair of arms is vertical and motionless, even though you are both trying hard to win. To begin winning, you want that pair of arms to rotate counterclockwise from your perspective. What must you do to make that happen?

The torque you exert on your friend’s arm must be greater in amount than the torque your friend exerts on your arm.
The angular acceleration of your arm must be greater than the angular acceleration of your friend’s arm.
The angular velocity of your arm must be greater than the angular velocity of your friend’s arm.
The counterclockwise torque you exert on the pair of arms must be greater in amount than the clockwise torque your friend is exerting on that pair.

8. You are making pizza and are spinning a ball of pizza dough in midair to make a larger and larger disk. As the diameter of the disk increases, you find it more difficult to change the disk’s angular velocity. Why?

The disk’s mass increases with its diameter, although its rotational mass remains unchanged.
Both the disk’s mass and rotational mass increase with its diameter.
The disk’s rotational mass increases with its diameter, although its mass remains unchanged.
The disk’s angular acceleration increases with its diameter, but its angular velocity remains unchanged.

9. Your car has a flat tire and you are using an automobile jack to lift the corner of the car so that you can change the tire. The jack involves a lever and you lift the corner of the car upward by pushing the handle of the lever downward. You notice that as the handle moves downward 10 inches, the corner of the car moves upward only 0.5 inches. Assuming that the jack is not wasting any energy, compare the downward force you exert on the jack handle to the upward force that the jack exerts on the car.

The jack’s upward force on the car is 20 times as large as your downward force on the jack handle.
The jack’s upward force on the car is 10 times as large as your downward force on the jack handle.
The jack’s upward force on the car is 0.5 times as large as your downward force on the jack handle.
Your downward force on the jack handle is 20 times as large as the jack’s upward force on the car.

10. Tower cranes are frequently seen in cities, where they are used to construct tall buildings. In a tower crane, a huge metal beam sits atop a vertical metal tower. The beam extends outward from the tower in two directions and it pivots about the top of the tower. A lifting cable hangs from one end of the beam and heavy weights hang from the other end of the beam. Since the lifting cable end of the beam is the only end that seems to do anything, what useful purpose does the weight-end of the beam serve?

The weight-end of the beam makes the beam more responsive to torques so that the crane operator can make it undergo more rapid angular accelerations about its pivot.
The weight-end of the beam ensures that the beam is approximately balanced about its pivot and experiences approximately zero torque due to gravity.
The beam pivots about its geometrical center, so it needs both ends in order for that geometrical center to located above the tower.
The weight-end of the beam places the beam’s center of gravity at that end and thus makes the beam more stable.

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