1 Physics -- Rotational Dynamics

A helicopter flies off, its propellers rotating. Why doesn't the body of the helicopter rotate in the opposite direction?

2 Answers

In the air, there's nothing holding the body of the helicopter still - so the rotor blades spin one direction, but the torque applied to spin the rotors tries to spin the body of the helicopter in the opposite direction. The little rotor on the tail helps keep the body of the helicopter pointed in the same direction.

A flat uniform disk of radius 2R has a hole of radius R removed from the center. The resulting annulus is then cut in half along the diameter. The remaining shape has mass M. What is the moment of ...

What is the purpose of the spin cycle of a washing machine? Explain in terms of acceleration components.

The spin cycle in washing machine spins the clothes in the washing machine making them move circular. the continious circular motion by the spin cycle gives centripetal and pseudecentripetal ( centrifugal ) force. after the completion of wash, the centrifugal force generated makes the water in the clothes get extracted due to high force ( caused by acceleration due to the spin cycle ) and make clothes dry. so, it primarily acts as moisture absorbent.

How might you determine experimentally the moment of inertia of an irregularly shaped body about a given axis?

Using the parallel axis theorem

We note the angular speed and also the system energy. Using these information we calculate the moment of inertia of irregular shaped body.

Io=2E/w²

When we have moment of inertia of irregular shaped body, we can then use parallel axis theorem to solve the moment of inertia if the body about the given axis.

I=Io+md²

In western astrology, there are 12 astrological ages corresponding to 12 zodiac signs. The completion of these 12 ages takes about 25,920 years. Knowing that as astrology stemmed from observational...

To maximize the moment of Inertia of a flywheel while minimizing its weight, what shape and distribution of mass should it have? Explain.

We know that the moment of inertia of any rotating body is directly proportional to the square of the distance from its rotational axis. In the case of a flywheel, the position of an individual particle from the rotational axis matters for determining its moment of inertia. So, in order to maximize the moment of inertia, while minimizing its weight, we have to make a flywheel with the maximum possible radius while having gaps in the body, i.e. having spoke-like structures to hold the wheel. A...

An Olympic high diver starts his dive with a double somersault while keeping his arms neatly tucked to his chest. But as he approaches the pool surfaces, his arms stretch over his head to a point a...

I, personally loved this problem. Though I am not that much interested in playing sports on my own, I love the beauty of physics behind each sport, and the way physics beautifies each sport.

Before approaching the question, let's know how diving is scored in Olympic games. The judges score looking at the starting position, approach, height, flight, and entry. The higher jump with a maximum number of somersaults and a perfect entry with a minimal splash is always considered the best one.

In the...

Why it is necessary to use shoulder straps and a lap belt in a car while a lap belt is sufficient in a plane, even though a plane moves much faster than a car?

A yo-yo falls to the bottom of its cord and then climbs back up. (a) Does it reverse its direction of rotation at the bottom? Explain your answer.(b) What "pulls" the yo-yo back to the top?

Get the answer from Yo Yo Master.

In cutting down a tree, a logger makes a cut on the side facing the direction in which the tree is to fall. Explain why. Would it be safe to stand directly behind the tree on the opposite side of t...

A student stands on a table rotating with an angular speed while holding two equal dumbbells at arm's length. He pulls his hands towards him, what's its consequence? But now, without moving anythin...

The angular speed of the student will increase. When the dumbbells are dropped on the floor, no external torque acts on the student and the turntable, and therefore the angular momentum must be conserved.
When the dumbbells are dropped, the rotational inertia of the student and turntable decreased. To compensate for the decrease in rotational inertia the angular speed of the student and the turntable must increase.

An arrow turns in flight so as to be tangent to its flight path at all times. However, a football (thrown with considerable spin about its long axis) does not do this. Why this difference in behavior?

The initial angular momentum due to the spin in the football ensures that any angular momentum due to the action of external forces does not change the direction of the initial angular momentum significantly. This allows the ball to reach the catcher with the same orientation as it was when thrown by the passer.
In case of the arrow, the lack of initial spin allows external forces such as gravity to change the direction of the angular momentum vector. This is accomplished by moving the arrow...

Why it is much more difficult to balance yourself on a bicycle that is at rest than on one that is rolling?

Balance on a bicycle is a matter of constantly correcting against falls, and it's easier when thespeed is higher because the inertia of moving forward overcomes the need for corrective actions.

Why does a long bar help a tightrope walker to keep his or her balance?

You are walking along a narrow rail and you start to lose your balance. If your start falling to the right, which way do you turn your body to regain balance? Explain

Explain in terms of angular momentum and rotational inertia, exactly how one "pumps up" a swing in the sitting position.

When one sits in the sitting position, the rotational inertia of the body decreases. Since no external torque acts on the swing during this, the change of angular momentum must be conserved. To ensure this, the decrease in rotational inertia of the body must be compensated by increase in angular velocity, and the increased angular velocity eventually pumps up the swing.

A person throws a bowling ball with radius 0.6 m so that at some point in time it moves 2 m/s leftward while rotating 4 rad/s counterclockwise about its center of mass. At this point in time, what ...

A toy electric top is comprised of two parts: a plastic disk of mass M and radius R, and a thin metal rod of mass m and L attached to the bottom side of the disk perpendicular at its center. A smal...

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