Physics I

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### Transcript

A spinning mass has the property of angular momentum and conservation of angular momentum is central to solving problems in rotational dynamics.

As explained in another video of this collection, an object's linear momentum does not change, that is Δp is zero until a net external force is applied.

The same conservation principle applies to angular momentum, denoted by the letter L. So ΔL is also zero until a net external torque is applied.

Here, we will first explain the concept of angular momentum and show how it is conserved using different examples. Then the video will demonstrate a lab experiment involving measurement of angular momentum for a spinning rod.

To understand angular momentum, let's consider a ball attached to string undergoing a rotational motion about an axis. The magnitude of angular momentum of this ball 'L' is r - the radius of the circle - times p, which is the translational momentum. Now p is mass times velocity, where velocity is the tangential velocity. The tangential velocity is the angular velocity 'ω' times r. The direction of angular momentum is given by the right-hand rule. If you curl the fingers of the right hand in the direction of rotation, then the extended thumb points in the direction of the angular momentum of the system.

Based on this formula and the principle of angular momentum conservation, we can predict that in the absence of net external torque, if r is reduced ω would increase, and if r is increased ω would decrease.

This principle of angular momentum conservation is evident in figure skating. With the arms out the skater rotates at one speed, but as soon as they bring their arms in, the rotation speed increases significantly.

Now that we have reviewed the principle of angular momentum conservation, let's see it in action in a physics lab. For the first demonstration, sit in a chair that can rotate freely and hold two weights out at arm's length. Ask another person to spin the chair. While spinning, bring the weights close to the chest and notice how the chair's speed of rotation increases.

As with the spinning ice skater, when the weights are held far from the body, the person on the chair has a high moment of inertia due to a relatively bigger r. Bringing the weights close to the body reduces the system's moment of inertia, and thus due to conservation of angular momentum, the speed of rotation increases.

For the second demonstration, again sit in a chair that can rotate freely and hold a bicycle wheel by the handles so its axis is vertical. Then spin the wheel counterclockwise, keeping the chair stationary. By the right-hand rule, the direction of the wheel's angular momentum vector is vertical, pointing up.

Flip the wheel so it is spinning clockwise when the axis is vertical again. Now its angular momentum points down. Notice how the chair spins in response.

The bicycle wheel, the person holding it and the chair make up a system of multiple objects. When the wheel alone is spinning, this system has a certain total angular momentum. Although the person holding the wheel applies a torque to flip it over, this torque originates within the system and the net external torque is zero.

With no external applied torque, angular momentum is conserved, meaning it does not change. Flipping the wheel reverses the direction of its angular momentum. In order to keep the total amount of angular momentum in the system conserved, the person and chair must spin, so that their combined angular momentum vector opposes that of the wheel.

As a result, the total angular momentum of the person, chair and flipped wheel must have the same magnitude and be in the same direction as the angular momentum of the wheel in its original position.

Next, let's see an experiment involving measurement of angular momentum of a spinning rod. For this, a falling weight pulls a string wound around an axle. The magnitude of the resulting torque is the tension in the string times the radius of the axle. This torque spins the axle, causing rotational acceleration of the rod attached to it. The rod's moment of inertia can be calculated from its mass M and length L.

The spinning rod's angular acceleration is equal to this torque divided by the rod's moment of inertia. With this information, it is possible to calculate angular velocity at any time from the equations for rotational kinematics.

Finally, using the rod's moment of inertia and angular velocity, the spinning rod's angular momentum will be determined at two points: when the weight has fallen halfway and when it has reached the end of its travel.

Before starting the experiment, measure the rod's length and mass then calculate its moment of inertia. Use a meter stick to determine the halfway point of the weight's downward travel. Mark this point with tape on the vertical beam. Attach 200 grams to the end of the string and wind it until the weight reaches the top.

Release the weight and measure the amount of time to reach the halfway point and the amount of time to reach the bottom. Record the results. Do this three times and use the average values to calculate the angular momentum at both points.

Increase the weight on the string to 500 grams. Perform the procedure four times and record the results. Then increase the weight to 1000 grams, repeat the procedure and record the results.

As the mass of the falling weight increases, the torque and angular acceleration on the axle of the spinning rod should increase proportionally. Theoretically, at any given time both the angular velocity and angular momentum should increase proportionally with this torque.

At any given distance that the weight had fallen, the angular momentum of the spinning rod should have been proportional to the square root of the weight's mass. The experiment showed that the angular momenta with the 500 gram weight were indeed approximately 1.6-or the square root of 5/2-times those of the 200 gram weight. Similarly, the momenta with the 1000 gram weight were approximately 1.4 -or the square root of 2-times those of the 500 gram weight.

Furthermore, for a given weight the torque and angular acceleration should be constant. Under this condition, the spinning rod's angular velocity should increase proportionally with the square root of the distance the weight falls. The final distance was double the distance at the half-way point, so the final angular momentum was 1.4-or the square root of 2-times the angular momentum at the halfway point.

The results from this experiment agree with theory and confirm the relationship between torque and angular momentum.

Angular momentum is an important property of rotating objects and its effects are at the core of many mechanical devices and day-to-day activities.

You must have noticed that it is easier to balance on a bike when it is in motion. The reason for that is angular momentum. When the wheels are in motion, they will have some amount of angular momentum with direction perpendicular to the frame. The larger the angular momentum the larger is the torque required to change the momentum, and therefore it is harder to tip the bike over.

Another system that uses conservation of angular momentum is helicopters with two rotors. Here the front rotor rotates the blades in a clockwise direction and the tail rotor rotates the blades in counter-clockwise direction. These rotations result in two opposing angular momenta, which cancel each other out...resulting in angular momentum conservation for the entire system. And this is what prevents the helicopter from spinning out of control.

You've just watched JoVE's introduction to angular momentum. You should now understand what angular momentum is, how it is conserved in various systems, and how it affects the behavior of rotating objects. As always, thanks for watching!