# Angular Momentum - Physics 101 / AP Physics 1 Review with Dianna Cowern | Summary and Q&A

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January 5, 2021
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Physics Girl
Angular Momentum - Physics 101 / AP Physics 1 Review with Dianna Cowern

## TL;DR

Learn about angular momentum and moment of inertia, and how they affect the motion of objects in physics.

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### Q: Why does water sprayed from a spinning sprinkler curve and trail behind?

The water curves and trails behind because of the conservation of angular momentum. As the sprinkler spins, the water droplets inertia causes them to resist changing their direction, resulting in the curved path.

### Q: Why does the rotational analog of mass, called moment of inertia, affect an object's spin?

Moment of inertia measures the object's resistance to rotational motion. The further the object's mass is from the axis of rotation, the greater its moment of inertia, making it harder to spin.

### Q: How is the moment of inertia calculated for objects with varying shapes?

For simple shapes like spheres or rings, the moment of inertia is calculated using their mass and radius. However, for irregular shapes, the moment of inertia is calculated by summing up the individual moments of inertia of all the mass elements using their mass and distance from the axis of rotation.

### Q: Can humans running in the same direction change the Earth's speed of rotation?

While humans running in the same direction would have angular momentum, the effect on the Earth's speed of rotation would be minuscule. It would result in a change of less than 0.0003% in the Earth's rotation speed.

## Summary & Key Takeaways

• Objects that are spinning will continue to spin unless acted upon by an external force, due to their angular momentum.

• Moment of inertia is the rotational analog of mass, and it determines how difficult it is to spin an object. It is calculated by multiplying the mass of an object by the square of its distance from the axis of rotation.

• The distribution of mass in an object affects its moment of inertia. Different shapes have different moments of inertia, with a hoop having the largest moment of inertia and a sphere having the smallest moment of inertia.