Unit Four Summary

Unit four unit summary

What did I learn this unit?

     Rotational inertia-

Rotational inertia is how hard or easy an object is to start or stop turning. An object with a larger rotational inertia will have more mass farther away from the axis of rotation; this is seen with the hoop and solid ball experiment. The key to a low rotational inertia is to have mass evenly distributed, or distributed closer to the axis of rotation. Angular momentum cannot be destroyed, only transferred. As a figure skater brings in her hands her rotational inertia lessens, she speeds up to balance her momentum back to its previous state..

·              Torque, and center of gravity

Torque causes rotation. A torque is created with a force and lever arm, and is inversely proportional. When you lower your force you must increase you lever arm to counter act the loss to keep the same torque. Unbalanced torqueses cause rotation. There is always a clock wise toque, and counter clockwise torque on an object.

-Center of gravity is the area of average mass, and where gravity pulls down; this is usually half way between the fulcrum and the edge of an object. When an objects center of gravity moves outside the base of support the object will fall over. Imagine a person trying to touch their toes while against a wall. To combat falling over a person will bend backwards as they lean over keeping their center of mass over their feet or base. Athletes bend their knees so they are less likely to be rotated off their base. They also widen their stance so they have a larger base of support making it harder to be rotated off their base of support.

·              Centripetal Force

Centripetal force makes an object stay in an arc. It is a center seeking force. It makes the object stay tangent to the axis of rotation. It also ensures that an object keeps moving while going into a turn. Centripetal force can be found by adding F weight + F support. Centripetal force is often mixed up with centrifugal force, which does not exist.

·              Rotational and tangential velocity

Rotational velocity is how many rotations an object completes in a certain amount of time, while tangential velocity creates a linear distance. A train wheel has tapered wheels, and though both ends have the same rotational velocity, the larger has more tangential velocity due to its longer radius. This longer radius is why a car with over size wheels will go faster than the speedometer reads. An example of something with the same tangential, but different rotational speeds would be a gear on a chain. Imagine a bike, the chain running at the same speed but the larger forward gear runs slower than the smaller rear gear.

     What are some things students may have forgotten while working on this unit?

-    Centrifugal force does not exist

·        -       An object will rotate if its center of gravity is moved from below its base.

            -     The farther the average mass is away from the axis of rotation the more rotational inertia it will have

              -     Inertia is not a force

Formulas from the unit

·         Angular momentum = (force) (velocity)

·         Torque = (Lever arm) (Force)

·         Balanced object = (Lever arm) (Force) = (Lever arm) (Force)

·         Centripetal force = F weight+ F support

How is this relevant to the real world?

            Rotation is everywhere, and is the reason earth is habitable. From turning a desk chair, loosening a bolt, or turning a car this unit can be applied to every situation even on the sports field. I enjoyed most learning about torque, but was also interested in rotational inertia. This unit was challenging, but every piece of material can be directly linked to the real world making it my favorite unit so far.

Podcast on torque