Unit 4 Reflection

In this unit I learned about:

1. Rotational and Tangential velocity
2. Rotational inertia
3. Conservation of angular momentum
4. Torque
5. Center of mass/gravity
6. Centripetal/Centrifugal force

Rotational velocity (angular speed) is the number of rotations or revolutions per unit of time. For example all parts of a marry go round share the same rotational velocity because they go around the axis of ration the same number of times.

Tangential velocity (linear speed) is greater on the outer edge of a rotating object than it is closer to the axis of rotation. For example a person siting closer to the edge of the merry go round is traveling a farther distance in the same amount of time as someone sitting closer to the axis of rations; therefore, they have a greater tangential velocity.

Rotational inertia is the property of an object to resist change in its rotational state. Rotational inertia depends on the distribution of mass, for the greater the distance the mass is from the axis of rotation the more rotational inertia there will be. For example an ice skater spinning around in one place with their arm spread will have more rotational inertia than an ice skater spinning with their arm drawn in towards their chest. As a result of less rotational inertia the skater has a greater rotational velocity.

Conservation of angular momentum states that in a system with no external force the momentum before and after a reaction are equal. The equation we used to solve for this is:  

rotational inertia (before) x rotational velocity (before) = rotational inertia (after) x rotational velocity (after).

Torque causes rotation. It is the product of a lever arm and the force that produces the rotation; 

torque = lever arm x force. Three ways to increase the torque on something is by increasing the lever arm, increase the force, or do both. Take a door for example, the door knob is place farthest away from the axis of ration because that way when there is a longer lever arm there is less force need to create a torque. If the door knob was placed closer to the hinges then it would require more force to close the door because there is a shorter lever arm.

Center of mass/gravity is the average position of an objects mass. One of the main questions involving center of mass is why do do wrestlers spread their legs apart and squat over? When they widen their stance their base of support is also wider meaning that more force will be need to push them until their center of mass is not over their base of support. Wrestlers also bend their legs because it lowers their bas of support, making it harder for the opponent to knock them over. This example is illustrated in the drawing below.

Centripetal/Centrifugal force. Centripetal force is a center seeking force that draws a body towards the center. For example when a car rounds a corner the friction between the tires and the road provides the centripetal force that keeps the car on the curved road. Centrifugal force is the apparent outward force on a rotating or revolving body. Pretend that you are in the car that is rounding the corning, when car first begins to turn you hit the side of the car door (door on the opposite side of the center of curvature). This is what we call the centrifugal force, it is not a real force but rather a feeling we have, for in actuality we hit the side door because we were once going straight and now being forced to turn in a different direction. This can be related to newtons 3rd law because you push out against the door, only because the door pushes you in. An illustration for this example is shown below.

What was most difficult for me in this unit was understanding the question about why train wheels are shaped the way they are, and how they stay on track. The correct answer to such a question is; because the wheels are connected by a shaft, their rotational speed must be the same, but when one wheel has a larger radius, that side has a greater linear velocity causing the train to curve in towards the center of the track.

I overcame this difficulty by talking the problem step by step with my teacher and asked questions concerning the difference between rotational and tangential velocity. What ultimately made the lightbulb click was understanding that every position on the wheels (small and big) are experiencing the same rotational velocity, and that the reason they stay on track is because the wheel with the larger part on the track is covering a greater distance than the part with the smaller radius on the track. This allows the wheels to balance out.

I worked hard when studying for quizzes and when searching for resources for my blog, and I hope that it will pay off when I take the unit test. My goal for next unit is to try and make longer blog descriptions whenever I post a link, for I think it will be beneficial to my viewers and myself. A connection from this unit that I made to everyday life is that whenever i'm on a roller coaster that goes upside down, the people don't fall out because the centripetal force isn't as strong as the velocity in which it travels at. 

A couple of my classmates and I created a podcast video discussing the definition of torque. Creating this video really helped me understand torque and I hope it does the same for you, enjoy!