Blogpost 4

A. Rotational and Tangential Velocity

• Factors of Circular motion
• Tangential motion
• Linear speed of something moving in a circle
• How fast an object with a circular path is moving
• Usually measured in m/s or km/h
• Rotational Speed

•  Also called angular speed
• Evaluates the number of full revolutions an object does over a period of time
• On a line from the center of a circle to a point on the outside, all objects on the line will have the same rotational speed
• If you begin at the center of a table and crawl outwards as it spins the rotational speed will remain the same
• Tangential speed is directly proportional to rotational speed at any fixed distance from the axis of rotation
• Examples
• 2 kids are on a merry go round. One is on the outside and one is on the inside. The one on the outside will have a greater tangential velocity but they both will have the same rotational velocity. This is because the make the same rotations in a period of time but the one on the outside will cover a much greater distance.
• 2 gears. One large and one small. They will both move one tick at a time, but since one tick is the same distance they will move at the same speed. On the other hand the smaller one will have a greater rotational velocity because it may have 10 ticks and the other has 30. So they move at the same tangential speed but the small one will make 3 full revolutions as the big one makes one.

B. Rotational Inertia

• The property of an object to resist changes in its rotational state of motion
• Greater rotational inertia makes it much harder for it to fall or rotate over
• Something rotating will continue rotation while something not rotating will remain at rest
• Dependent on mass
• More specifically mass distribution
• The further the mass is from the axis of rotation the greater the rotational inertia will be making it harder for the object to rotate
• Tightrope walkers use long poles to take advantage of this
• The pole has a great rotational inertia and the longer the pole the harder it would be for them to rotate
• An object with a high rotational inertia is harder to get sped up, but once they begin rotating they have a tendency to continue rotating
• A long baseball bat is much harder to swing but it will continue to rotate once it begins to rotate
• A ring will lose to a solid marble regardless of their mass because a hoop has a great rotational inertia
• It is much harder to get spinning

C. Conservation of Angular Momentum

• If no external net torque acts on a rotating system, the angular momentum of that system remains constant
• With the weights far away from the axis of rotation he moves at a slow speed but when he pulls the weights in close to him, he speeds up
• Though he goes much faster the angular momentum remains the same
• If he were to put the weights back out, he'd slow down again
• Pulling the weights in makes him easier to spin because 
• Similarly, a diver in the air
• While flipping divers will sometimes "tuck" 
• Brings legs in towards chest and curls up with a "cannonball"
• Allows you to rotate much faster because you decrease your rotational inertia
• As he goes up he will go slow
• As he tucks, his rotational speed will increase
• As he leaves the tuck, his rotational speed will go back down
• Throughout the entire flip his angular momentum remained the same

D. Torque

• Rotational counterpart of force
• Torque causes a rotation or twist of an object
• Defined as torque= lever arm x force
• Lever arm is the part of the system that provides leverage
• The shortest distance between the applied force
• Torque is directly proportional to both lever arm and force
• The ratio of effect lever arm an force have on torque is 1:1
• They have equal effects on torque
• 2 children on a seesaw

• One is twice the force as the other
• The smaller one will need a bigger lever arm so must be further from the center of the seesaw

E. Center of Mass/Gravity

• Center of Mass
• CM
• The average position of all the mass that makes up an object
• A symmetrical object will have its center of mass directly in the center
• An irregular object will have its center of mass closer to the bigger side
• The center of mass doesn't have to touch the object at all
• Center of Gravity
• CG
• Same as CM since mass and weight are proportional
• The center of mass cannot go beyond the base of support or the object will rotate/fall over
• This is why athletes are told to crouch and bend their knees.
• Much harder for them to fall over 
• Much harder for them to get knocked over

F. Centripetal/Centrifugal Force

• Centripertal
• Center seeking
• Pulls inward toward the center of a circle
• Tin can whirling on a string, you must keep pulling the string
• String omits the centripetal force
• Centripetal force depends on the mass of the object
• Also depends on tangential speed and radius of the circle
• Centrifugal Force
• Center fleeing
• Not real 
• NEVER THE ANSWER
• NOT WHAT CAUSES YOU TO KEEP GOING STRAIGHT IN A TURNING CAR
• Turning car is by properties of inertia
• Nothing made you stop, so you continued going straight