# Rotational Mechanics Ch. 11.

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Rotational Mechanics Ch. 11

11.1 Torque Torque produces rotation. NOT a force
List times when you make an object turn or rotate NOT a force To make an object move you apply a force, force causes acceleration Torque is produced when a force is applied with leverage The longer the handle the more leverage can be applied

11.1 continued Lever arm-the distance from the turning axis to the point of contact Torque = Fd W=Fd d in torque is ┴ to the force d in work is parallel to the force Units= newton-meter

11.2 Balanced Torques Use a triple beam balance to weight different masses What relationships do you notice? Torque in 2 different directions Counter clockwise torque opposes clockwise torque If the two are equal the scale will be balanced F1d1=F2d2

11.2 Balance Torque Cont. 200N 2 m 2 m τ= 400 N-m τ= 400 N-m 200N 2 m

11.3 Torque and Center of Gravity
Why can’t a person stand against the wall and then bend forward without falling forward? Your feet are not beneath your center of gravity This creates torque τ= Fd How could the torque be balanced so that a person standing against the wall doesn’t fall over? answer Why do pregnant women get back pain?

11.3 When a football is kicked or throw why does is rotate end over end instead of spiral? If a force is applied off center then torque is created thus causing the ball to move end over end How do you get it to spiral? answer

11.4 Rotational Inertia What is inertia?
Rotational inertia- the resistance of an object to changes in its rotational motion An object rotating wants to keep rotating, an object that is not rotating wants to continue to not rotate Torque is required to change the rotational state of motion Rotational inertia depends on mass And the distribution of mass The greater the distance between the axis and the mass the greater the rotational inertia

11.4 Think about a baseball bat
Why do some people use a shorter bat than other people? What does it mean to “choke up”? Why would a baseball player need to “choke up”? Compare the legs of a giraffe with that of a mouse List some difference How does rotational inertia apply?

11.4 Formulas for rotational inertia: pg. 157
If both cylinders were rolling down a ramp which would have more acceleration? (same mass, same radius)

11.5 Rotational Inertia & Gymnasics
Human body has 3 axes of rotation Median (z) Transverse (x) Longitudinal (y) All at 90° angle to one another Rotational inertia is least about the longitudinal axis thus it is the easiest rotation to perform To increase longitudinal inertia extend an arm or a leg while spinning A summersault rotates about the transverse axis Least when in tuck position Greatest when fully extended A cartwheel rotates about the median axis

11.6 Angular Momentum Angular momentum- inertia of rotation
Vector quantity Has direction and magnitude Rotational velocity- when a direction is assigned a speed Angular momentum= rotational inertia x rotational velocity Angular momentum = I x ω

11.6 An external net torque is required to change angular momentum
A moving bicycle is easier to balance on because the wheels have angular momentum there fore more torque is required.

11.7 conservation of angular momentum
Law of conservation of angular momentum- if no unbalanced external torque acts on a rotating system, the angular momentum of that system is constant With no external torque, the product of rotational inertia and rotational velocity at one time will be the same as at any other time Initial conditions What happens to V as you increase the velocity bar but keep the radius the same? What happens to V as you increase the radius but keep the velocity the same? What happens to the snowman as you change the radius under run time controls?

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