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Rotational Mechanics.  Torque produces rotation.  List times when you make an object turn or rotate  NOT a force  To make an object move you apply.

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Presentation on theme: "Rotational Mechanics.  Torque produces rotation.  List times when you make an object turn or rotate  NOT a force  To make an object move you apply."— Presentation transcript:

1 Rotational Mechanics

2  Torque produces rotation.  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

3  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

4  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  F 1 d 1 =F 2 d 2

5 200N 2 m 200 N 400 N 1 m2 m τ= 400 N-m

6  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?

7  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

8  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

9  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?

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

11  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

12  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

13  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.

14  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  ssci/physical/giambattista/ cam/cam.html ssci/physical/giambattista/ cam/cam.html  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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