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 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

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

http://science.howstuffworks.com/fpte4.htm

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

200N 200N

2 m 2 m

200 N 400 N

1 m2 m

τ= 400 N-m

τ= 400 N-m

τ= 400 N-m

τ= 400 N-m

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?

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

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

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?

Formulas for rotational inertia: pg. 157

If both cylinders were rolling down a ramp which would have more acceleration? (same mass, same radius)

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

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

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.

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

http://www.mhhe.com/physsci/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?