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PPT – Chapter 2: The laws of motion, Part II PowerPoint presentation | free to download - id: 486b4d-YTgzM

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Chapter 2 The laws of motion, Part II First two

chapters Introduce the language of

physics Subsequent chapters Explore objects and

underlying physical concepts

Homework 2.1 due Tuesday, Feb. 9 in class

(Jill Bjerke) Exercises 4, 5, 6, 7, 8, 9, 11,

12, 16, 17 Problems 1, 3, 4, 5, 6

Announcements

- Homework 1.3 due Tuesday, Feb. 2
- Web page for class is http//www.wfu.edu/gutho

ldm/Physics110/phy110.htm - Bring i-clicker to class
- You are allowed 30 missed points in the i-clicker

total score ( 160 points) - Homework solutions are posted on web page

(password protected)

PHYSICS 110 TUTOR SESSIONS (in OLIN 101, class

room) Tutor Jillian Bjerke Maggie

Baldwin Session 1 Mo, 4-6 pm (Jill) Session

2 We, 4-6 pm (Jill) Session 3 Th, 5-7 pm

(Maggie)

Chapter 2.1 Wind turbines, rotational motion

Concepts

Demos and Objects

- Wind turbines (rotating wheel)
- Opening rusty screws
- rotating objects
- pirouettes
- levers

- angular displacement
- angular velocity
- angular acceleration
- moment of inertia (rotational mass)
- Torque
- Newtons I. II. law for rotational motion
- levers
- mechanical advantage

Windturbines (this chapter is more about

rotational motion than wind turbines, generators

to create electricity come later)

Observations about wind turbines

- Wind turbines are symmetrical and balanced
- A balanced wind turbine rotates smoothly
- An unbalanced turbine settles heavy-side down
- Most wind turbines have three blades
- Wind turbines start or stop spinning gradually
- Wind turbines extract energy from the wind and

convert it into electrical energy

i-clicker-1

- A diver does a somersault dive (spinning dive).

First, she is tightly tugged in, then extends. - Is the diver spinning when (right before) she

hits the water? - Yes, she still spins (slower).
- No, she will stop spinning.
- Not enough information.

Physics Concept

- Rotational Inertia
- In the absence of an external net torque,
- A body at rest tends to remain at rest.
- A body thats rotating tends to continue

rotating.

We ignore friction for the time being

Physical Quantities for rotational motion

- Angular Position an objects orientation (angle

with respect to reference, i.e. horizontal - Angular Velocity its change in angular position

with time - Torque a twist or spin (more later)

Angular rotation or Angular position, q

- SI unit of angular rotation is the radian
- One radian is 180/p 57.3
- Rotation requires an axis of rotation

Angular velocity w

- SI unit radians per second or just 1/sec
- An other unit rotations per minute (not an SI

unit). - Measure of how fast an object spins
- Angular velocity is a vector!
- Use right hand rule to determine direction of

vector - Align right thumb with axis
- Align fingers with rotational movement
- Thumb points into direction of angular velocity

vector

Angular velocity is a vector

Right-hand rule for determining the direction of

this vector.

Every particle (of a rigid object)

- rotates through the same angle,
- has the same angular velocity,
- has the same angular acceleration.

i-clicker-2

- What is the angular velocity of earths motion

around its own axis? - 1 Year
- 1 Day
- 1 revolution/year
- 1 revolution/day
- 0

Newtons First Law of Rotational Motion

A rigid object thats not wobbling and that is

free of outside torques rotates at a constant

angular velocity.

- Rotational Inertia
- In the absence of external torques,
- A body at rest tends to remain at rest.
- A body thats rotating tends to continue rotating.

Center of mass

When an object is rotating freely (no fixed

axis), it rotates about its center of mass

Center of Mass

- The point about which an objects mass balances
- A free object rotates about its center of mass

while its center of mass follows the path of a

falling object

Where is the center of mass of these objects?

How do we start something spinning??? We have to

apply a torque to it

- We need a
- pivot point
- lever arm
- applied force
- Torque force x lever arm

Lever arm is perpendicular to applied

force (non-perpendicular force will produce

smaller torque)

Torque is a vector

It has a direction and a magnitude

- Use the right hand rule to figure out the

direction of the torque - Thumb is torque, t
- Index finger is lever, r
- Middle finger is Force, F

i-clicker-4

A mechanic is trying to open a rusty screw on a

ship with a big ol wrench. He pulls at the end

of the wrench (r 0.5 m) with a force F 500 N

at an angle of 90.

F

- What is the net torque the mechanics is applying

to the screw? - 500 Nm
- 0.5 m
- 250 Nm
- 250 N
- 90 N

Some objects are harder to spin than others.

Moment of inertia (rotational mass)

- The moment of inertia or rotational mass is a

measure of an objects rotational inertia, its

resistance to change in angular velocity - Analogous to mass (translational inertia)

- Moment of inertia depends on
- mass of object
- and mass distribution (where the mass sits with

respect to axis) - the axis about which the axis rotates

i-clicker Which object is hardest to rotate??

A. B. C.

Physical Quantities

- Angular Position an objects orientation
- Angular Velocity its change in angular position

with time - Torque a twist or spin
- Angular Acceleration its change in angular

velocity with time - Moment of Inertia measure of its rotational

inertia

Newtons Second Law of Rotational Motion

The torque exerted on an object is equal to the

product of that objects moment of inertia times

its angular acceleration. The angular

acceleration is in the same direction as the

torque. Torque Moment of Inertia Angular

Acceleration

Physics Concept

- Net Torque
- The sum of all torques on an object.
- Determines that objects angular acceleration.

Mechanical advantage

- A 200 N child can support a 400 N child
- How does a crowbar work?
- How does a bottle opener work?

(No Transcript)

Summary Angular and linear quantities

Linear motion

Rotational motion

Angular position (angle) q

Position x

Angular velocity w

Velocity v

Angular Acceleration a

Acceleration a

Torque

Force

Newton 2

Newton 2

Rotational mass I

Mass m