**Brainstorm**

**I clickers**

**Research**

**Motion in 1-D**

**Friction**

Kinematics

Vectors

Components

Important

Details

(cc) photo by Metro Centric on Flickr

(cc) photo by Franco Folini on Flickr

(cc) photo by jimmyharris on Flickr

Forces!!!

(cc) photo by Metro Centric on Flickr

NEWTON's

LAWS

1st Law

2nd Law

F=ma

if F = 0

then a = o

and v = ?

Static Equilibrium

Dynamic Equilibrium

gravity & weight

Types of Friction

Drag and Velocity

Equilibrium

Weight

Friction

Drag

F=ma

Equilibrium

Sum of forces = Zero

Static

Velocity also = zero

NOT MOVING

Dynamic

Velocity is constant

IS MOVING

Note:

Only works on a particle model, just a dot,

we'll deal with rods and massive objects later

Using F=ma

Two steps:

1. Find Acceleration

2. Use kinematics to find motion

Components, Components, Components

Lots of Algebra

Free body diagrams

Use acceleration in equations

Hints: Look for knowns (time, distance, velocities)

Repeat same steps

each time, always!!

Weight, Gravity, Mass

Newton's law of gravity

Yet we have said F = mg

G

Weight w = mg

ALWAYS

What about elevators?

- An apparent weight!

3 Types

Take a closer

Look

SUM it all up!!

**QUIZ**

TIME

TIME

If an objects Net Force

is Zero then

A. Its not moving

B. It has constant velocity

C. Either / or Depends

In using Newton's Laws in

problems it is good to use:

A. FBD

B. Vector Components

C. Algebra

D. Kinematic equations

E. All the above, in order!!

If I wanted to have an apparent weight double my normal weight

what should my acceleration be?

A. 2 g

B. g

C. 0

D. -g

E. -2g

(g can change though)

**"Could you please go over static vs dynamic equilibrium?"**

**" Can we go over the figure 6.1 with the man weighing himself in the accelerating elevator?"**

**"Is it appropriate to say "apparent weight" when talking about objects accelerating? "**

**" how do we measure mass?"**

**"Why is the cup "Accelerating" with the astronaut? from the pre-quiz questions?"**

**A student stands on a scale in an elevator that is accelerating at 2.5 m/s2. If the student has a mass of 78 kg, to the nearest newton what is the scale reading?**

Static Friction

Static friction is probably the most complicated.

It is the force of friction if the object is not moving. This is the same idea as static equilibrium.

It can range from zero, up to a maximum value.

It takes the form of:

Where n is the normal force

and mu is the coefficient of

Static friction.

Things to note with this form of friction

It is not equal to that maximum value unless

it is just starting to slip.

Thus when these problems are written they look

like:

"what minimum force is require to start a book sliding"

or

"What is the maximum angle that a car can park if it is

not to slide..."

This range however is 1-to-1 linear

from zero up to that max value.

Kinetic Friction

Most common, constant, smaller than max static friction

Always opposes motion. Has the value of:

One often sees graphs for this like the following

Force push

Friction

static

at rest

kinetic

accelerating

max

F

fs

slope = 1

Rolling Friction

Smallest.

Closer to Static (breaks bonds vertically)

Here is a little table to compare these coefficients for you.

Static

1.00

0.80

0.10

0.5

0.12

0.10

Kinetic

0.80

0.60

0.05

0.2

0.06

0.03

rolling

0.02

0.002

Material

Rubber on concrete

Steel on Steel (dry)

Steel 0n Steel (lubricated)

Wood on wood

Wood on Snow

Ice on Ice

Causes of Friction

Rolling Friction

Kinetic Friction

Static Friction

Bonds form between the surfaces, max value is when bonds are broken.

Some bonds, not nearly as much

Rather surfaces 'protrude' into the other surface, and then gets 'scrapped off'

This is why tread wears out.

Similar to static, but the bonds are broken vertically

Similar to kinetic in that not nearly as many bonds form

With this visualization comes lubricants.

Liquids often help make the atoms not touch each other, but filling the space between

the substances, causing them to move by without

forming bonds.

In this text they use a simplified model that accounts for

Size (cross sectional, needs to be .001 - 10 m)

Speed must be relatively small

Just moving through atmosphere close to earth

DRAG

Complicated force. Always opposes motion.

Depends on speed and viscosity. It takes different forms.

C is a constant that describes how Aerodynamic the object is (ranges form 0-1.5)

rho is the the density of the material (air)

A is cross sectional area

v is velocity

Terminal Velocity

As the speed of an object increases, (due to force of gravity) the Drag force increases. Eventually these forces equal and cancel.

This removes the acceleration and that final speed reached is call the terminal velocity.

Fg = D

DRAG and Terminal Velocity

Which is the largest force of friction?

a. Static

b. Kinetic

c. Rolling

d. Max static

e. Can't be determined

Which is true when acting on an object?

c) Both acceleration and drag depend on mass

d) Neither acceleration nor drag depend on mass

a) Drag depends on mass, but acceleration does not.

b) Acceleration depends on mass, but drag does not