Transcript Friction, Action Reaction Pairs, Center of Mass

FUN SIDE OF MECHANICS
By Jonathan Abbott
WHAT IS IT?

What is Mechanics?


Mass


The tendency for an object to resist change in motion
Newton’s First Law


How much gravity pulls on a object
Inertia


Property of an object: how much stuff is in an object
Weight

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It is the study of Motion.
An object in motion stays in that motion until acted
upon by an outside force
Newton’s Second Law

Force = Mass x Acceleration
WHAT IS IT?

Force
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Net force


Displacement/Time: a rate of change in position
Acceleration


Vector pointing from start to end
Velocity


Distance/time
Displacement


The sum of all the forces on an object
Speed


A push or a pull
A rate of change in velocity
Scalar vs Vector

Scalar: Only magnitude, Vector: magnitude and
direction.
BONUS QUESTIONS

What is i-Hat, j-Hat, and k-Hat?



They are unit vectors.
Unit vectors have a magnitude of 1 unit and point in
a particular direction.
What is Simple harmonic Motion? (What are two
examples I gave last week).
Simple Harmonic Motion is when an object oscillates
(like a sine or cosine function)
 One example is idling on a unicycle
 Another example is a mass on a spring.

PRACTICE QUESTIONS
A car initially traveling at 6 m/s accelerates at
constant acceleration of 3 m/s2 until it reaches the
speed limit of 18 m/s (~40 mph). What distance
does it travel during this time?
 Answer: 48m


We can use average velocity to get this answer too!
A soccer ball is kicked at an angle Θ above the
horizontal at velocity v. What will be the range of
the soccer ball if it travels over flat ground?
 Answer: Δx = v2 /g * sin (2 Θ)

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So what angle will give the maximum range?
TWO TYPES OF FRICTION: STATIC AND
KINETIC
 Static
friction is when two objects are
not scraping with each other (not
moving/sliding).
 Kinetic
friction is when the two objects
are scraping with each other
(moving/sliding).
IDENTIFY THE TYPE(S) OF FRICTION
Kinetic
between
Skateboard
and Rail
Static
between
Feet and
Skateboard
IDENTIFY THE TYPE(S) OF FRICTION
kr.com/photos/question_everything/3893946197/.
Static
between
Wheels and
road
COEFFICIENT OF FRICTION
The
force of friction is given by:
Ffriction = μ * N
μ is the “coefficient of friction”
N is the normal force. (Do you
remember what that was?)
COEFFICIENT OF FRICTION
The coefficient of static friction is usually greater
than the coefficient of kinetic friction.
 Usually the coefficient of friction is less than 1.
 The coefficient of friction only depends on the two
objects in contact

WHAT IS THE BEST WAY
TO MOVE THIS ROCK?
Jerk it so that it
starts to move.
Then keep it
moving
http://www.flickr.com/photos/michaelima
ge/530418902/
WHY MIGHT WET ROADS BE A PROBLEM?

Guess what the coefficient of friction is between
tires and a dry road


Guess what the coefficient of friction is between
tires and a wet road


Answer: about .7
Answer: about .4
Source: http://hyperphysics.phy-astr.gsu.edu/hbase/mechanics/frictire.html
WHAT IS
THE BEST
WAY TO
STOP A
CAR
MOVING
ON ICE?
Pump the
brakes!
http://www.flickr.
com/photos/egazel
le/3111507422/siz
es/z/in/photostrea
m/
LAB! 

We are going to test the coefficient of friction for
a whole bunch of objects.
c
Θ
a
b
IDENTIFY ALL OF THE ACTION-REACTION
PAIRS
1
2
3
FREE BODY DIAGRAMS, WITH FRICTION!
CENTER OF MASS
The center of mass is where a force (like gravity)
acts on an object.
 We care about the center of mass because next
week when we discuss balance, the location of the
center of mass is very important.

WHERE IS THE CENTER OF MASS?
2m
1m