f un s ide of m echanics by jonathan abbott. w hat i s it ? what is mechanics? it is the study of...
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FUN SIDE OF MECHANICSBy Jonathan Abbott
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WHAT IS IT?
What is Mechanics? It is the study of Motion.
Mass Property of an object: how much stuff is in an object
Weight How much gravity pulls on a object
Inertia The tendency for an object to resist change in motion
Newton’s First Law An object in motion stays in that motion until acted
upon by an outside force Newton’s Second Law
Force = Mass x Acceleration
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WHAT IS IT? Force
A push or a pull Net force
The sum of all the forces on an object Speed
Distance/time Displacement
Vector pointing from start to end Velocity
Displacement/Time: a rate of change in position Acceleration
A rate of change in velocity Scalar vs Vector
Scalar: Only magnitude, Vector: magnitude and direction.
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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.
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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) So what angle will give the maximum range?
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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).
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IDENTIFY THE TYPE(S) OF FRICTION
Kinetic between Skateboard and Rail
Static between Feet and Skateboard
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IDENTIFY THE TYPE(S) OF FRICTION
Static between Wheels and road
www.flickr.com/photos/question_everything/3893946197/.
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COEFFICIENT OF FRICTION
The force of friction is given by:
Ffriction = μ * Nμ is the “coefficient of friction”N is the normal force. (Do you
remember what that was?)
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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
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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/michaelimage/530418902/
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WHY MIGHT WET ROADS BE A PROBLEM?
Guess what the coefficient of friction is between tires and a dry road Answer: about .7
Guess what the coefficient of friction is between tires and a wet road Answer: about .4
Source: http://hyperphysics.phy-astr.gsu.edu/hbase/mechanics/frictire.html
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WHAT IS THE BEST WAY TO STOP A CAR MOVING ON ICE?
http://www.flickr.com/photos/egazelle/3111507422/sizes/z/in/photostream/
Pump the brakes!
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LAB!
We are going to test the coefficient of friction for a whole bunch of objects.
a
bcΘ
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3
2
1
IDENTIFY ALL OF THE ACTION-REACTION PAIRS
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FREE BODY DIAGRAMS, WITH FRICTION!
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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.
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WHERE IS THE CENTER OF MASS?
2m
1m