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Fun Side of Mechanics

Fun Side of Mechanics. By Jonathan Abbott. 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

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Fun Side of Mechanics

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  1. Fun Side of Mechanics By Jonathan Abbott

  2. 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

  3. 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.

  4. 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.

  5. 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?

  6. 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).

  7. Identify the type(s) of friction Kinetic between Skateboard and Rail Static between Feet and Skateboard

  8. Identify the type(s) of friction Static between Wheels and road www.flickr.com/photos/question_everything/3893946197/.

  9. 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?)

  10. 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

  11. 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/

  12. 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

  13. What is the best way to Stop a car moving on ice? Pump the brakes! http://www.flickr.com/photos/egazelle/3111507422/sizes/z/in/photostream/

  14. LAB!  • We are going to test the coefficient of friction for a whole bunch of objects. b c Θ a

  15. Identify all of the Action-Reaction Pairs 1 2 3

  16. Free Body Diagrams, with friction!

  17. 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.

  18. Where is the center of Mass? 2m 1m

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