fun side of mechanics

FUN SIDE OF MECHANICSBy 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

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

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.

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) So what angle will give the maximum range?

TWO TYPES OF FRICTION: STATIC AND KINETICStatic 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

Static between Wheels and road

www.flickr.com/photos/question_everything/3893946197/.

COEFFICIENT OF FRICTIONThe 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?)

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/michaelimage/530418902/

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

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!

LAB! We are going to test the coefficient of friction

for a whole bunch of objects.

a

bcΘ

3

2

1

IDENTIFY ALL OF THE ACTION-REACTION PAIRS

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