Describing Motion

Aristotle on Force A force is required to keep an object moving 2000 years pass….

Galileo 1564-1642 Lived from 1564-1642 AD He made a big discovery that proved Aristotle was wrong Dropped two weight objects and they both landed at the same time So Aristotle must have been wrong about forces being necessary to keep objects moving. Galileo said that force is required to START an object moving, but none is required to keep it moving (when there isn’t friction)

Inertia Objects in motion want to stay in motion Objects at rest want to stay at rest Unless acted on by another force.

Mass and Inertia Mass is actually a measurement of inertia. More mass = more inertia Less mass = less inertia

Check it out… https://www.youtube.com/watch?v=vfnt8Sdj7cs Inertia Demos: Redo inertia “magic trick” with hoop and pennies/dice Paint stick and newspaper karate chop Hammer and bowling ball Card and poker chips Magic Broom Meter vs Newton-Meter drop

Lesson Recap A ball rolling on a pool table slowly comes to a stop. How would Aristotle explain this behavior? How would Galileo explain it? How would you explain it?

Speed Speed is a scalar quantity Formula: 𝑆𝑝𝑒𝑒𝑑= 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑡𝑖𝑚𝑒 Scalar means it is just a number without direction Formula: 𝑆𝑝𝑒𝑒𝑑= 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑡𝑖𝑚𝑒

Velocity Velocity is a vector quantity Formula: Vector means it has both a number AND direction Formula: 𝑉𝑒𝑙𝑜𝑐𝑖𝑡𝑦= ∆ 𝑑𝑖𝑠𝑝𝑙𝑎𝑐𝑒𝑚𝑒𝑛𝑡 ∆ 𝑡𝑖𝑚𝑒

Velocity Δ means “change in” which is calculated = (final – initial) ( - ) or (+) in physics just indicates direction

Average vs. Instantaneous “Average” velocity is a measure of the total distance traveled in a certain period of time. “Instantaneous” velocity is the exact speed at a specific point in time

Distance vs. Displacement Distance (scalar) Is how much ground an object has covered Displacement (vector) Is the overall change in position Watch this! https://www.youtube.com/watch?v=V8hJhTE3bUk Now let’s try it! Practice Questions—write on white boards http://www.physicsclassroom.com/class/1DKin/Lesson-1/Distance-and-Displacement

Graphs Position vs. Time graphs--- Slope = velocity value

Acceleration (vector) The formula for acceleration is: 𝐴𝑐𝑐𝑒𝑙𝑒𝑟𝑎𝑡𝑖𝑜𝑛= ∆ 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 ∆ 𝑡𝑖𝑚𝑒

Graphs In a velocity vs. time graph SLOPE = ACCELERATION value

Translations Recall that on a displacement vs. time graph SLOPE = VELOCITY value But what about the slope of a velocity vs. time graph? What are the units? Distance divided by the time squared Example m/s2 What does the measurement mean? It means the object is traveling fanster each second than it did in the second before

Summary of how to represent motion in graphs