1. How Can You Describe Motion?
Chapter 1, Lesson 2; pF14-21

2. Observing Motion Observing Motion Describing motion scientifically
Change in object’s position compared to objects at rest
Describing motion scientifically
Measure position or distance
Measure time
Relationship between distance and time
Speed

3. Identifying Frame of Reference
Sometimes it is difficult to tell which object is in motion
Ex. Heliocentric model vs. Geocentric model
Helio = “sun”
Geo = “Earth”
Centric = “center”
See sun move across our sky
Moon move across the sky
Stars move across the sky
People think Earth is the center

4. Identifying Frame of Reference
People use Earth to measure motion
Earth’s equator rotating
1675 km/hr (1041 mi/hr)
Earth orbits sun
100,000 km/hr (62,000 mi/hr)
Solar system orbits galaxy center
792,000 km/hr (492,000 mi/hr)
Sometimes necessary to use more distant frames of reference

5. Identifying Frame of Reference
Motion is observed in comparison to objects that seem to be standing still
Ex.
Block falls from table
Moves past top of the table
Changing distance from the top of the table
Table is the frame of reference
Frame of reference: something you do not expect to move

6. Identifying Frame of Reference
Can’t observe motion without a frame of reference
Sitting in car,“feel” like you are moving forward
Car beside you moved backwards
Sitting in plane, “feel” like you aren’t moving?
Look outside and you are moving toward a runway
Observing motion depends on observer’s frame of reference
Person walking east in west bound train
Motion of walking person inside train
Motion of walking person outside train

7. Calculating Speed Measuring motion often requires measuring time
Speed: measure of how far something moves during a period of time
Speed is a measurement at one instant of time
Cars speed up and slow down
Glaciers & dolphins speed up & slow down too
Ratio of distance moved to elapsed time
Examples / Units:
Humans: 65 km/hr
Light: 300,000 km/s
San Andreas Fault: 2 cm/yr

8. Calculating Speed Difficult to measure speed in an instant of time
Average speed: measure of the total distance the object has moved divided by total elapsed time
Drive from Prairie City to Chicago is 315 mi
The trip takes about 5 hrs
The average speed is 63 mi/hr

9. Velocity Velocity: speed of an object in a particular direction
Why do the following cars have different velocities?
Ex 1: Two cars pass each other in opposite lanes of 2-way highway. Both cars traveling at 88 km/hr?
Different directions
Ex. 2: Two cars pass each other in same lane of 2 lane highway. One car traveling at 60 km/hr the other at 80 km/hr?
Different speeds
What will happen to the following cars?
Ex.3: Two cars traveling at the same velocity?
Will always be same distance from each other

10. Velocity Scientists sometimes use arrows to represent velocity
Length represent speed measurement
Direction it points represents direction measurement
Velocity is circular motion constantly changing
Direction is constantly changing
Speed may remain constant

11. Velocity Changes in velocity caused by applied forces
Car speeds up, slows down, changes direction
Force has acted on it
Your body moves backwards, or forwards, or to the side
Your body resists the force or change in motion

12. The Laws of Motion Evolution of the laws Aristotle Galileo Newton
Moving objects needed a force, push or pull, to keep it moving
Galileo
Moving objects keep moving in straight line until something interfered with it
Newton
Built on Galileo’s work

13. The Laws of Motion
Laws of motion give us a way to measure & predict motion & the effect of forces
Hold true on Earth
Used to describe how objects move on Earth
Laws can only be tested under controlled conditions
Too many other forces acting on objects
Hold true in rest of the universe
Used to describe how planets move around the sun

14. The First Law of Motion First Law of Motion
“An object at rest tends to stay at rest, and an object in motion tends to stay in motion in a straight line until an outside force acts on it”
Objects don’t change velocity unless acted on by force
Examples:
Hockey puck sitting on ice
Puck sliding across the ice
Objects in motion slow down on Earth
Outside force, friction, acts on them

15. The First Law of Motion “Law of Inertia”
Inertia: tendency of an object to resist a change in its motion
Property of car that makes it difficult to start rolling
Property of car that makes it difficult to stop rolling

16. Second Law of Motion Second Law of Motion
“Force = Mass x Acceleration”
Unbalanced forces change motion of objects
Unbalanced force: force without an equal force in the opposite direction
Changes velocity – speed, direction, or both
Acceleration: rate of change in velocity
Depends on mass of object & size of force
Greater force = greater acceleration
Greater mass = smaller acceleration

17. Second Law of Motion Unbalanced forces change motion of objects
Examples:
Object at rest
Object moves in direction of force
Object moving in same direction as force
Object moves faster in same direction
Object moving in the opposite direction as force
Object slows down or stops

18. Second Law of Motion Unbalanced forces change motion of objects
Examples:
1 person pushing on back of car might get it to move
2 people pushing can get it rolling faster
1 person pushes from from – 1 from back may cancel out
Rate of change in car’s velocity depends on:
Mass of car
Size of forces applied to it
Different results when you push on toy car vs. real car

19. Second Law of Motion Unbalanced forces change motion of objects
Amusement Park Examples:
Bumper car bumps into stationary car slowly?
Stationary car moves a little bit
Bumper car bumps into stationary car fast
Stationary car moves a lot more
Acceleration depends on force applied by car in motion
Bumper car bumps into real car
Bumper car moves backwards (car = more mass & inertia)

20. Third Law of Motion Third Law of Motion
“For every action force, there is an equal and opposite reaction force”
Forces always come in pairs
Examples: (Action force?) (Reaction force?)
Pushing on desk / Pulling of Door
Sit on a chair
Skateboarder pushes with a toe
Swimmer pushes on water
Gravitational forces
Parachutist falling to Earth

21. Summary Motion is a change in the position of an object
Speed is a measure of an object over time
Velocity is a measure of speed in a direction
There are three laws that describe motion
An object at rest stays at rest and an object in motion stays in motion
Unbalanced forces change the velocity of an object. The rate of this change in velocity or acceleration depends on the size of the force and the mass of the object
For every action force, there is an equal and opposite reaction force

22. Homework
Read Lesson 2, Chapter 1, pF14-21
Answer ?s on Google Form