1. Physics
Describing Motion

2. Aristotle 384-322 BC One of the first to study motion
Stated there were two kinds of motion
Natural Motion
Violent Motion

3. Natural Motion Due to “nature” of object –
the combination of the 4 elements:
Objects wanted to get to “natural place”
Ex: Smoke rises to be with air; Rocks fall to be with Earth
Straight up and Down (on Earth)
Circular (in space)
Believed different rules applied to the heavens, which were made of quintessence (perfect unchanging substance)

4. Violent Motion Due to pushes and pulls (FORCES) Imposed motion
Ex: Person pushing a cart
Wind blowing a ship
EXTERNAL CAUSE, not due to “nature” of object

5. Galileo 1564-1642, showed Aristotle was wrong
Showed heavy and light objects fall at same rate
Forces are not needed to keep objects in motion
Inertia: Tendency of objects to resist change in motion
Objects at rest tend to stay at rest, Objects in motion tend to stay in motion until another force acts on it.
Tested his ideas through experimentation
End of philosophizing about ideas
Beginning of modern science

6. Mass – A Measure of Inertia
Amount of Inertia (resist in change of motion) depends on amount of mass
Mass: Amount of matter in an object; measured in kilograms (kg)
DIFFERENT THAN WEIGHT!
Weight: Force on an object due to gravity
Weight and mass are directly proportional
Double the Mass, Double the Weight!
Which bucket would be harder to push? WHY?

7. Newtons Newton (N) = standard unit of force 1kg = 9.8N on Earth
1kg = 2.2 pounds (lb)
9.8N was rounded to 10

8. Density
Measure of “compactness” – how much mass is squeezed into a given space
Density = mass / volume
What has more mass:
1kg rocks or 1 kg feathers?
Which liquid is the most dense?
Which solid is the most dense?
What has more weight:
1kg rocks or 1 kg feathers?
Which liquid is the least dense?
What is more dense:
1kg rocks or 1 kg feathers?
Which solid is the least dense?

9. Net Force Net Force: Combination of all forces on an object
Vector Quantities: Forces shown by arrows
Have both magnitude (how much) and direction (which way)

10. The Equilibrium Rule When the net force on something is equal to zero
Tension = Stretching Force
Is this diagram in equilibrium?

11. If the Normal Force is less than the Weight, what happens?
Support Force
The force that supports an object against gravity – often called normal force
If the Normal Force is equal to the Weight, is the object in equilibrium?
If the Normal Force is less than the Weight, what happens?

12. Equilibrium of Moving Things
Equilibrium = state of no change
An object moving at a constant speed in a straight line is in equilibrium
Friction: Force that occurs opposite of motion when objects are in contact

13. Friction Occurs for Solids, Liquids, and Gases
ALWAYS acts opposite the direction of movement
Ex: Air resistance: Force of friction acting on an object as it moves through the air

14. Causes of Friction
Due to surface bumps and “stickiness” of the atoms on the surfaces of materials
Ex: Ridges in fingers allow you to grab things
When your hands are wet, water fills the ridges, you can’t hold on to things as well
Shooting Stars
Meteors are heated to the point of burning when entering our atmosphere due to hitting gas molecules
Earthquakes
Occur when the friction that is holding rock slabs together is overcome and plates slide past each other

15. Vector Addition Parallelogram Law:
Triangle Method (always head to tail)

16. Vector Practice
Free-Body Diagram: A sketch showing only the forces on the selected particle.

17. Speed and Velocity Speed: Distance covered per amount of time traveled
Units: km/hr; mi/hr; cm/min; km/day
Preferred unit: m/s
Instantaneous Speed: Speed at a given instant
Ex: Speedometer
Average Speed: averages all of the instantaneous speeds
Velocity: Speed AND Direction
Ex: 60mi/hr North; 30km/hr West
Is it possible to have constant speed but NOT constant velocity?

18. Motion is Relative
When we discuss speed or velocity, we mean RELATIVE to something else

19. Relative Velocities Define coordinate system Direction of bus = +x
+x y

20. Relative Velocities Relative to street vbus= +8m/s
Relative to bus vyou = +3m/s

21. Relative Velocities Vector Diagram: vyou=3m/s vbus= 8m/s
vyou, relative to street=11m/s

22. Relative Velocities Relative to street vbus= +8m/s
Relative to street vyou = -3m/s

23. Relative Velocities Vector Diagram: vyou= -3m/s vbus= 8m/s
vyou, relative to street= 5m/s

24. Acceleration Rate at which velocity changes with time
Magnitude (speeding up OR slowing down)
Direction
Units: m/s2
Ex: Gravity = 9.8m/s2