1. Newton’s Laws of Motion
Chapter 4
Newton’s Laws of Motion

2. Stationary Earth Earth is heavy It looks like the sky is moving
Easier to imagine the sky can move
It looks like the sky is moving
We have no sensation of our motion
Rotating Earth would make objects fly off of the surface
Stellar Parallax

3. Stellar Parallax
Hipparchus 150 BC
June
Sun
January

4. Aristotelian Universe Terrestrial Realm
Composition predicts motion
natural tendencies
Fire and Air tend to rise
Earth and Water tend to sink
Overall tendency to seek rest
Objects are corruptible (changing)

5. Aristotle attempted to understand motion by classification.
1. ARISTOTLE ON MOTION
Aristotle attempted to understand motion by classification.
Two Classes:
Natural and Violent

6. Aristotelian Universe Celestial Realm
Celestial Objects composed of Aether
Self luminous but does not consume
Motion is constant, circular
Objects are incorruptible (not changing)
Meteors and comets were phenomena of the Earth’s atmosphere

7. The falling speed of an object was supposed to be proportional to its weight.
Galileo’s Experiment

8. Nicholas Copernicus
(Niklas Koppernigk)
Developed a mathematical model for a Sun-centered solar system

9. Galileo Galilei
Among the first to turn a telescope to the sky
Developed the Scientific Method
Believed in the popularization of science
Developed the Law of Inertia

10. He tested with planes. Demo - Ball and incline plane
The change in speed depended on the slope of the incline.
Seemed that the ball was trying to achieve the same vertical height.

12. Isaac Newton

13. Newton’s Laws The 1st Law
A body continues to move as it has been moving unless acted upon by an external force.

14. Newton’s First Law No mention of chemical composition.
No mention of terrestrial or celestial realms
Force required when object changes motion.
Acceleration is the observable consequence of force acting.

15. Net Force Force is a vector
We must add all the forces acting on the body
5 N
10 N
5 N
5 N
5 N
0 N
5 N
10 N
5 N

16. Equilibrium When the net force is zero (SF = 0) Acceleration is zero
Velocity may not be zero

17. Mass and Inertia
Inertia is a property of a body that resists changes in motion
Mass is a measure of the amount of matter in a body
Inertia and Mass are the same concept
Units – Kg or slugs

18. Weight Force of gravity pulling on the mass of the body
Units – Newtons or Pounds
A newton is about the weight of a small apple
One kg weighs 2.2 lbs.
W = mg
g = acceleration due to gravity = 32 ft/s2 = 9.8 m/s2

19. Inertia Demonstrations
Demo - Table setting
Demo - Bottle, hoop, and chalk

20. The 2nd Law Newton’s Laws
The Sum of the Forces acting on a body is proportional to the acceleration that the body experiences
S F  a
S F = (mass) a

21. 2. NEWTON'S 2nd LAW OF MOTION

22. If the net force is perpendicular to the velocity, the direction of the velocity changes.
Acceleration is always in the direction of the net force.

23. When Acceleration Is Zero - Equilibrium
Static Equilibrium
Velocity is zero
Examples:
Scales pushing up
Normal up
Weight down
Computer setting on a table
Weighing yourself on a set of scales
Hanging from a tree
Tree
pulling up
Weight down
Car parked on an incline
Normal
Friction
Weight down
Weight down

24. Dynamic Equilibrium Velocity is nonzero and constant Examples:
Driving at constant velocity
Normal up
Friction
Force from road
Weight down

25. Newton’s Laws The 3rd Law
For every action force there is an equal and opposite reaction force
(You cannot touch without being touched)

26. Oppose motion or the tendency of motion
Dissipative Forces
Oppose motion or the tendency of motion
Friction
Air Resistance

27. Friction Depends upon... Static friction (no motion)
Materials that are in contact
Forces that press surfaces together (Normal forces)
Static friction (no motion)
Proportional to force used until maximum is reached
Kinetic friction (motion)
Nearly constant for normal speeds

28. Friction
Static F f
Kinetic f F

29. Air Resistance Terminal Velocity! Depends on
Surface Area
Velocity
When weight = air resistance we have
SF= 0 Þ a = 0
Weight
Air Resistance
Terminal Velocity!