1. Newton’s First Law of Motion—Inertia
Chapter 4

2. Aristotle (4th Century B.C.)
Believed in two different types of motion: natural motion & violent motion
Natural Motion – either straight up or straight down, objects would seek out their natural resting places (it was “natural” for heavy things to fall and light things to rise)
Violent Motion – imposed motion, result of forces that pushed or pulled
The proper state of objects was that of rest.

3. Aristotle

4. Copernicus ( A.D.)
Because Earth was already in its natural resting place, it could not move (there was no force big enough to move Earth)
Copernicus concluded that the only way to make sense of the way the planets move is to assume that Earth and the other planets move around the sun
He worked in secret to avoid being persecuted by the church

5. Copernicus

6. Galileo The foremost scientist of the late-Renaissance in Italy
Outspoken in his support of Copernicus, later put on trial and confined to house arrest
A force is any push or pull
Friction – the force that acts between materials that touch as they move past each other
If friction were absent, an object would need no force to remain in motion

7. Galileo

8. Galileo’s Inclined Planes
Noted that a ball that rolls down an inclined plane picks up speed
When the same ball rolls up the inclined plane, it slows down
When a ball rolls on a flat horizontal surface, it rolls at near constant velocity
Stated that with the absence of friction, the ball would roll forever

9. Galileo’s Inclined Planes Cont.
If you have two inclined planes face each other, a ball rolled down one plane will reach nearly the same height as it rolls up the other plane
He noticed that the ball ended up at the same height, even is the plane was elongated or at a different angle
Inertia – the property of a body to resist change (the tendency of a moving body to keep moving and every material object resists change to its state of motion)

10. Inclined Planes

11. Newton’s First Law
Every object continues in a state of rest, or of motion in a straight line at constant speed, unless it is compelled to change that state by forces exerted upon it.
An object will keep doing what it’s already doing
Toss an object out the door of the International Space Station, and it will keep moving at the speed you threw it at forever (no friction in space)

12. Newton’s 1st Law

13. The Law of Inertia

14. The Law of Inertia

15. Pioneer and Voyager
After they initially were sent into space, these two spacecraft utilize Newton’s Law of Inertia to move through space
They are now cruising outside of our solar system

16. Mass – A Measure of Inertia
The amount of inertia an object has depends on its mass—which is roughly the amount of material present in the object
Mass is NOT volume, the measure of space that an object takes up
Mass is NOT weight, the force of gravity on an object
Mass is a measure of the inertia that an object exhibits in response to any effort made to start it, stop it, or otherwise change its state of motion
Mass and weight may not be the same, but they are proportional to each other
Weight = mass x acceleration due to gravity
Measured in Newtons (N)
A 1-kg bag of nails weighs 9.8 N on the surface of Earth (2.2 lbs.)

17. Net Force
In the absence of a net force, objects do not change their state of motion
If you push with equal and opposite forces on opposite sides of an object at rest, it will remain at rest
Net Force – the combination of all forces acting on an object
We use force diagrams to figure out the net force

18. Force Diagrams

19. Equilibrium
If only the force of gravity was acting on an object at rest, the object would constantly be in free fall
The fact that the object is at rest means that another force must be acting upon it
The other force exactly balances out the weight of the object and produces a net force of zero
The other force is called the support force, or normal force
Equilibrium – the net force on an object is zero

20. Equilibrium

21. Vector Addition of Forces
We can use the same vector addition techniques on forces as we did on velocities!
Force, like velocity has a magnitude and a direction
For any pair of scales, ropes, or wires supporting a load, the greater the angle from the vertical, the larger the tension force in them
The resultant of the tension forces in a rope must be equal and opposite to the load being supported

22. Vector Addition of Forces

23. Assignment (Due Wednesday 10/7)
Read Chapter 4 (pg )
Do Ch. Assessment #21-39 (pg )
Appendix F #1-10 (pg )