2. Aristotle on Motion Force causes motion
Natural motion and unnatural motion
Natural motion—not caused by forces
Smoke and boulders—up, down
Planets and the heavens—circular
Unnatural motion— requires force, push or pull
People push or pull a cart
Thought the earth did not move
4th century BC

3. Copernicus and the Moving Earth
Changed the view of the world
Earth and other planets revolve around the sun
Avoided persecution by hiding his discoveries
Close to death he released his ideas for print
De Revolutionibus

4. Galileo on Motion
Championed Copernicus’ ideas, was persecuted and put on trial
Proved through experimentation force is not needed to keep an object moving
Explained friction (a force that opposes motion)
Every object resists change to its state of motion—Inertia!

5. Galileo on Motion
Galileo showed that experiments are better than logic in testing knowledge

6. Speed vs Velocity Speed is how fast Speed = distance/time
Velocity is how fast and in what direction (a vector quantity)
Constant speed means steady speed. Something with constant speed doesn’t speed up or slow down.
Constant velocity means both constant speed and constant direction. (Motion in a straight line at constant speed)
Unless stated otherwise, all speeds are relative to the surface of the Earth. Motion is relative.
Example: When you sit on a chair, your speed is zero relative to the Earth but 30 km/s relative to the sun.

7. Acceleration The rate at which the velocity is changing
Applies to decreases as well as increases in speed
Change may be in magnitude, direction, or both
Acceleration = change of velocity/time interval

8. Chapter 4: Newton’s First Law of Motion--Inertia

9. A Vector Quantity
Vector: a quantity with magnitude (size) and direction
Some Examples:
Displacement
Velocity
Acceleration
Force
An arrow is used to represent the magnitude and direction of a vector quantity.
Length of arrow: magnitude
Direction of the arrow: direction

10. Newton’s Law of Inertia
Newton’s First Law—Every object continues in a state of rest, or of motion in a straight line at a constant speed, unless it is compelled to change that state by forces exerted upon it.
Things tend to keep doing what they are already doing.

11. Examples
A hockey puck on the surface of slippery ice moves a long way until ice and air friction finally stops it.
If you toss an object in a vacuum of outer space (no friction) it will move forever in a straight-line path.

12. Mass—A Measure of Inertia
Mass is the amount of material in an object, or a measure of its inertia
The more mass..the greater inertia and the more force it takes to change its state of motion
Mass is not volume
Volume—a measure of space, cubic cm, cubic m, and liters
Mass is measured in kilograms
Which is more difficult to set into motion?

13. Mass—A Measure of Inertia
Mass is not weight
Weight is a measure of the gravitational force acting on an object
Depends on location
Mass and weight are proportional
Weight on Mars 40% of that on Earth.
Jump 2.5 times higher!
Talk about hang time!
On Mars
On Earth

14. Mass—A Measure of Inertia
One Kilogram weighs 9.8 Newtons
The unit of force is the Newton—N
Learn how to convert kg to N and vice versa
Weight = mass x acceleration due to gravity or weight = mg
Weight = mg

15. Net Force
Net force—The combination of all forces acting on an object

16. 50 N
30 N
20 N
15 N
25 N
45 N
300 N
Net force=352

17. Net force—The combination of all forces acting on an object
300
190
Net force=355
200 N
200 N

18. 760 N
Fnet = 2776 N
2670 N
140 N
60 N
80 N
100 N
180 N
2670 N
2670 N
300
190
Net force=355
200 N
200 N

20. Pythagorean Theorem
a2 + b2 = c2
Paddling = 650 N
Current = 400 N
Wind = 200 N

21. Current = 400 N
650N-400N =250N
Paddling = 250 N
Fnet = 320 N
Wind = 200 N
Pythagorean Theorem
a2 + b2 = c2

23. Bull blast = 1050 N
Horizontal Force = 325 N
Gravity = 535 N

24. Bull blast = 1050 N
Fnet = 609 N
Bull blast - gravity = 515 N
Horizontal Force = 325 N
Gravity = 535 N

25. 525 N
15,250 N
9,875 N

26. 14,725 N
Fnet= 17,730 N
9,875 N

27. Equilibrium Support force—normal force: at right angles to
Equilibrium is when net force is equal to zero—the object is in a state of rest
Compression

28. Equilibrium How about tension?
Tension is shared equally among vertical supports
2 N
2 N
4 N

29. Vector Addition of Forces
Equilibrium is when net force is equal to zero—the object is in a state of rest
Tension increases as the angle of the supports increases