1. CHAPTER – 4 Newton’s First Law
4.1 Aristotle on Motion
Aristotle was a fourth century b.c. Greek scientist that stated motion is divided into two types:
1. natural motion
2. violent motion

2. Natural Motion =
Either straight up or straight down motion. Objects will seek their natural resting place.
Boulders on the ground compared to smoke, clouds or fog in the air.
Heavy things will fall and light things will rise.

3. Circular motion was natural for “the heavens” (all celestial bodies, planets, moons, stars, … etc) since there was no beginning nor end. Since these motions were considered natural they were not caused by forces.

4. Violent Motion
Motion is a result of forces acting on an object by either pushing or pulling the object. Therefore, the motion has an external cause.
Example:
A cart being pulled by a horse, a ship being pushed by the wind.

5. For over 2000 years the mindset was that if an object was moving “against its nature” then some force was acting on it.
The proper state of an object was at rest, unless being pushed or pulled.
It was not considered possible that there was enough force to move the Earth, so the Earth must not move.

6. 4.2 Copernicus and the Moving Earth
Nicolaus Copernicus ( ) theorized that the Earth and other planets orbited a stationary sun.
Copernicus did not publish his theory for fear of being put in jail or even possibly receiving the death sentence.
Q: WHY ?

7. A: During this particular time period “The Church” deemed that the Earth was the center of the universe, and everything circled (orbited) the Earth. Any other theory would be considered blasphemy, with the result being jailed or put to death.
Q: Did Copernicus ever publish his work?
A: YES, Copernicus finally published his works/theories on May 24, Ironically, this was also the day he died.

8. 4.3 Galileo on Motion
Galileo Galilei ( )

9. Galileo supported the theories of Copernicus
Galileo supported the theories of Copernicus. As a result, he was put on house arrest. He was permitted to carry out experiments but not publish any of his findings.
Galileo’s greatest contribution to physics was his idea that a force is not required to keep an object moving.

10. Force =
Any push or pull
Friction =
The force that acts between materials that touch as they move past each other.
Even surfaces that are very smooth have some amount of friction between them. Nothing is completely frictionless.
If friction did not exist, a moving object would never stop moving and our ever day lives would be completely different.

11. SKETCH A ball rolling down an incline will increase speed. Q: Why?
A: Because gravity is constantly pulling the ball downward, towards the center of the Earth.

12. A ball rolling up an incline will decrease speed and eventually stop.
SKETCH
Q: Why?
A: Because gravity is constantly pulling the ball downward, towards the center of the Earth.

13. A ball rolling on a very smooth surface (bowling ball/alley) will roll nearly at a constant speed for a very long time.
SKETCH
Q: Why will the ball eventually slow down?
A: Gravity is pulling only in a downward direction,  some other force (FRICTION) is acting between the two surfaces and friction in the form of air resistance is causing the ball to slow down and eventually stop.

14. Therefore, if the ball were set in motion on a perfectly flat surface and there were NO forces of friction, including air resistance, the ball would continue to roll forever. NO FORCE would be required to keep the ball rolling.

15. SKETCH, dual incline
If the ball is released at point A it will roll down the ramp and up the other ramp to position B which is at an equal height to point A, were the ball was originally released, NEGLECTING FRICTION.
With friction, the smoother the surfaces are the closer the ball will roll to the height at which it was released.

16. SKETCH Incline 3 & 4
Incline 4 is at a lesser angle than 3 but is a longer ramp.
Q: What will happen when the ball is released at point A, neglecting friction?
A: The ball will reach the same height at which it was released on the opposite ramp.
With friction, the smoother the surface the closer the ball will reach point B.

17. Galileo stated it was the “natural” tendency for objects that are moving to continue to move, resisting a change in the object’s motion.
Inertia=
The resistance to a change in an object’s motion.
Galileo’s findings about motion and his concept of inertia, discredited Aristotle’s theory of motion.

18. 4.4 Newton’s Law of Inertia
Sir Isaac Newton ( ) was born on Christmas day the same year Galileo died.

19. Newton developed the Three Laws of Motion that still hold true today
Newton developed the Three Laws of Motion that still hold true today. His three laws completely changed the beliefs of science for the previous 2000 years.
Newton’s First Law (aka: Law of Inertia) states that an object at rest will remain at rest and an object in motion will remain in motion, with the same speed and direction, until some net force acts on the object. In other words, the object wants to keep on doing what it is already doing.

20. SKETCH
If a ball is thrown horizontally with a velocity of 40m/s it would travel in a straight line for ever at 40m/s until some outside/net force acted on the ball.
Q: What force(s) are responsible for the ball to travel on its downward curved path and eventually hit the ground?
A: Gravity will cause the ball the ball to fall in a vertical direction only. The frictional force of air resistance would cause the ball to slow down in both the horizontal and vertical direction.

21. Q: If there were no gravity, what would happen to the ball when it was thrown?
A: The ball would travel in a perfectly straight line, however, air resistance would eventually cause the ball to slow down and stop.

22. If you slid a hockey puck on the sidewalk it would slow down and stop rather quickly. If you slid a hockey puck across smooth ice it would slide a lot farther.
Q: WHY?

23. A: Because there is a difference in the force of friction between the two different surfaces.
The force that acts to resist (oppose) the relative motion, or attempted motion, of objects, materials, or surfaces that are in contact with each other.

24. Q: What would happen if you threw a baseball from the space station in “outer space” ?
A: The ball would travel in a perfectly straight line at the same speed forever. Because of the ball’s inertia and lack of any net force acting on the ball.

25. 4.5 Mass-A Measure of Inertia
The property of an object to resist a change in its motion.
The amount of inertia an object has depends on the amount of mass it has, more mass = more inertia.

26. Mass =
The amount of matter an object has.
Mass is measured in kilograms (kg).
Volume =
The measurement of the amount of space an object takes up. Measured in liters (L).
Weight =
The measure of the force of gravity on an object’s mass.

27. Q: If you were to shake a bowling ball back and forth on Earth, on the moon, or in outer space would you notice any difference between any of the locations?
EXPLAIN, why/why not.

28. An object’s mass can never change no matter were it is, on Earth, the moon or in outer space (zero gravity/weightlessness). The mass would be the same every where. Therefore the object would have the same amount of inertia whether the object is on Earth, the moon or in outer space.

29. Therefore, if you were to shake a bowling ball on Earth, it would take the same amount of force to shake the bowling ball on the moon or in outer space because the bowling ball has the same amount of mass/inertia in all three locations.

30. An object’s weight depends on were the object is located
An object’s weight depends on were the object is located. A 100 lb cheerleader would weigh only lbs on the moon and nothing in space.

31. Mass and weight are proportional to each other, the more mass an object has the more weight it will have provided there is a gravitational force acting on it.
Q: Does a 20kg bowling ball have twice the inertia, twice the mass, twice the weight and twice the volume as a 10kg bowling ball in the exact same location?
A: inertia = yes weight = yes
mass = yes volume = yes

32. Q: Does a 40kg bag of potatoes have twice the inertia, twice the mass, twice the weight and twice the volume as a 20kg bowling ball when weighed in the same location?
A: inertia = Yes
mass = Yes
weight = No
volume = No

33. USA standard of mass =
Slug ( = 32.2lbs)
The rest of world standard unit of mass =
kilogram (kg)
USA standard unit of weight =
Pound (lb)
The rest of world standard unit of weight =
Newton (N)

34. Common Conversions Used
1kg = 2.21lbs = 9.8N
1lb = 0.45kg = 4.45N

35. Example - 1
7kg of wood = _____ lbs
7kg lbs
X kg = lbs

36. Example - 2 What is the mass (metric) of 23lbs of apples? 23lbs 1kg
X lbs = 10.4kg

37. Example - 3 What is the weight (metric) of 17kg of blueberries?
17kg N
X 1kg = N

38. 4.6 Net Force
Net Force =
The combination (sum) of all the forces acting on an object.
If the net force acting on an object is zero, the object will keep on doing what it is already doing. If it is at rest, it will remain at rest, if it is in motion, it will remain in motion at the same speed and direction until some net force acts on it.

39. If an object does have a net force acting on it, the object will accelerate in the direction of the net force.

40. Net Force = 0

41. Net Force = some value (x)

42. Various examples of Net force = 0 and net force = some value (x)

43. 4.7 Equilibrium Draw a book laying on a desk.
Q: What forces are acting on a book laying on a desk?

44. The support force or force of the table is called the normal force
The support force or force of the table is called the normal force. The normal force (FN) is the upward force that opposes gravity. The normal force is drawn perpendicular to the surface of the object. Therefore, on a horizontal surface the FN would be = to the force of gravity acting on the object.

45. FBD =
Free Body Diagram
How is an FBD drawn?
An object is represented by a dot with all the forces on it. The vector forces are drawn with their tails touching the dot and their tips radiating from the dot in the proper directions.
EXAMPLES:

46. Equilibrium =
When the net force acting on an object = 0

47. When equilibrium is reached there can be NO CHANGE in an object’s motion because there are NO net forces acting on it.

48. FBD EXAMPLES A rock thrown in space. A rock thrown in this room.
A penny resting on a desk.
A penny being pushed at a constant velocity across a desk.
A penny being pushed at a constant acceleration a cross a desk.
A penny that was pushed on a desk the instant it leaves the top of the desk.

49. Text page 52 Figure 4.12, example of a girl hanging on a trapeze bar.
Draw sketch
The tension in each arm/rope will be equal to exactly ½ the weight of the girl.
What is the force in the girl’s arm if the girl holds on with one arm?
Examples with spring scales & masses. Use single and double scales.

50. PRE SECTION 4.8
DEMONSTRATION:
with possible bonus points

51. 4.8 Vector Addition of Forces
Refer to transparencies and/or sketches on board with regard to changing forces in ropes/chains as the angles between them change.