1. Newton’s First Law of Motion
Forces & The Law of Inertia

2. To aid your understanding:
Aristotle's Ideas of Motion
Galileo's Concept of Inertia
Newton's First Law of Motion
Net Force and Vectors
The Equilibrium Rule
Support Force
Equilibrium of Moving Things
The Moving Earth

3. Aristotle's Ideas of Motion
Natural motion
Straight up or straight down for all things on Earth
Ex: Falling Leaves
Beyond Earth, motion is circular
Ex: The Sun and Moon continually circle Earth.
Violent motion
Produced by external pushes or pulls on objects
Ex: Wind imposes motion on ships.

4. Galileo's Concept of Inertia
Galileo demolished Aristotle's assertions in the 1500s.
Galileo's discovery:
Objects of different weight fall to the ground at the same time in the absence of air resistance.
A moving object needs no force to keep it moving in the absence of friction.

5. Galileo's Concept of Inertia
Force
is a push or a pull.
Inertia
is a property of matter to resist changes in motion.
depends on the amount of matter in an object (its mass).

6. Galileo's Concept of Inertia
Balls rolling on downward-sloping planes pick up speed.
Balls rolling on upward-sloping planes lose speed.
So a ball on a horizontal plane maintains its speed indefinitely.
If the ball comes to rest, it is not due to its "nature," but due to friction.

7. Galileo's Concept of Inertia CHECK YOUR NEIGHBOR
The use of inclined planes for Galileo's experiments helped him to
eliminate the acceleration of free fall.
discover the concept of energy.
discover the property called inertia.
discover the concept of momentum
C. discover the property called inertia. 7

8. Galileo's Concept of Inertia CHECK YOUR ANSWER
The use of inclined planes for Galileo's experiments helped him to
eliminate the acceleration of free fall.
discover the concept of energy.
discover the property called inertia.
discover the concept of momentum.
Comment:
Note that inertia is a property of matter, not a reason for the behavior of matter.
Inertia is a property of matter.
It is not a reason for the behavior of matter. 8

9. Newton's First Law of Motion
Isaac Newton was born soon after Galileo died.
In 1665, at the age of 23, Newton stated his 3 Laws of Motion which we still study today.
Newton was VERY young when he made this analytical discovery.

10. Newton's First Law of Motion
Every object continues in a state of rest or of uniform speed in a straight line unless acted on by a nonzero net force.
The LAW of INERTIA
The key word in this law is continues: An object continues to do whatever it happens to be doing unless a force is exerted upon it.
If it is at rest, it continues in a state of rest.
If an object is moving, it continues to move without turning or changing its speed.

11. Newton’s 1st Law – Law of Inertia

12. Inertia INERTIA – a resistance to change in its state of motion
Inertia is a “property” of ALL matter----it is NOT a FORCE which acts on matter

13. Inertia Think about what happens when you suddenly stop a car.
What does your body do?

14. Inertia

15. Inertia
What would happen to a ladder placed on top of a truck if it suddenly came to a screeching halt?

17. Inertia Your body continues to move in the same direction.
You are RESISTING change in your state of motion: INERTIA!

18. Mass – A measure of inertia
MASS – the amount of material present in an object
The amount of INERTIA an object has is dependent on its mass---1st Law of Motion
Greater mass; Greater the inertia

19. Newton’s 1st Law of motion is affected by 2 things:
1) Any type of force (net force, friction, weight, restoring force, etc.)
2) Inertia---which will keep the body doing what it was already doing—resistance to change

20. Net Force Vector quantity
a quantity whose description requires both magnitude (how much) and direction (which way)
can be represented by arrows drawn to scale, called vectors
length of arrow represents magnitude and arrowhead shows direction
Examples: force, velocity, acceleration

21. Net Force
Definition:
The combination of all the forces that act on an object is the NET FORCE—called the resultant force of vectors.

22. Net Force
Net force is the combination of all forces that act on an object.
Example: Two 5-N pulls in the same direction produce a 10-N pull (net force of 10 N). If the pair of 5-N pulls are in opposite directions, the net force is zero.

23. Net Force CHECK YOUR NEIGHBOR
A cart is pulled to the right with a force of 15 N while being pulled to the left with a force of 20 N. The net force on the cart is
5 N to the left.
5 N to the right.
25 N to the left.
25 N to the right.
A. 5 N to the left. 23

24. Net Force CHECK YOUR ANSWER
A cart is pulled to the right with a force of 15 N while being pulled to the left with a force of 20 N. The net force on the cart is
5 N to the left.
5 N to the right.
25 N to the left.
25 N to the right.
The two forces are in opposite directions, so they subtract.
The direction is determined by the direction of the larger force.
When you take a 15-N force to the right added to a 20-N force to the left, because the two forces are in opposite directions, they in fact subtract, rather than add.
So, the larger 20-N force to the left is cut back by the 15-N force to the right, so that the remaining force, that is, the net force, is only 5 N to the left. 24

25. Net Force CHECK YOUR NEIGHBOR
What is the net force acting on the box?
15 N to the left
15 N to the right
5 N to the left
5 N to the right
D. 5 N to the left ? 25

26. Net Force CHECK YOUR ANSWER
What is the net force acting on the box?
15 N to the left
15 N to the right
5 N to the left
5 N to the right
When you take a 10-N force to the right added to a 5-N force to the left, because the two forces are in opposite directions, they in fact subtract, rather than add.
So, the larger 10-N force to the right is cut back by the 5-N force to the left, so that the remaining force, that is, the net force, is only 5 N to the right. 26

27. NET FORCES—TYPES OF:
There are 3 types of “net forces”.
1) Force vectors moving in the same direction, the resultant is found by “adding” the force vectors. 2) Force vectors move in “opposite” direction, the resultant is found by “subtracting” the force vectors to find the resultant and the direction is the same as the larger of the 2 force vectors.

28. Equilibrium – When Net Force Equals Zero
NET FORCES—TYPES OF:
When an object is at rest, with the net force on it being zero, we say it is in a state of EQUILIBRIUM.
Equilibrium – When Net Force Equals Zero

29. RESULT OF NET FORCES

30. Vectors Vector quantity has magnitude and direction.
is represented by an arrow.
Example: velocity, force, acceleration
Scalar quantity
has magnitude ONLY.
Example: mass, volume, speed

31. Forces
Are vector quantities since they have both “direction” and “magnitude”
Any push or pull
4 characteristics:
Net force (forces combining)
distance
found in pairs
work opposite to each other

32. Instrument and Units
The instrument used to measure force is called a spring scale.
Mass is measured with a triple beam balance &/or an electronic balance.
Volume is measured with a graduate cylinder
Time with a stop-watch

33. Force Force is measured in SI units called NEWTONS
9.8 Newtons (N) of force = 1 kg of mass acted on by gravity so,
1 kg ⋅ gravity = 9.8 N
One newton , 1N, is the force needed to accelerate one kilogram of mass at the rate of one meter per second squared in direction of the applied force so,
1 N = 1 kg⋅m/s2

34. Mass, Volume, and Weight
Volume is the measure of space an object takes up.
Weight is the measure of gravitational attraction an object has to Earth.
Mass is NOT the same as weight!!

35. Types of Forces
8 different types of forces which can act on objects to produce motion

36. Types of forces
8 different types of forces which can act on objects to produce motion
1. Air Resistance Force
2. Applied Force
3. Spring Force
4. Frictional Force
5. Gravitational Force
6. Electrical Force
7. Normal Force
8. Magnetic Force

37. What causes air resistance?
Air resistance force
What is air resistance?
Basically, it is friction between an object and the air (gas), and also in fluids (liquids). Objects moving through air encounter forces and experience conditions similar to objects traveling through liquids. f
What causes air resistance?
All matter is made from atoms and/or molecules. The air is no exception. When something moves through the air, it bumps into the atoms and molecules.
Pressure---is a force but, it is based on the amount of area in contact

38. Applied force What is applied?
An applied force is a force that is applied to an object by a person or another object. If a person is pushing a desk across the room, then there is an applied force acting upon the object. The applied force is the force exerted on the desk by the person.

39. Spring force What is spring force?
Is when an object is pushed together, then
when pressures taken away it pops back to
its normal self.
The restoring force which acts on any type of spring is also called the elastic force.
Direction is horizontal to “restore” the stretch of the spring
Hooke’s Law

40. What causes frictional force?
What is frictional force?
Frictional force is present everywhere in our daily life. It is simply impossible to reduce it completely. Friction is an imposed force which will act “opposite” the motion of the object.
Without friction, an object would continue moving forever on a level surface.
What causes frictional force?
The causes of the resistive force of friction are molecular adhesion, surface roughness, and the plowing effect. Adhesion is the molecular force resulting when two materials are brought into close contact with each other resulting in heat or deformation.

41. Newton’s fiery of gravitational forces :
“Every particle of matter in the universe attracts every other particle with a force that is directly proportional to the product of the masses of the particles and inversely proportional to the square of the distance between them.”
Objects rolling downhill tend to speed up (accelerate)
Objects rolling uphill tend to slow down (decelerate)
What about those on an even surface?

42. Gravity as a Force Causes Motion
Objects on a level surface do not have “horizontal” acceleration but may have “vertical” acceleration depending on whether dropped or acted upon by a force-which is “gravity”
Horizontal motion is considered to be “constant” when forces do not act on the object

43. Weight Weight is the pull of gravity on an object
Direction is always “downward”---positive value
Attraction between 2 objects with earth as 1 of the objects.
W = mg (mass is scalar) just gives the magnitude of the weight, which is positive. The direction of the weight vector is downward. (Note that g is a positive constant.) Often when you talk about the weight of something you just mean the magnitude.

44. What is electrical force?
The attractive or repulsive interaction between any two charged objects is an electric force. Like any force, its affect upon objects.

45. Normal force What is normal force?
The normal force is the support force exerted upon an object that is in contact with another stable object. For example, if a book is resting upon a surface, then the surface is exerting an upward force upon the book in order to support the weight of the book. On occasions, a normal force is exerted horizontally between two objects that are in contact with each other. For instance, if a person leans against a wall, the wall pushes horizontally on the person.
force that balances the force of an object on the surface—also called the perpendicular force
The table’s force pushing back on the book.
Normal force is equal to the weight of the book

46. Support Force
Support force (normal force) is an upward force on an object that is opposite to the force of gravity.
Example: A book on a table compresses atoms in the table, and the compressed atoms produce the support force.

47. Understanding Support Force
When you push down on a spring, the spring pushes back up on you.
Similarly, when a book pushes down on a table, the table pushes back up on the book.

48. Support Force CHECK YOUR NEIGHBOR
When you stand on two bathroom scales with one foot on each scale and with your weight evenly distributed, each scale will read
your weight.
half your weight.
zero.
more than your weight.
B. half your weight. 48

49. Support Force CHECK YOUR ANSWER
When you stand on two bathroom scales with one foot on each scale and with your weight evenly distributed, each scale will read
your weight.
half your weight.
zero.
more than your weight.
Explanation:
You are at rest, so F = 0.
Forces from both scales add to cancel your weight.
Force from each scale is one-half your weight.
You are at rest on the scales, so F = 0. The sum of the two upward support forces is equal to your weight.
Your weight is distributed over the two scales. So, each scale reads half your weight. 49

50. Magnetic force What is magnetic force?
Magnetic force is the same as gravitational and electrical forces in that no one knows truly what it is. Magnetic force is different from gravitational and electrical forces in that its potential and momentic energy is at the expense of an electrical field in time.

51. Other Forces Found On Matter
Other types of forces which can also be considered equilibrant forces are:
Thrust - up ward force against gravity and air resistance
Tension/Taut - horizontal force with acts against air resistance to pull something tight
Both have a net force of “0”

52. The Equilibrium Rule: Example
A string holding up a bag of flour
Two forces act on the bag of flour:
Tension force in string acts upward.
Force due to gravity acts downward.
Both are equal in magnitude and opposite in direction.
When added, they cancel to zero.
So, the bag of flour remains at rest.

53. The Equilibrium Rule
The vector sum of forces acting on a nonaccelerating object equals zero.
In equation form: F = 0.
The red arrows represent force vectors. The sum of the two upward force vectors minus the sum of the three bottom force vectors, equals zero. We say the forces cancel to zero, and the system of Burl, Paul, and the staging is in equilibrium.

54. The Equilibrium Rule CHECK YOUR NEIGHBOR
The equilibrium rule, F = 0 (read as ‘sum of Forces is equal to 0’, applies to
vector quantities.
scalar quantities.
Both of the above.
None of the above.
A. vector quantities. 54

55. The Equilibrium Rule CHECK YOUR ANSWER
The equilibrium rule, F = 0, applies to
vector quantities.
scalar quantities.
Both of the above.
None of the above.
Explanation:
Vector addition accounts for + and – quantities. So, two vectors in opposite directions can add to zero.
Vector addition takes into account the signs of quantities. So quantities, such as forces can be either positive or negative. In this way, two forces (forces), one positive and the other negative, can add to zero. But there is no way that two masses (scalars) can add to zero. 55

56. Equilibrium of Moving Things
Equilibrium: a state of no change with no net force acting
Static equilibrium
Example: hockey puck at rest on slippery ice
Dynamic equilibrium
Example: hockey puck sliding at constant speed on slippery ice

57. Equilibrium of Moving Things
Equilibrium test: whether something undergoes change in motion
Example: A crate at rest is in static equilibrium (no change in motion).
Example: When pushed at a steady speed, it is in dynamic equilibrium (no change in motion).

58. Equilibrium of Moving Things CHECK YOUR NEIGHBOR
A bowling ball is in equilibrium when it
is at rest.
moves steadily in a straight-line path.
Both of the above.
None of the above.
C. Both of the above. 58

59. Equilibrium of Moving Things CHECK YOUR ANSWER
A bowling ball is in equilibrium when it
is at rest.
moves steadily in a straight-line path.
Both of the above.
None of the above.
Explanation:
Equilibrium means no change in motion, so there are two options:
If at rest, it continues at rest.
If in motion, it continues at a steady rate in a straight line.
For an object to be in equilibrium, it does not necessarily have to be at rest.
It can be in equilibrium as long as it is not changing its motion.
If it is initially at rest it must continue to remain at rest.
If it is initially in motion it must continue in motion at a steady rate in a straight-line path. 59

60. Equilibrium of Moving Things CHECK YOUR NEIGHBOR
You push a crate at a steady speed in a straight line. If the friction force is 75 N, how much force must you apply?
More than 75 N.
Less than 75 N.
Equal to 75 N.
Not enough information.
C. equal to 75 N 60

61. Equilibrium of Moving Things CHECK YOUR ANSWER
You push a crate at a steady speed in a straight line. If the friction force is 75 N, how much force must you apply?
More than 75 N.
Less than 75 N.
Equal to 75 N.
Not enough information.
Explanation:
The crate is in dynamic equilibrium, so, F = 0.
Your applied force balances the force of friction.
The crate is in dynamic equilibrium. So there is no NET force acting on it.
So you must apply a force that balances out the force of friction. Which means it must be equal and opposite to the force of friction. So if Friction is 75 N to the left, the Applied Force from you is 75 N to the right. 61

62. The Moving Earth
Copernicus proposed that Earth was moving, circulating the Sun.
This idea was refuted by people.
Example: If Earth moved, how could a bird swoop from a branch to catch a worm?
Solution: As it swoops, due to inertia, it continues to move sideways at the speed of Earth along with the tree, worm, etc.

63. The Moving Earth CHECK YOUR NEIGHBOR
You are riding in a vehicle at a steady speed and toss a coin straight upward. Where will the coin land?
Behind you.
Ahead of you.
In your hand.
There is not enough information.
C. in your hand 63

64. The Moving Earth CHECK YOUR ANSWER
You are riding in a vehicle at a steady speed and toss a coin straight upward. Where will the coin land?
Behind you.
Ahead of you.
In your hand.
There is not enough information.
Explanation:
Due to the coin's inertia, it continues sideways with the same speed as the vehicle in its up-and-down motion, which is why it lands in your hand.
C. in your hand 64

65. Vocabulary Newton’s 1st Law Equilibrium New Force Force Kilogram
Mass
Normal/Support Force
Newton
Equilibrium
Force
Friction
Inertia
Law of Inertia
Weight