1. Forces and Newton’s Laws
Chapter 12
Forces and Newton’s Laws

2. Section 12-1
Forces

3. On page 77 in your notebook, write each question and Answer the Following: 12.1 Objectives
1. How do forces affect the motion of an object?
Before:
After:
2. What are the 4 main types of friction?
3. How do gravity and air resistance affect a falling object?
4. In what direction does Earth’s gravity act?
5. Why does a projectile follow a curved path?

4. What is a Force? A force is a push or a pull that acts on an object.
A force can cause a resting object to move, or it can accelerate a moving object by changing the object’s speed or direction.

5. Measuring Force Forces are often easy to measure. (Spring Scale)
Force is measured in newtons, N
1 Newton is the force that causes a 1 Kg mass to accelerate at a rate of 1 m/s2.
You can use an arrow to represent the direction and strength of a force.
The direction of an arrow represents the direction of the force.
The length of the arrow represents the strength, or magnitude of the force.

6. Combining Forces
You can combine force arrows to show the result of how forces combine.
Adding: =
Subtracting: =
Equal and opposite:
= 0

7. Net force is the overall force acting on an object after all the forces are combined.
Balanced = when the forces are equal and opposite; no net force; no motion.
Unbalanced = the object will accelerate.

8. Friction
All objects are subject to friction, a force that opposes the motion of objects that touch as they move past each other.

9. 4 main types of Friction Static Friction
: the friction that acts on objects that are not moving.
: always acts in the direction opposite to that of the applied force.
Sliding Friction
: a force that opposes the direction of motion of an object as it slides over a surface.

10. Rolling Friction
: the friction that acts on rolling objects.
Fluid Friction
: opposes the motion of an object through a fluid (cake batter)

11. Gravity Gravity is a force that acts between any 2 masses.
It is an attractive force; it pulls objects together.
Earth’s gravity acts downward toward the center of Earth.
9.8 m/s2 on Earth.

12. Gravity causes objects to accelerate downward, whereas air resistance acts in the direction opposite to the motion and reduces acceleration.
Terminal velocity is the constant velocity of a falling object when the force of air resistance equals the force of gravity. (not going any faster or slower)

13. Projectile Motion When you throw an object, it curves.
This curved path is an example of projectile motion.
Projectile Motion is the motion of a falling object after it is given an initial forward velocity.
Air resistance and gravity are the only forces acting on the object.

14. Review
FORCE PACKET

15. Go back to the 12-1 objectives (pg 74) you did and re-answer them.
How do forces affect the motion of an object?
What are the 4 main types of friction?
How do gravity and air resistance affect a falling object?
In what direction does Earth’s gravity act?
Why does a projectile follow a curved path?

16. Section 12-2
Newton’s Laws

17. On Page 110, Write the Questions and Answer the Following:
12.2 objectives
1. How does Newton’s 1st law relate change in motion to a zero net force?
Before:
After:
2. How does Newton’s 2nd Law relate force, mass, and acceleration?
3. How are weight and mass related?

18. Newton’s 1st Law
Every Motion you Observe or Experience is Related to a Force.
Sir Isaac Newton described the relationship between motion and force in three laws that we now call Newton’s laws of motion.
If you slide a book across a rough surface, the book will eventually come to a rest.
If you slide a book across a smooth surface, the book will slide much farther before stopping.

19. Friction causes the book to come to stop
Friction causes the book to come to stop. Without friction, the book would continue to slide forever.
This is an example of Newton’s First Law…..
An object at rest remains at rest and an object in motion maintain its velocity unless it experiences an unbalanced force.

20. You experience Newton’s first law every time you ride in a car and it comes to a stop. You feel your body continue to move forward. The seatbelt acts as an unbalanced force needed to bring you to a stop as the car stops.
Inertia is the tendency of an object at rest to remain at rest or, if moving, to continue moving with a constant velocity.

21. All objects have inertia because they resist changes in motion
All objects have inertia because they resist changes in motion. An object with very little mass can be accelerated with a small force. But it takes a much larger force to accelerate a large mass.

22. Newton’s Second Law
Newton’s Second Law Describes What Happens When the Net Force Acting on an Object is Zero.
The object either remains at rest or continues moving with constant velocity.
But what happens when the net force acting on an object is not zero?

23. Newton’s Second Law describes the effect of this unbalanced force on the motion of an object.
His second law describes the relation between mass, force, and acceleration:
The unbalanced force acting on an object equals the object’s mass times its acceleration.

24. Mathematically it is written as follows:
F = ma
Force = mass x acceleration
Think of pushing an empty shopping cart versus a full shopping cart. Which one takes more force to push? The one with more mass takes a much larger force to accelerate.
When two masses are the same, a greater force provides a greater acceleration.
The acceleration is always in the direction of the net force.

25. Force is Measured in Newtons.
The SI unit for force is the Newton (N).
1 Newton = 1 Kg x 1m/s2
1 lb. = N
1 N = lbs.

26. Examples:
1. Zookeepers lift a stretcher that holds a sedated lion. The total mass of the lion and the stretcher is 175 kg, and the lion’s upward acceleration is m/s2. What is the unbalanced force necessary to produce this acceleration of the lion and stretcher?
F = ?
m = 175 kg
a = m/s2
F = ma
F = (175)(0.657)
F = N

27. Examples:
2. What is the net force necessary for a 1.6 x 103 kg car to accelerate forward at 3.5 m/s2?
F = ?
m = 1.6 x 103 kg
a = 3.5 m/s2
F = ma
F = (1600)(3.5)
F = 5600N

28. Examples:
3. A softball accelerates downward at 9.8m/s2. If the gravitational force acting on the ball is 2.9 N, what is the softball’s mass?
F = 2.9N
m = ?
a = 9.8 m/s2
M = F/a
M= 2.9 / 9.8
m = kg

29. Examples:
4. A cruise ship and its passengers have a combined mass of 12,875kg. If the cruise ship experiences an unbalanced force of 762 N pushing it forward, what is the cruise ship’s acceleration?
F = 762 N
m = 12,875 kg
a = ?
a = F / m
a = 762 / 12,875
a = m/s2

30. Free Fall and Weight
When Gravity is the Only Force acting on an Object, it is said to be in Free Fall.
The free fall acceleration of an object is directed toward the center of the Earth.
Because Free Fall acceleration results from the force due to gravity, it is often abbreviated as the letter g.
Near Earth’s Surface, g is approximately equal to 9.8 m/s2.

31. Free Fall Acceleration near Earth’s Surface is Constant.
All objects near Earth’s Surface accelerate at the same rate, regardless of their mass.
This means you can drop two objects with different masses from the same height and they will hit the ground at the same time.
We will assume that all objects on Earth accelerate at exactly 9.8 m/s2.

32. Newton’s Second Law shows us that acceleration depends on both the force acting on an object and its mass.
A heavier object experiences a greater gravitational force than a lighter object. But heavier objects are also harder to accelerate because it has more mass. The extra mass of the heavy object exactly compensates for the additional gravitational force.

33. Weight Equals Mass Times Free-Fall Acceleration.
The force on an object due to gravity is called its weight.
On Earth, your weight is simply the amount of gravitational force exerted on you by Earth.
If you know the free-fall acceleration, g, acting on a body, you can use F = ma to calculate the body’s weight.

34. Weight equals mass times free-fall acceleration.
Weight = mass x free-fall acceleration
W = mg
The SI unit for weight is the Newton, (N).

35. Weight is Different from Mass.
Mass and Weight are easy to confuse.
Mass is a measure of the amount of matter in an object.
Weight is the gravitational force an object experiences due to its mass. The weight of an object depends on gravity, so a change in an object’s location will change the object’s weight.
(Think outer space vs. earth vs. other planets)

36. Gravity influences the shapes of living things
Gravity influences the shapes of living things. On land, large animals must have strong skeletons to support their mass against gravity.
For organisms living in water, the downward force of gravity is balanced by the upward forces of the water. For many creatures, strong skeletons are not necessary. Think of a jellyfish.

37. Go back to the objectives on page 101 for 12-2 and answer them.
How does Newton’s 1st law relate change in motion to a zero net force?
How does Newton’s 2nd Law relate force, mass, and acceleration?
How are weight and mass related?

38. Newton’s 3rd Law and Momentum
Section 12-3
Newton’s 3rd Law and Momentum

39. 12-3 Objectives: On page 118, write and Answer the Following:
1.What is Newton’s 3rd Law?
Before:
After:
2.What is needed for an object to have a large momentum?
3.How is momentum conserved?

40. Newton’s Third Law Forces Always Occur in Pairs.
Normally there are action and reaction type forces that act upon every object.
Newton’s third Law is the called the law of action and reaction.
For every action force, there is an equal and opposite reaction force.

41. Newton’s third law implies that forces always occur in pairs
Newton’s third law implies that forces always occur in pairs. But the action and reaction force of a force pair act on different objects.
The action and reaction forces also occur at the same time, but they do not cancel out because they are acting on different objects.
When you kick a soccer ball, the ball and the foot exert equal and opposite forces on one another.

42. III. Momentum A. Moving Objects have Momentum.
: Lighter objects have less momentum than heavier objects.
: A car is easier to stop than a train because the car has less momentum.
: Momentum is calculated by simply multiplying an object’s mass by its velocity.
Momentum = mass x velocity
p = mv
Units: mass  kg
velocity  m/s
momentum  kg m/s

43. III. Momentum : Like velocity, momentum also has direction.
: An object’s momentum is in the same direction as its velocity.
: The more mass an object has, the greater the momentum.
: The faster an object is moving, the greater the momentum.
Greater Momentum Less Momentum
1. High Mass Small Mass
2. High Velocity 2. Small Velocity

44. Examples
1. Calculate the momentum of a 6.0 kg bowling ball moving at 10m/s down the alley.
p = ?
m = 6.0kg
v = 10m/s
p = mv p = (6.0)(10)
p = 60 kg m/s down the alley

45. Examples
2. Calculate the momentum of a 75 kg speed skater moving forward at 16m/s.
p = ?
m = 75kg
v = 16m/s
p = mv p = (75)(16)
p = 1200 kg m/s moving forward

46. Examples
3. Calculate the momentum of a 135 kg ostrich running north at 16m/s.
p = ?
m = 135kg
v = 16m/s
p = mv p = (135)(16)
p = 2160 kg m/s running north

47. Examples
4. Calculate the momentum of a 5 kg baby on a train moving eastward at 72 m/s.
p = ?
m = 5kg
v = 72m/s
p = mv p = (5)(72)
p = 360 kg m/s moving east

48. Examples
5. Calculate the momentum of an 80g kitten running to the left at 6.5m/s.
p = ?
m = 80g = 0.080kg
v = 6.5m/s
p = mv p = (0.080)(6.5)
p = 0.52 kg m/s moving left

49. Examples
6. Calculate the momentum of a 48,500g passenger on a train stopped on the tracks.
p = ?
m = 48,500g = 48.5kg
v = 0m/s
p = mv p = (48.5)(0)
p = 0 kg m/s

50. III. Momentum B. The Law of Conservation of Momentum
: Imagine that 2 cars with different masses and traveling with different velocities collide head on.
: Can you predict what will happen after the collision?
: Momentum can be used to predict the motion of the cars after the collision.
: This is because in the absence of outside influences, the momentum is conserved.

51. III. Momentum
: In other words, the total momentum of the two cars before a collision is the same as the total momentum after the collision.
: Cars can bounce off of each other to move in opposite directions.
: If the cars stick together after a head-on collision, the cars will continue in the direction of the car that originally had the greater momentum.

52. Examples
1. A 6.0 kg bowling ball moving at 10m/s down the alley hits a 1.0kg pin at rest. After the collision, the 6.0kg bowling ball is traveling at 5m/s. How fast is the pin moving?
ptotal before = ptotal after
pball before + ppin before = pball after + ppin after
(6.0)(10) + (1.0)(0) = (6.0)(5) + (1.0)(v)
= v
60 = 30 + v
30 = v
v = 30 m/s

53. Examples
2. A 75kg speed skater moving forward at 16m/s hits another 65kg skater moving forward at 10m/s. If they collide and hold on to each other, how fast are they moving now? ptotal before = ptotal after
psketer before + pskater before = pskaters after
(75)(16) + (65)(10) = ( )(v)
= 140v
1850 = 140v
13.2 = v
v = 13.2 m/s

54. Examples
3. A person fires a 90kg cannonball from a 2000kg cannon (both start at rest). The cannonball reaches a speed of 30m/s. How fast is the cannon traveling?
ptotal before = ptotal after
Pcannonball + Pcannon before = pcannonball after + pcannon after
( )(0) = (90)(30) + (2000)(v)
0 = v
-2700 = 2000v
-1.35 = v v = m/s