1. Acknowledgements © 2013 Mark Lesmeister/Pearland ISD
This work is licensed under the Creative Commons Attribution-ShareAlike 3.0 Unported License. To view a copy of this license, visit or send a letter to Creative Commons, 444 Castro Street, Suite 900, Mountain View, California, 94041, USA.
Selected graphics and problems from OpenStax College. (2012, June 12). College Physics. Retrieved from the Connexions Web site:
Cartoons from Looney Tunes Movie Collection, © 2005 Warner Brothers Entertainment. Used under the fair use doctrine for educational purposes.
Selected questions from

2. Pearland High School Physics
The Laws of Motion
Pearland High School Physics
Ask “What is the natural state of motion?” “What is the natural state of a student’s motion?”

3. FORCE Force is a push or pull exerted on some object.
Forces cause changes in velocity such as:
Start moving, stop moving or change direction.
The SI unit for force is the Newton.
1 Newton = 1 kg m/s2

4. Types of Forces Forces can act through contact or at a distance.
Contact Force – physical contact between two objects
Field Force – does not involve physical contact between two objects.
Example include:
electrical forces
magnetic forces
the force of gravity

5. The Law of Inertia
Part 1

6. Observation #1
An object at rest remains at rest, unless something makes it move.
Put piece of dry ice on table. (Or have students put an airpuck on the air tables, with no air flowing.) Ask why it doesn’t move. Answers might be because it doesn’t have any “force”. This is approximately what most people in the ancient world believed.
Aristotle believed that motions were either natural or forced. An object naturally moved to its correct place and then stayed at rest. Heavy things, being composed of “earth”, naturally moved toward the Earth. An outside agent was needed to move the thing away from its natural position and natural state of rest.

7. Is rest the natural state of an object?
Picture a ball on a table in a moving train car.
STOP
There are problems with the idea that the natural state of an object is rest, and that an object requires an outside agent to keep moving. The ball is sitting on the table. Is it at rest? Or is it moving? (It is at rest relative to the table. It is moving relative to the Stop sign.) In a train going at a constant speed, the object is moving to someone outside the train. So the idea that rest is the natural state of an object is untenable, since “rest” depends on what you look at.

8. Observation #2
An object in motion continues in motion with constant velocity, unless something makes it change its velocity.
Constant velocity means constant speed in the same direction.
Show dry ice again (or have students push on an airpuck with the air on.) Push slowly. As it moves ask if I have to keep pushing it. Will its motion change? When? So, it will stay moving in one direction until made to do something else. So, what is the key idea here? Is something necessary for an object to move, or change its motion? So, something is needed to cause something to speed up or slow down, but not to keep moving.
(1)
Now push a cart against another cart with magnets out (or have students let puck hit wires). Ask what is happening when it hits the barrier? Did it change its velocity? How?
(2)

9. Combining Observations 1 & 2
An object left alone will not change it’s velocity. Something must cause a change in velocity.
A force is something that causes an acceleration or change in velocity
Change in speed
Change in direction.

10. Objects and Systems
An object is something that has no internal structure, or that we can treat as having no internal structure. Ex: Electron
A system is an object or collection of objects grouped together for study. Ex. Atoms
An electron can be considered an object. A proton is a system, consisting of objects exerting forces on each other. Everything made of atoms is also a system.

11. External and Internal Forces
An object cannot exert a force on itself.
Internal forces have no effect on the motion of a system as a whole.
Only external forces are considered in Newton’s Laws.
Demonstrate by sitting in a rolling chair and trying to push yourself across the room (pushing yourself, not against another object) that an internal force cannot change motion.) Explain that we’ll find out why we can ignore internal forces later on.

12. Observation #3
An object will not change its velocity unless a net external force acts on it.
Then ask about tug of war. Ask if force was being applied to the rope. Ask why it didn't move. Elucidate it’s unbalanced or net forces that matter.
(1)

13. Newton’s First Law Objects do not change their motion without a cause.
Forces are what cause changes in motion.
It is the net external force acting on an object that determines whether it will change motion.

14. Newton’s First Law
An object at rest remains at rest, and an object in motion continues in motion with constant velocity, unless the object experiences a net external force.
A net external force is required to change velocity.

15. Inertia
Another way to say the First Law is to say that objects have inertia.
Inertia is the tendency of objects to resist changes in motion.
The amount of inertia an object has is determined by its mass.
Demonstration 3 with physics books and paper.

16. What is happening?
This is the ball in the train car again. The ball appears to start moving for no reason.
Does this violate Newton’s First Law?
What’s happening in (b)? Is Newton’s First Law being violated? (No, it is being demonstrated. No outside horizontal forces act on the ball, so it moves a constant rate. The *train car* on the other hand is being accelerated.

17. Inertial Reference Frame
An inertial reference frame is one in which Newton’s First Law holds.
Accelerating reference frames are not inertial.

18. Inertial reference frame
The Earth is not an inertial reference frame.
Large scale motions on the Earth will appear to curve without a force.
This is because the Earth is rotating.
This includes
Large air and ocean currents.
Long range missiles.
(1)
Why do hurricanes get their motion?
(2)

19. Newton’s 2nd Law

20. Force SI unit of force is the Newton (N). A force is a vector.
1 N = lb
1 lb. = N
A force is a vector.
It has a magnitude, measure in N or lbs.
It acts in a particular direction.
(1)
Say that forces are measured in Newtons.
Do demonstration 2 with a long and short piece of rope. Ask why more force had to be applied in one situation. So clearly, direction matters.

21. Forces P P P P A force is the interaction of two objects.
There are four fundamental interactions, in other words four fundamental forces.
4 kg
This slide shows the four fundamental interactions, which the students have studied before. Have them guess what is going to happen in each case. The first interaction shows two protons, indicated by the letter P, separated by a distance of less than 2 fm. Ask the students to predict which way the protons will move. Students should make their predictions publically using hand signals. Then show that they attract, and explain how this related to the existence of nuclei, whose protons would repel according to the electric force.
Next, show the Carbon-14 atom, and explain that another force is responsible for causing a neutron to decay into a proton and an electron. Ask what the nucleus will be then (hint, it has 7 protons now). Then show the beta decay and the formation of Nitrogen.
Then demonstrate the electric force and gravity force, each time asking students to predict the motion before you show it.
Carbon-14
Nitrogen P
1 kg P P P e-
2 fm
2 fm

22. Four fundamental forces
Gravitational forces
Strong nuclear forces
Weak nuclear forces
Electromagnetic forces

23. Types of forces
Contact forces- result from physical contact between two objects.
Field forces- force that can exist between two objects even in the absence of physical contact.
E.g. gravity, electric forces
Objects may be in contact, they just don’t have to be.
Ask what I had to do to get dry ice moving. (Touch it.) Is a touch necessary for a force to act? (Yes, no, whatever.) Then drop ball and ask what caused ball to accelerate. (Gravity.) Did earth touch the ball? Did it still accelerate?
(1), (2)

24. Common Forces The force of gravity (Fg) pulls straight down.
The force of friction (Ff) occurs between two objects that can slide against each other.
It opposes the relative motion of the surfaces.

25. Common Forces
The normal force (FN) is the support force from a surface.
It is called “normal” because it is always perpendicular to the surface.
The tension (FT) is the force in a rope or string.
The tension is the same in every part of a rope.
Ask why does the book not fall through the table. Are their forces acting on it? Elucidate that both gravity and the “force of the table” act on it. Do demo- Normal force.
(1)
The word normal means perpendicular in mathspeak. Note how we always draw forces going out from the body on which they act, even if they actually point in.
(2)

26. Newton’s Second Law
The acceleration experienced by an object is directly proportional to and in the same direction as the net force that acts on it, and inversely proportional to the mass of the object.
Using a cart and fan, demonstrate Newton’s 2nd Law.

27. Free-body diagram
Free-body diagrams consider just one object and the forces that act on it.
To draw a free body diagram
Draw a dot to represent the object.
Draw and label vector arrows representing all the forces acting on the object.
All the vectors should be shown as acting at a single point.
Assign Worksheet 1 for homework

28. Newton’s Second Law in Component Form
Force and acceleration are vectors, which can be broken into components.
Newton’s Second Law can be applied in each component direction separately.

29. Practice problem 1
Space shuttle astronauts experience accelerations of about 35 m/s2 during takeoff. What magnitude of force does a 75 kg astronaut experience during an acceleration of this magnitude? (answer in the correct significant figures)
Answer: 2600 N

30. Practice problem 2
A 7.5 kg bowling ball initially at rest is dropped from the top of an 11 m building. It hits the ground 1.5 s later. Find the net force on the bowling ball while it is in the air, including direction and magnitude. Down is negative.
Answer: -74 N

31. Practice Problems 3
A 2.0 kg otter starts from rest at the top of a muddy incline 85 cm long and slides to the bottom in 0.50 s. What net external force acts on the otter?

32. Agent-Object Notation
To avoid confusion, we will often identify forces with two subscripts.
The first subscript indicates what agent is applying the force.
The second subscript indicates to what object the force is applied.
FE-M

33. Weight/Mass Relationship
Weight is the magnitude of the force of the Earth’s gravity on an object.
The force of gravity is shown in diagrams as FE-O or Fg.
Mass is a measure of the amount of matter in an object.
Weight and mass are proportional.
The constant of proportionality near the surface of the Earth is g = 9.81 N/kg.
The weight of an object is often written as mg.
Do weight/mass proportionality demonstration with a force meter, as set of hooked masses, and preferably a graphing program (like LoggerPro).

34. Gravitational Mass vs. Inertial Mass
The property of an object that determines how much weight it has at a certain place is called its gravitational mass.
The property of an object that determines its resistance to changes in motion is called its inertial mass.
Experiments have confirmed that these two properties are the same.
Both weight, i.e. the amount of gravity, and inertia, i.e. resistance to change in motion, seem to depend on the same property of an object, the mass.
There is no reason classically why they should be the same. In Einstein’s General Theory of Relativity, the reason for this equivalence is explained.

35. Inclined Planes
A box slides down two smooth rails with no friction. Find the acceleration of the box. q

36. FN θ
Instead of using axes that are horizontal and vertical, we use a system with the x axis parallel to the plan, and the y axis perpendicular to the plane. This way the acceleration will be entirely along the x-axis, in the positive x axis as I have drawn it here. Also, now all the forces, including friction when we start to include it, will lie along an axis, with the exception of gravity. But, gravity will always have the same two components. Once we figure out what those components are, we can use them in every incline plane problem.
Here, the two forces acting on the block are gravity and the normal force. There is no friction. So, the free body diagram is as shown above.
The two components of gravity are shown above with dotted lines. From geometry, it is easy to show that the angle of incline is the same as the angle θ shown above. The angle we want to use is always the angle between the force of gravity and the perpendicular, or y, axis. The components of gravity are then Fgx = mg sin θ and Fgy = mg cos θ. As long as we use this angle, the components of gravity will always be the same.
In the y-direction, there is no acceleration, since the block does not move perpendicular to the incline. The y direction is in equilibrium, so we have
Σ FY = 0
FN - mg cos θ = 0
FN = mg cos θ
In the x-direction, we can immediately apply Newton’s Second Law to find the acceleration.
Σ Fx = m a
mg sin θ = m a
g sin θ = a
This is the formula we are trying to find.
Fg = mg

37. The law of action and reaction
Section 3

38. Which has more force?
When the boxer hits the bag, which has more force, the boxer on the bag or the bag on the boxer?

39. Newton’s Third Law
If Object A exerts a force on Object B, then B exerts a force on Object A that is equal in magnitude but opposite in direction.
Do Newton’s 3rd Law lab as brief discovery activity. Then go over above. Ask what’s bigger, the force of a bug on the windshield, or the force of the windshield on the bug.
©2012 OpenSTAX College

40. Newton’s Third Law The two forces are called an action-reaction pair.
The two forces do not balance each other, since they act on different objects.
Do Newton’s 3rd Law lab as brief discovery activity. Then go over above. Ask what’s bigger, the force of a bug on the windshield, or the force of the windshield on the bug.
©2012 OpenSTAX College

41. Action-Reaction Pairs
Identify all the action-reaction pairs involved in a ball sitting on a table.

42. Action-Reaction Pairs
Identify all the action-reaction pairs involved in a ball sitting on a table.

43. Basic Problem Solving With Newton’s Laws

44. Acknowledgements © 2013 Mark Lesmeister/Pearland ISD
This work is licensed under the Creative Commons Attribution-ShareAlike 3.0 Unported License. To view a copy of this license, visit or send a letter to Creative Commons, 444 Castro Street, Suite 900, Mountain View, California, 94041, USA.
Selected graphics and problems from OpenStax College. (2012, June 12). College Physics. Retrieved from the Connexions Web site:
Selected questions from LearnAPPhysics.com, © 2009 Richard White
Select problems from Serway and Faughn, Holt Physics, © 2002 Holt Rinehart Winston

45. Equilibrium model
Objects that are at rest or moving with constant velocity are in equilibrium.
According to Newton’s First Law, objects in equilibrium have a net external force that equals 0.
Δv = 0, ∑ F = 0
∑ F = 0, Δv = 0

46. Constant Force Model vs. Equilibrium Model
Object will be at rest or move with constant velocity.
Position vs. time graph-
∑ F = constant.
Object will accelerate in the direction of the net force.
Position vs. time graph- x t x t x t or

47. Problem Solving Tips
Often the problem will be equilibrium in one direction and constant acceleration in another.
You may have to use the constant acceleration model either before or after finding the acceleration.

48. Solving Equilibrium Problems
Givens and Unknowns:
Sketch and label a diagram of the object and its surroundings.
Enclose your object in a boundary to help identify outside forces.
Draw a free body diagram. If there is motion, choose one axis in the direction of motion.
Identify all forces that act on the object, and draw them on the diagram.
Model: Equilibrium
Method
Apply Newton’s 1st Law in component form.
Fnet = 0 so ΣFx = 0 and ΣFy=0

49. Solving Constant Force (Acceleration) Problems
Givens and Unknowns:
Sketch and label a diagram of the object and its surroundings.
Enclose your object in a boundary to help identify outside forces.
Draw a free body diagram. If there is motion, choose one axis in the direction of motion.
Identify all forces that act on the object, and draw them on the diagram.
Model: Constant Force
Method
Apply Newton’s 1st and 2nd Laws in component form.
Fnet = ma so ΣFx = max and ΣFy= may

50. Example 1: Equilibrium
Find the tension in each rope, as a function of the angle.
30o
5.0 kg

51. Solution to Example 1
Givens and Unknowns- as shown in diagrams to the right.
Model:
Equilibrium
Method:
𝐹 𝑥 =0 and 𝐹 𝑦 =0
F1 =?
Fg=mg
F2y =F2 sinq
F1y =F1 sinq
F1x =F1 cos q
F2x =F2 cos q

52. Solution to Example 1, Continued
Implement
From the horizontal equation
From the vertical equation
F1 =?
Fg=mg
F2x =F2 cos q
F2y =F2 sinq
F1y =F1 sinq
F1x =F1 cos q

53. Example 2: Constant Force
A truck is pulling a trailer with a force of 2200 N. Resistive forces opposed to the motion of the trailer total 1200 N. The trailer has a mass of 500 kg. How long will the trailer take to reach 20 m/s?

54. Constant Force Example
Givens and Unknown- as in diagram to the right, and
m = 500 kg
v0 = 0 and v = 20 m/s
t = ?
Model:
Equilibrium in y, constant force (so constant acceleration) in x.
Method: FN
FTruck-Trailer = 2200 N
Ffriction-Trailer = 1200 N
Fg=mg

55. Example 2 Solution Implement FN FTruck-Trailer = 2200 N
Ffriction-Trailer = 1200 N
Fg=mg

56. Additional Practice, Newton’s Laws
From OpenSTAX College Physics:
Normal, Tension and Other Forces, Exercises 2, 4 and 6.
Problem Solving Strategies, Exercises 2,4, 10,