1. Dynamics: Newton’s Laws of Motion
Chapters
Dynamics: Newton’s Laws of Motion

3. Key Terms Equilibrium, net force is zero.
Net force, the vector sum of all forces acting on an object
Force a push or pull that accelerates and object (Newton, N = 1 kg-m/s2)
Friction, a force that acts to resist relative motion
Inertia (mass), resistance of an object to change its state of motion
Kilogram, unit of mass.
Law of inertia is Newton’s first Law of motion
Normal force, the upward force balancing the weight and any additional downward force on an object.
Weight, the force on a body due to the gravitational attraction of another body (Earth)

4. Up to now we dealt with motion in terms of velocity and acceleration
Up to now we dealt with motion in terms of velocity and acceleration. However there is a deeper question, what makes objects accelerate?
The answer is that it is force, it is helpful to develop a qualitative understanding of forces to fully understand motion.

5. Force
A force is a push or pull. An object at rest needs a force to get it moving; a moving object needs a force to change its velocity. A change in velocity means there is a non-zero acceleration.
The magnitude of a force can be measured using a spring scale.

6. Newton’s First Law of Motion
Newton’s first law is often called the law of inertia.
Every object continues in its state of rest, or of uniform velocity in a straight line, as long as no net force acts on it.

7. Force
Not all forces result in motion. For example you can push against a wall and the wall does not move.

8. Mass
Mass is the measure of inertia of an object. In the SI system, mass is measured in kilograms.
Mass is not weight:
Mass is a property of an object. Weight is the force exerted on that object by gravity.
If you go to the moon, whose gravitational acceleration is about 1/6 g, you will weigh much less. Your mass, however, will be the same.

9. Newton’s First Law of Motion
Every object remains in the same state of rest or motion at constant velocity unless acted on by a net external force
Bicycle Does 100mph! - YouTube

10. Newton’s First Law: Equilibrium
Equilibrium the net force on an object is zero
                                                                                        

11. Balanced forces

12. Objects at rest remain at rest unless acted on by a net force.
                                                               
                            
A lot of inertia! The large train resists changing its motion.
Very little inertia. The small baby carriage has very little resistance to changes in motion.
Since the train is so huge, it is difficult to change its speed. In fact, a large net force is required to change its speed or direction.
Since the baby carriage is so small, it is very easy to change its speed or direction. A small net force is required to change its speed or direction.

13. Mass vs. Weight
Mass is a measure of inertia, it has the SI units of kilogram (kg).
Weight is mass x gravitational acceleration, the SI units for weight are Newtons (N).
Note an object’s weight is not constant, on the moon an object weighs ~1/6th of it’s weight on Earth. This is due to the difference in gravitational acceleration.

14. Object
Mass
electron rest mass
x kg
proton
x kg
neutron rest mass
x kg
smallest virus
9.5 x kg
typical red blood cell
9 x kg
typical rain drop
2 x 10-6 kg
mosquito
5 x 10-5 kg
dime (U.S.)
2.268 x 10-3 kg
ping-pong ball
2.5 x 10-3 kg
quarter (U.S.)
5.750 x 10-3 kg
golf ball
4.6 x 10-2 kg
tennis ball
5.7 x 10-2 kg
baseball
1.45 x 10-1 kg
volley ball
2.55 x 10-1 kg
soccer ball
4.25 x 10-1 kg
basketball
6.25 x 10-1 kg
baseball bat
8.5 x 10-1 to 1.02 kg

15. Object
Mass
bowling ball
7.2 kg
wheel and tire (small car)
13 kg
average woman
58 kg
average man
70 kg
polar bear (male)
5.9 x 102 kg
typical automobile
1.5 x 103 kg
elephant
3.8 x 103 kg
blue whale (large)
1.4 x 105 kg
747 (empty)
1.6 x 105 kg
JFK class aircraft carrier
7.3 x 107 kg
living matter on Earth
3.6 x 1014 kg
Moon
7.354 x 1022 kg
Earth
x 1024 kg
Sun
1.99 x 1030 kg
Milky Way galaxy
4 x 1041 kg
universe
1 x 1053 kg

16. Question Why is mass more fundamental than weight?
Mass is a measure of inertia, which quantifies the amount of matter. Weight is the force of gravity, i.e. weight depends on the local gravitational acceleration.
How To Draw A Tablecloth Off From Under A Dinner Service - YouTube

17. If the car were to abruptly stop and the seat belts were not being worn, then the passengers in motion would continue in motion. Assuming a negligible amount of friction between the passengers and the seats, the passengers would likely be propelled from the car and be hurled into the air. Once they leave the car, the passengers becomes projectiles and continue in projectile-like motion.
Now perhaps you will be convince of the need to wear your seat belt. Remember it's the law - the law of inertia
Invisible Forces: Ned Kahn | Science, The Arts | Video | PBS LearningMedia

18. Newton’s Second Law of Motion
Newton’s second law is the relation between acceleration and net force. Acceleration is proportional to force and inversely proportional to mass.
The sum of all forces is the net force .

19. Force and Acceleration
Three possible ways to accelerate an object

20. F a m Magic Triangle Cover up variable you are asked to find
Read off equation F
F = m x a m = F ÷ a a = F ÷ m a m x

21. Force and Net Force
Force is a push or pull that accelerates an object.
The net force is the vector sum of all forces acting on an object. Be sure to use vector addition rules when adding forces.

22. Newton’s 2nd Law
                      
       
                               
                            
                                  

23. Newton’s 2nd Law
                                a = F / m
                                a  / 2= F / 2m
                                 a / 3 = F / 3m
Note Newton’s second law quantifies the first law: it takes more force to accelerate more massive objects and it shows the linear relationship between acceleration and net force.

24. Newton’s 1st and 2nd Laws

25. Equilibrium

26. Newton’s Second Law of Motion
Force is a vector, so is true along each coordinate axis.
The unit of force in the SI system is the Newton (N).
Note that the pound is a unit of force, not of mass, and can therefore be equated to Newtons but not to kilograms.

28. The above diagram shows what is occasionally a difficult concept to believe. Many students find it difficult to see how 10 N + 10 N could ever be equal to 10 N

29. Net Force The net force is 10N to the right The Net force is zero

30. more examples

32. What is Fnet? 0N
5N, left
15N, up

33. Applications Involving Friction, Inclines
On a microscopic scale, most surfaces are rough. The exact details are not yet known, but the force can be modeled in a simple way.
For kinetic – sliding – friction, we write:
is the coefficient of kinetic friction, and is different for every pair of surfaces.

34. Applications Involving Friction, Inclines

35. Applications Involving Friction, Inclines
Static friction is the frictional force between two surfaces that are not moving along each other. Static friction keeps objects on inclines from sliding, and keeps objects from moving when a force is first applied.

36. Applications Involving Friction, Inclines
The static frictional force increases as the applied force increases, until it reaches its maximum. Then the object starts to move, and the kinetic frictional force takes over.
What is Friction - Physics for kids - YouTube

37. Balanced Forces
A rightward force is applied to a 6-kg object to move it across a rough surface at constant velocity. The object encounters 15 N of frictional force. Use the diagram to determine the gravitational force, normal force, net force, and the applied force (use g=10m/s2).                                 
Fgrav = mg
= 6kg*10m/s2
= 60 N
Fnet = ma = 0, (a=0)
Fnorm = Fgrav = 60 N
Fapp = Ffrict = 15 N

38. On two different occasions during a high school soccer game, the ball was kicked simultaneously by players on opposing teams. In which case (Case 1 or Case 2) does the ball undergo the greatest acceleration? Explain your answer
Case 2 results in the greatest acceleration. Even though the individual forces are greater in Case 1, the net force is greatest in Case 2. Acceleration depends on the net force; it is not dependent on the size of the individual forces.

39. Newton’s Third Law of Motion
Any time a force is exerted on an object, that force is caused by another object.
Newton’s third law:
Whenever one object exerts a force on a second object, the second exerts an equal force in the opposite direction on the first object.

40. Newton’s Third Law of Motion
A key to the correct application of the third law is that the forces are exerted on different objects. Make sure you don’t use them as if they were acting on the same object.

41. Newton’s Third Law of Motion
Rocket propulsion can also be explained using Newton’s third law: hot gases from combustion spew out of the tail of the rocket at high speeds. The reaction force is what propels the rocket.
Note that the rocket does not need anything to “push” against. That is why rocket propulsion works in outer space.

42. Newton’s Third Law of Motion
Helpful notation: the first subscript is the object that the force is being exerted on; the second is the source.
This need not be done indefinitely, but is a good idea until you get used to dealing with these forces.

43. Newton’s Third Law
When an object exerts a force on a second object, the second object exerts an equal but opposite force on the first object. Often, these forces are called action and reaction, note both forces occur simultaneously.

44. More Action-Reaction Forces
Note the rocket pushes on the gases it expels when the fuel is burned. This is why rocket propulsion can work in space, which does not provide anything to push against

45. Identifying Action Reaction Pairs

46. Example
What is the recoil acceleration of a 60 kg person on ice skates pushing off a wall with a 30N force?

47. Answer
Newton’s third law tells us the force on the skater is also 30N.
Using a = F/m we get
a = 30N/60kg
= 0.5 m/s2

48. Question
While driving down the road, a firefly strikes the windshield of a bus and makes a quite obvious mess in front of the face of the driver. This is a clear case of Newton's third law of motion. The firefly hit the bus and the bus hits the firefly. Which of the two forces is greater: the force on the firefly or the force on the bus?
The forces are equal and opposite

49. Arnold Strongman and Suzie Small pull on opposite ends of a rope in a tug of war. The greatest force exerted on the rope is by a) Arnold b) Suzie c) ... both the same

50. Question
Since action-reaction forces are equal and opposite why don’t they cancel and result in a net force of zero?

51. Answer
The action force is the force object 1 exerts on object 2. The reaction force is the force object 2 exerts on object 1. So although they are equal in magnitude and opposite in direction they are acting on different objects. Therefore the net force on either one of the objects is not zero for a given action-reaction pair of forces.
Newton's Third Law of Motion: Astronauts in Outer Space | Science | Classroom Resources | PBS Learning Media

52. Free Body Diagrams
A free body diagram is a pictorial representation often used by physicists and engineers to analyze the forces acting on a body of interest. A free body diagram shows all forces of all types acting on this body. Drawing such a diagram can aid in solving for the unknown in a problem.
The sketch of the free body need include only as much detail as necessary. Often a simple outline is sufficient. Depending on the analysis to be performed and the model being employed, just a single point may be the most appropriate

56. Solving Problems with Newton’s Laws – Free-Body Diagrams
When a cord or rope pulls on an object, it is said to be under tension, and the force it exerts is called a tension force.

57. Weight – the Force of Gravity; and the Normal Force
Weight is the force exerted on an object by gravity. Close to the surface of the Earth, where the gravitational force is nearly constant, the weight is:

58. Weight – the Force of Gravity; and the Normal Force
An object at rest must have no net force on it. If it is sitting on a table, the force of gravity is still there; what other force is there?
The force exerted perpendicular to a surface is
called the normal force. It is exactly as large as needed to balance the force from the object (if the required force gets too big, something breaks!)

59. Weight – the Force of Gravity; and the Normal Force
The Normal force is not the action-reaction pair to the gravitational force known as weight. The reason is that it is also acting on the object. Action-reaction pairs act on different objects.
Note, if you are standing on a bathroom scale, the scale reading gives the value of the normal force acting on you from the scale. If you are standing on a scale which is on the horizontal ground, then this normal force is just equal to your weight. However,  if the floor is inclined, or if the scale happens to be in an elevator which is accelerating, then the scale reading will not be equal to your weight!  In this case, the scale reading that is, the normal force is sometimes called the apparent weight.

60. Forces When a box sits on the table what forces are at work?
The normal force is the force the table exerts on the box in this case the weight of the box. To make the net force be zero
(b) The normal force is directed upward with a force equal to the weight of the book + 40N.

61. In the diagram, Otis's 80-kg is traveling with constant speed (A), accelerating upward (B), accelerating downward (C), and free-falling (D) after the elevator cable snaps.

62. Fnorm equals Fgrav Fnorm = 784 N Fnorm > Fgrav by 400 N
Fnet = m*a
Fnet = 0 N
Fnet = 400 N, up
Fnet = 400 N, down
Fnet = 784 N, down
Fnorm equals Fgrav
Fnorm = 784 N
Fnorm > Fgrav
by 400 N
Fnorm = 1184 N
Fnorm < Fgrav
Fnorm = 384 N
by 784 N
Fnorm = 0 N

64. Terminal Velocity

65. Unbalanced Forces
An applied force of 50 N is used to accelerate an object to the right across a frictional surface. The object encounters 10 N of friction. Determine the normal force, the net force, the mass, and the acceleration of the object (use g= 10m/s2)
                                                                   
Fnorm = 80 N
m = 80N/10 m/s2
= 8 kg
Fnet = 50N-10N
= 40N
a = Fnet/m
= 40N/8kg
a = 5m/s2

66. Equilibrium
If an object is at equilibrium, then the forces are balanced. Balanced is the key word which is used to describe equilibrium situations. Thus, the net force is zero and the acceleration is 0 m/s/s.

67. If an object is at rest and is in a state of equilibrium, then we would say that the object is at "static equilibrium.“
Science of NHL Hockey
Force A B C
Magnitude
3.4 N
9.2 N
9.8 N
Direction
161 deg.
70 deg.
270 deg

68. Friction: Inclined Plane
Applet: Simple Inclined Plane

69. Summary of Chapter 4 and 5
Newton’s first law: If the net force on an object is zero, it will remain either at rest or moving in a straight line at constant speed.
Newton’s second law:
Newton’s third law:
Weight is the gravitational force on an object.
The frictional force can be written:
(kinetic friction) or (static friction)
Free-body diagrams are essential for problem-solving