1. The History of Dynamics

2. The Greeks
Natural motion was caused by some internal quality of an object that made it seek a certain “preferred” position without any application of force.

3. The Greeks Unnatural motion was anything else.
Unnatural motion was thought to require applied force to be sustained.

4. The Greeks Natural motions were divided into two categories:
Terrestrial (near the earth)
Celestial (in the heavens)

5. The Greeks
Aristotle taught that an object’s “heaviness” determined how “vigorously” it sought its natural place.

6. Galileo
began by collecting facts and establishing a description of motion
This is called kinematics.
Galileo then inductively developed workable theories of dynamics.

7. Galileo
Experiments showed that the rate at which an object falls is not proportional to its size or mass.
Astronauts later verified his theory on the moon.

8. Ad hoc
a hypothesis based on conjecture rather than observation, usually in an attempt to explain a natural phenomenon

9. Inertia
Galileo’s experiments of “unnatural” motion indicated that the “natural” state of motion of an object could include moving as well as resting.

10. Galileo’s Principle of Inertia:
An object will continue in its original state of motion unless some outside agent acts on it.

11. A push causes a change in an object’s motion.
Inertia
A moving object does not require a continuous push to maintain a constant velocity!
A push causes a change in an object’s motion.

12. Newton built on the work of others studied gravitation Principia
only in recent decades have scientists discovered any exceptions to his work

13. Forces
BJU Press Art
baseball: Physical Science text p. 105

14. Summing Forces Forces are often described as “pushes” and “pulls.”
Forces are vectors.
Forces can be added just as vectors are added.
BJU Press Art
baseball: Physical Science text p. 105

15. Summing Forces Notation: ΣF ≡ F1 + F2 + ... + Fn
The Greek capital letter sigma (Σ) is used to indicate a sum.
BJU Press Art
baseball: Physical Science text p. 105

16. Summing Forces If forces are balanced... ΣF = 0
...and no change in motion will occur.
BJU Press Art
baseball: Physical Science text p. 105
ΣF = 0 ↔ ΣFx = 0 and ΣFy = 0

17. Unbalanced Forces will change an object’s state of motion
there may be two, or more than two, forces which are unbalanced
BJU Press Art
baseball: Physical Science text p. 105

18. Unbalanced Forces
To find the sum of unbalanced forces, you add the force vectors acting upon the object.
This usually involves finding and adding the vector components.
BJU Press Art
baseball: Physical Science text p. 105

19. Equilibrant Force
a force that balances one or more other concurrent forces

20. Equilibrant Force
a vector having the same magnitude as the vector sum of the other unbalanced forces but pointing in the opposite direction
Fequil. = -ΣFother

21. Equilibrant Force
If the sum of all forces on an object is zero, then any unknown force must be the equilibrant of all the known forces.

22. Weight the force of gravity acting on an object
a vector pointing straight downward
often notated Fw

23. Types of Forces
All forces are classified as either fundamental forces or mechanical forces.
There are four fundamental forces.

24. Fundamental Forces Gravitational force
proportional to the masses of interacting objects
can exert its influence over theoretically infinite distances

25. Fundamental Forces Gravitational force
all objects exert gravitational force on all other objects

26. Fundamental Forces Electromagnetic force
used to explain both magnetism and electricity
a long-range force
a short-range force

27. Fundamental Forces Strong nuclear interaction force
Weak nuclear interaction force

28. Classification of Forces
Noncontact Forces
gravity
electromagnetic forces
sometimes called “action-at-a-distance” forces

29. Classification of Forces
Noncontact Forces
field theory attempts to explain these
virtual particles have been offered as an explanation

30. Classification of Forces
Contact Forces
transmitted only by physical contact between objects
include the following:

31. Classification of Forces
tensile (pull things apart)
compressive (push things together or crush)
torsion (twist)

32. Classification of Forces
friction (oppose motion between two objects in contact)
shear (cause layers within matter to slide past one another)

33. Measuring Forces instruments used include: spring scale load cell
pressure gauge

34. Measuring Forces instruments used include: ballistic pendulum
accelerometer
force table

35. Newton’s Laws of Motion

36. Newton’s Laws These are the central principles of dynamics.
Their proper use requires an understanding of what a system is.

37. It is isolated from its surroundings.
Systems
In physics, a system is whatever is inside an imaginary boundary chosen by the physicist.
It is isolated from its surroundings.

38. Newton’s 1st Law
A system at rest will remain at rest, and a moving system will move continuously with a constant velocity unless acted on by outside unbalanced forces.

39. Newton’s 1st Law
If all external forces on a system are balanced, then its velocity remains constant; the acceleration is zero.

40. Newton’s 1st Law
If all forces acting on a system are not balanced, then a nonzero resultant force exists and the velocity changes, resulting in an acceleration.

41. Stated mathematically:
Newton’s 1st Law
Stated mathematically:
ΣF = 0 ↔ a = 0
or equivalently:
ΣF ≠ 0 ↔ a ≠ 0

42. Newton’s 1st Law Friction is a force that causes motion to change.
Inertia is the tendency for a system to resist a change in motion.

43. Without unbalanced forces, objects tend to move in straight lines.
Newton’s 1st Law
Mechanical equilibrium occurs when the sum of all forces on a system is zero.
Without unbalanced forces, objects tend to move in straight lines.

44. Newton’s 2nd Law the most general of the three laws
gives a working definition of force and a way to measure such force

45. Newton’s 2nd Law
The acceleration of a system if directly proportional to the sum of the forces (resultant force) acting on the system and is in the same direction as the resultant.

46. Stated mathematically:
Newton’s 2nd Law
Stated mathematically:
a = ΣF m
or equivalently:
ΣF = ma

47. This is how the Newton, a derived unit, is defined.
Newton’s 2nd Law
A resultant force of 1 N, when applied to a mass of 1 kg, produces an acceleration of 1 m/s².
This is how the Newton, a derived unit, is defined.

48. Newton’s 2nd Law
component equations:
ΣFx = max
ΣFy = may
ΣFz = maz

49. Newton’s 3rd Law
If system X exerts a force on system Y, then Y exerts a force of the same magnitude on X but in the opposite direction.
FX→Y = -FY→X

50. Newton’s 3rd Law forces have four properties that relate to this law:
All forces occur in pairs.
Each force in an action-reaction pair has the same magnitude.

51. Newton’s 3rd Law
Each force acts in the opposite direction in line with the other force of the pair.
Each force acts on a different system.

52. Weight and Mass
The force of planetary gravitational attraction on an object is called its weight, Fw.
Weight is directly proportional to mass.
Fw = am

53. Weight and Mass Since this gravitational acceleration is downward:
Fw = mg
g = m/s²
The magnitude of an object’s weight vector is |mg|.

54. Weight and Mass
The weight vector, like the gravity vector, points straight down (toward the center of the earth).
In scalar component form:
Fwy = mgy

55. Weight and Mass
Mass is measured on scales and reported in units of kg or g.