2The GreeksNatural motion was caused by some internal quality of an object that made it seek a certain “preferred” position without any application of force.
3The Greeks Unnatural motion was anything else. Unnatural motion was thought to require applied force to be sustained.
4The Greeks Natural motions were divided into two categories: Terrestrial (near the earth)Celestial (in the heavens)
5The GreeksAristotle taught that an object’s “heaviness” determined how “vigorously” it sought its natural place.
6Galileobegan by collecting facts and establishing a description of motionThis is called kinematics.Galileo then inductively developed workable theories of dynamics.
7GalileoExperiments 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.
8Ad hoca hypothesis based on conjecture rather than observation, usually in an attempt to explain a natural phenomenon
9InertiaGalileo’s experiments of “unnatural” motion indicated that the “natural” state of motion of an object could include moving as well as resting.
10Galileo’s Principle of Inertia: An object will continue in its original state of motion unless some outside agent acts on it.
11A push causes a change in an object’s motion. InertiaA moving object does not require a continuous push to maintain a constant velocity!A push causes a change in an object’s motion.
12Newton built on the work of others studied gravitation Principia only in recent decades have scientists discovered any exceptions to his work
13ForcesBJU Press Artbaseball: Physical Science text p. 105
14Summing Forces Forces are often described as “pushes” and “pulls.” Forces are vectors.Forces can be added just as vectors are added.BJU Press Artbaseball: Physical Science text p. 105
15Summing Forces Notation: ΣF ≡ F1 + F2 + ... + Fn The Greek capital letter sigma (Σ) is used to indicate a sum.BJU Press Artbaseball: Physical Science text p. 105
16Summing Forces If forces are balanced... ΣF = 0 ...and no change in motion will occur.BJU Press Artbaseball: Physical Science text p. 105ΣF = 0 ↔ ΣFx = 0 and ΣFy = 0
17Unbalanced Forces will change an object’s state of motion there may be two, or more than two, forces which are unbalancedBJU Press Artbaseball: Physical Science text p. 105
18Unbalanced ForcesTo 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 Artbaseball: Physical Science text p. 105
19Equilibrant Forcea force that balances one or more other concurrent forces
20Equilibrant Forcea vector having the same magnitude as the vector sum of the other unbalanced forces but pointing in the opposite directionFequil. = -ΣFother
21Equilibrant ForceIf the sum of all forces on an object is zero, then any unknown force must be the equilibrant of all the known forces.
22Weight the force of gravity acting on an object a vector pointing straight downwardoften notated Fw
23Types of ForcesAll forces are classified as either fundamental forces or mechanical forces.There are four fundamental forces.
24Fundamental Forces Gravitational force proportional to the masses of interacting objectscan exert its influence over theoretically infinite distances
25Fundamental Forces Gravitational force all objects exert gravitational force on all other objects
26Fundamental Forces Electromagnetic force used to explain both magnetism and electricitya long-range forcea short-range force
27Fundamental Forces Strong nuclear interaction force Weak nuclear interaction force
28Classification of Forces Noncontact Forcesgravityelectromagnetic forcessometimes called “action-at-a-distance” forces
29Classification of Forces Noncontact Forcesfield theory attempts to explain thesevirtual particles have been offered as an explanation
30Classification of Forces Contact Forcestransmitted only by physical contact between objectsinclude the following:
31Classification of Forces tensile (pull things apart)compressive (push things together or crush)torsion (twist)
32Classification of Forces friction (oppose motion between two objects in contact)shear (cause layers within matter to slide past one another)
33Measuring Forces instruments used include: spring scale load cell pressure gauge
34Measuring Forces instruments used include: ballistic pendulum accelerometerforce table
36Newton’s Laws These are the central principles of dynamics. Their proper use requires an understanding of what a system is.
37It is isolated from its surroundings. SystemsIn physics, a system is whatever is inside an imaginary boundary chosen by the physicist.It is isolated from its surroundings.
38Newton’s 1st LawA 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.
39Newton’s 1st LawIf all external forces on a system are balanced, then its velocity remains constant; the acceleration is zero.
40Newton’s 1st LawIf all forces acting on a system are not balanced, then a nonzero resultant force exists and the velocity changes, resulting in an acceleration.
41Stated mathematically: Newton’s 1st LawStated mathematically:ΣF = 0 ↔ a = 0or equivalently:ΣF ≠ 0 ↔ a ≠ 0
42Newton’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.
43Without unbalanced forces, objects tend to move in straight lines. Newton’s 1st LawMechanical equilibrium occurs when the sum of all forces on a system is zero.Without unbalanced forces, objects tend to move in straight lines.
44Newton’s 2nd Law the most general of the three laws gives a working definition of force and a way to measure such force
45Newton’s 2nd LawThe 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.
46Stated mathematically: Newton’s 2nd LawStated mathematically:a =ΣFmor equivalently:ΣF = ma
47This is how the Newton, a derived unit, is defined. Newton’s 2nd LawA 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.
48Newton’s 2nd Lawcomponent equations:ΣFx = maxΣFy = mayΣFz = maz
49Newton’s 3rd LawIf 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
50Newton’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.
51Newton’s 3rd LawEach force acts in the opposite direction in line with the other force of the pair.Each force acts on a different system.
52Weight and MassThe force of planetary gravitational attraction on an object is called its weight, Fw.Weight is directly proportional to mass.Fw = am
53Weight and Mass Since this gravitational acceleration is downward: Fw = mgg = m/s²The magnitude of an object’s weight vector is |mg|.
54Weight and MassThe weight vector, like the gravity vector, points straight down (toward the center of the earth).In scalar component form:Fwy = mgy
55Weight and MassMass is measured on scales and reported in units of kg or g.