QODQOD
What is force?What is force?
Forces Forces & the & the
Laws of MotionLaws of Motion
Physics IPhysics I
Mr. PadillaMr. Padilla
Aristotle (~400 B.C.)Aristotle (~400 B.C.)
Divided motion into 2 Divided motion into 2 typestypes NaturalNatural ViolentViolent
Natural – Straight up Natural – Straight up or straight down.or straight down.
Objects seek their Objects seek their “natural” resting place“natural” resting place Clouds – UpClouds – Up Rocks – DownRocks – Down Heavens – Perfect Heavens – Perfect
Circles around EarthCircles around Earth
Violent – imposed Violent – imposed motionmotion
A result of forces that A result of forces that pushed or pulled. Had pushed or pulled. Had an external cause.an external cause.
These ideas held for These ideas held for about 2000 years.about 2000 years.
Copernicus (1473-1534)Copernicus (1473-1534)
Nicholas Copernicus was the first to Nicholas Copernicus was the first to reason that the Earth & planets moved reason that the Earth & planets moved around the sun.around the sun.
Heavily persecuted for his ideas and Heavily persecuted for his ideas and worked in secret to hide them.worked in secret to hide them.
His work was published for the first time His work was published for the first time on the day he died.on the day he died.
GalileoGalileo
Believed Copernicus Believed Copernicus and so was placed and so was placed under house arrestunder house arrest
ForceForce – any push or – any push or pullpull
FrictionFriction – any force – any force that acts between that acts between materials as they materials as they move past each othermove past each other
Only when friction is Only when friction is present is a force present is a force needed to keep an needed to keep an object in motionobject in motion
Discredited Aristotle’s Discredited Aristotle’s ideas of motionideas of motion
Changes in MotionChanges in MotionForces can act through contact or at a Forces can act through contact or at a
distancedistanceContact forces – result from the physical Contact forces – result from the physical
contact between 2 objects.contact between 2 objects.Ex: Jordan pushes KevinEx: Jordan pushes Kevin
Field forces – does not involve physical Field forces – does not involve physical contact.contact.Ex: Gravity, electric fields Ex: Gravity, electric fields Field forces were developed to explain how Field forces were developed to explain how
an object can exert force on another without an object can exert force on another without touching.touching.
NewtonNewton
Newton’s 1Newton’s 1stst Law of Motion Law of MotionEvery object continues in a state of rest, or of Every object continues in a state of rest, or of
motion in a straight line at a constant speed, motion in a straight line at a constant speed, unless it is compelled to change that state by unless it is compelled to change that state by forces exerted upon it.forces exerted upon it.
An object in motion stays in motion, an An object in motion stays in motion, an object at rest stays at rest.object at rest stays at rest.
AKA law of inertia AKA law of inertia
Inertia in ActionInertia in Action
Inertia: the reluctance of any body to a Inertia: the reluctance of any body to a change in the state of motion.change in the state of motion.
Book on a tableBook on a tableAir hockey tableAir hockey tableTable cloth & dishesTable cloth & dishesWhere would they go?Where would they go?
Ball thrown in outer spaceBall thrown in outer spaceCar without a driving forceCar without a driving forcePlanets without the sun’s gravityPlanets without the sun’s gravity
Mass vs. WeightMass vs. Weight
The inertia of an The inertia of an object depends on its object depends on its massmass How much material is How much material is
in the objectin the object The more mass = The more mass =
more inertiamore inertia Mass is a measure of Mass is a measure of
the inertia of an the inertia of an object.object.
Mass is not weight.Mass is not weight. Weight – force of Weight – force of
gravity of an objectgravity of an object Weight can change in Weight can change in
different environments: different environments: mass, and therefore mass, and therefore inertia, remain inertia, remain constant.constant.
Mass & Weight are Mass & Weight are proportional.proportional.
WeightWeight
Weight can be calculated by taking an Weight can be calculated by taking an objects mass times gravityobjects mass times gravityW = mgW = mg
The SI unit for weight is The SI unit for weight is kg (x) m/skg (x) m/s22 = kgm/s = kgm/s22 or Newtons (N) or Newtons (N)
A mass of 1kg has a weight of 9.8N, or A mass of 1kg has a weight of 9.8N, or 2.2lbs2.2lbs
Mass ExampleMass Example
Felicia, the ballet dancer, has a mass of Felicia, the ballet dancer, has a mass of 45.0 kg. a) What is Felicia’s weight on 45.0 kg. a) What is Felicia’s weight on Earth? b) What is Felicia’s mass on Earth? b) What is Felicia’s mass on Jupiter, where acceleration due to gravity Jupiter, where acceleration due to gravity is 25.0 m/sis 25.0 m/s22? c) What is Felicia’s weight on ? c) What is Felicia’s weight on Jupiter?Jupiter?
Net forceNet force
Force is a vector quantity.Force is a vector quantity.Has magnitude and direction.Has magnitude and direction.
Net force – When considering Newton’s 1Net force – When considering Newton’s 1stst Law, you must think of net forces not just Law, you must think of net forces not just forces.forces.
Net Force – combination of all forces Net Force – combination of all forces acting on an object.acting on an object.
All individual forces can be added and All individual forces can be added and treated as a single (net) forcetreated as a single (net) force
FBDFBD
A free body diagram A free body diagram can be drawn to can be drawn to represent the forces represent the forces acting on an object.acting on an object.
A FBD shows vectors A FBD shows vectors for forces acting on a for forces acting on a single objectsingle object
EquilibriumEquilibrium
What forces act on a book at rest on a What forces act on a book at rest on a table?table?At least 2 – weight and a support (normal) At least 2 – weight and a support (normal)
forceforce If the net force acting on an object is zero, If the net force acting on an object is zero,
it is in equilibrium.it is in equilibrium.No change in state of motion.No change in state of motion.
Normal force – always acts at a right angle Normal force – always acts at a right angle to the surface.to the surface.
QODQOD
Determine your weight, in Newtons, on the Determine your weight, in Newtons, on the other planets in our solar system.other planets in our solar system.Mercury 3.59 m/sMercury 3.59 m/s22
Venus 8.87 m/sVenus 8.87 m/s22
Mars 3.77 m/sMars 3.77 m/s22
Jupiter 25.0 m/sJupiter 25.0 m/s22
Saturn 11.08 m/sSaturn 11.08 m/s22
Uranus 10.67 m/sUranus 10.67 m/s22
Neptune 14.07 m/sNeptune 14.07 m/s22
AccelerationAcceleration
So far: a = So far: a = ΔΔV/tV/tThis is the definition of aThis is the definition of a
The cause of acceleration is force.The cause of acceleration is force.Applying a force (or net force) to an object Applying a force (or net force) to an object
causes it to acceleratecauses it to accelerateMore force causes more acceleration (direct)More force causes more acceleration (direct)
Mass resists the accelerationMass resists the accelerationmore mass causes less acceleration more mass causes less acceleration
(inversely)(inversely)
22ndnd Law Law
Acceleration produced by a net force on Acceleration produced by a net force on an object is directly proportional to the an object is directly proportional to the magnitude of the net force, is in the same magnitude of the net force, is in the same direction as the net force, and is inversely direction as the net force, and is inversely proportional to the mass of the objectproportional to the mass of the object..
OrOra ~ (net force)/massa ~ (net force)/mass a = F/ma = F/mBetter yet… FBetter yet… Fnetnet = ma = ma
ExamplesExamples
A force of 30.N is A force of 30.N is applied to a 12kg applied to a 12kg mass, how fast does mass, how fast does it accelerate?it accelerate?
How much force is How much force is needed to accelerate needed to accelerate a 700.kg vehicle at a a 700.kg vehicle at a rate of 4.0m/srate of 4.0m/s22??
F = maF = ma 30.N = 12kg(a)30.N = 12kg(a) a = 30N/12kga = 30N/12kg a = 2.5 m/sa = 2.5 m/s22
F = maF = ma F = (700.kg)(4.0m/sF = (700.kg)(4.0m/s22)) F = 2800NF = 2800N
More Great ExamplesMore Great Examples Ben and Sam push on Ben and Sam push on
opposite sides of Chris. opposite sides of Chris. Ben pushes with 400.N of Ben pushes with 400.N of force, Sam with 200.N. force, Sam with 200.N. What is the net force on What is the net force on Chris?Chris?
If Chris has a mass of If Chris has a mass of 75kg, what will his 75kg, what will his acceleration be?acceleration be?
A constant force exerted A constant force exerted on an object, will result in on an object, will result in what kind of motion?what kind of motion?
FFnetnet = = ΣΣF acting on ChrisF acting on Chris FFnetnet = 400.N – 200.N = 400.N – 200.N FFnetnet = 200.N = 200.N
F = maF = ma 200N = (75kg)a200N = (75kg)a a = 200N/75kga = 200N/75kg a = 2.7 m/sa = 2.7 m/s22
Constant force produces Constant force produces constant acceleration.constant acceleration.
Free Fallin’Free Fallin’
In a vacuum all things fall at the same rate due In a vacuum all things fall at the same rate due to gravity. to gravity.
In reality, air resistance provides friction and In reality, air resistance provides friction and causes some things to slow down. causes some things to slow down.
All objects reach a point where the force of All objects reach a point where the force of gravity pulling them down is equal to the force of gravity pulling them down is equal to the force of air resistance pushing back on them.air resistance pushing back on them.
This is called terminal velocity.This is called terminal velocity. Humans = 150-200 km/h in free fallHumans = 150-200 km/h in free fall 15-20 km/h with a parachute.15-20 km/h with a parachute.
ForcesForces
Forces are not a thing by themselves, but Forces are not a thing by themselves, but part of an interaction.part of an interaction.
Forces always occur in pairs.Forces always occur in pairs.One force is called the action force, the One force is called the action force, the
other is called the reaction force.other is called the reaction force.
Newton’s Third LawNewton’s Third Law
Whenever one object exerts a force Whenever one object exerts a force on a second object, the second object on a second object, the second object exerts an equal and opposite force on exerts an equal and opposite force on the first object.the first object.
To every action, there is an equal and To every action, there is an equal and opposite reaction.opposite reaction.
Action/Reaction PairsAction/Reaction Pairs
When a hammer hits When a hammer hits a nail, which is the a nail, which is the action and which is action and which is the reaction force?the reaction force?
Identify the Identify the action/reaction forces action/reaction forces when you walk.when you walk.
Action: Object A Action: Object A exerts force on exerts force on object Bobject B
Reaction: Object B Reaction: Object B exerts force on exerts force on object Aobject A
Different MassesDifferent Masses
Forces objects exert on each other are Forces objects exert on each other are always equal in strength and in opposite always equal in strength and in opposite directions.directions.
Newton’s second law (F=ma) still applies.Newton’s second law (F=ma) still applies. If the masses are greatly different, then If the masses are greatly different, then
the object with the smaller mass will the object with the smaller mass will undergo greater acceleration. undergo greater acceleration.
ExamplesExamples A gun with a mass A gun with a mass of 20kg shoots out a of 20kg shoots out a .04kg bullet. The .04kg bullet. The gun kicks back gun kicks back at .2m/sat .2m/s22, at what , at what rate does the bullet rate does the bullet accelerate?accelerate?
F = FF = F ma = mama = ma (20kg)(.2m/s(20kg)(.2m/s22) = ) =
(.04kg)a(.04kg)a 4N = (.04kg)a4N = (.04kg)a 100m/s100m/s22 = a = a
Calculate the kick of a Calculate the kick of a 50kg cannon that 50kg cannon that shoots a 3kg shoots a 3kg cannonball out with cannonball out with an acceleration of an acceleration of 12m/s12m/s22..
Action - ReactionAction - Reaction
A car accelerates along a road. Strictly A car accelerates along a road. Strictly speaking, what is the force that moves the speaking, what is the force that moves the car?car?
We know that Earth pulls on the moon. We know that Earth pulls on the moon. Does the moon also pull on the Earth? If Does the moon also pull on the Earth? If so, which pull is stronger?so, which pull is stronger?
Can you identify the action and reaction Can you identify the action and reaction forces of an object falling in the vacuum of forces of an object falling in the vacuum of outer space?outer space?
CancellationCancellation
Do action/reaction forces cancel each Do action/reaction forces cancel each other out? other out?
Create a system to determine net force on Create a system to determine net force on an object.an object.
Ex: Boy on a skateboard, pulling against a Ex: Boy on a skateboard, pulling against a girl standing on the ground.girl standing on the ground.
Draw systems, identify action reaction Draw systems, identify action reaction forces.forces.
FrictionFriction Friction is a force that Friction is a force that
affects motion.affects motion. It acts when materials are It acts when materials are
in contact with one in contact with one another and always another and always opposes motion.opposes motion.
Friction is due to the Friction is due to the irregularities in the two irregularities in the two surfaces.surfaces.
Constant speed = no net Constant speed = no net force so friction = driving force so friction = driving forceforce
Different surfaces have Different surfaces have different amounts of different amounts of friction between them (friction between them (μμ))
Friction is not restricted to Friction is not restricted to solids , it also occurs in solids , it also occurs in fluidsfluids This can be felt even at This can be felt even at
slow speedsslow speeds Air resistance is friction Air resistance is friction
acting on something acting on something moving through air.moving through air.
FrictionFriction
Two types of FrictionTwo types of FrictionStatic friction (FStatic friction (Fss))
Restrictive force when object is not in motionRestrictive force when object is not in motionFFss = -F = -Faa
Kinetic friction (FKinetic friction (Fkk))Restrictive force when object is in motionRestrictive force when object is in motionNet external force acting on an object is equal to the Net external force acting on an object is equal to the
difference between applied force and force of kinetic difference between applied force and force of kinetic frictionfriction
F - FF - Fkk
Kinetic friction is less than static friction.Kinetic friction is less than static friction.
FrictionFriction
The force of friction is proportional to the The force of friction is proportional to the normal force.normal force.Easier to push a chair across the floor at a Easier to push a chair across the floor at a
constant speed than to push a heavy desk at constant speed than to push a heavy desk at the same speed.the same speed.
Coefficient of Friction is a Ratio Coefficient of Friction is a Ratio of Forcesof Forces
Defined as the ratio between the normal force Defined as the ratio between the normal force and the force of friction between two surfaces.and the force of friction between two surfaces.
Coefficient of Kinetic FrictionCoefficient of Kinetic Friction
μμkk = = FFkk
FFnn
Coefficient of Static FrictionCoefficient of Static Friction
μμss = = FFs,maxs,max
FFnn
If the value of If the value of μμ and the normal force on and the normal force on the object are known, then the magnitude the object are known, then the magnitude of the force of friction can be calculated of the force of friction can be calculated directly.directly.
FFff = = μμFFnn
Coefficients of Friction
Material μμss μμkk steel on steelsteel on steel 0.740.74 0.570.57Aluminum on steelAluminum on steel 0.610.61 0.470.47Rubber on dry concreteRubber on dry concrete 1.01.0 0.80.8Rubber on wet concreteRubber on wet concrete ---------- 0.50.5Wood on woodWood on wood 0.40.4 0.20.2Glass on glassGlass on glass 0.90.9 0.40.4Waxed wood on wet snowWaxed wood on wet snow 0.140.14 0.10.1Waxed wood on dry snowWaxed wood on dry snow ---------- 0.040.04Metal on metal (lubricated)Metal on metal (lubricated) 0.150.15 0.060.06Ice on iceIce on ice 0.10.1 0.030.03Teflon on teflonTeflon on teflon 0.040.04 0.040.04Synovial joints in humansSynovial joints in humans 0.010.01 0.0030.003
Air Resistance is a form on friction
Whenever an object moves through a fluid medium (air or water), that fluid provides a resistance to the object’s motion.Car at low speed = FR is roughly proportional
to the car’s speed in the opposite directionCar at higher speed = FR is roughly
proportional to the square of the car’s speed in the opposite direction
When the magnitude of FR equals the magnitude of the force moving the car forward, the net force is zero and the car moves at a constant speed.
ExamplesExamples A 24 kg crate initially at rest on a horizontal A 24 kg crate initially at rest on a horizontal
floor requires 75 N horizontal force to set it in floor requires 75 N horizontal force to set it in motion. Find the coefficient of static friction motion. Find the coefficient of static friction between the crate and the floor.between the crate and the floor.
A student moves a box of books by attaching A student moves a box of books by attaching a rope to the box and pulling with a force of a rope to the box and pulling with a force of 90.0 N at an angle of 30.090.0 N at an angle of 30.0oo. The box of . The box of books has a mass of 20.0 kg, and the books has a mass of 20.0 kg, and the coefficient of kinetic friction between the coefficient of kinetic friction between the bottom of the box and the sidewalk is 0.50. bottom of the box and the sidewalk is 0.50. Find the acceleration of Find the acceleration of the box.the box.
Ch. 4 Mixed Review AnswersCh. 4 Mixed Review Answers45. 55 N to the right45. 55 N to the right
47. 13 N down the incline47. 13 N down the incline
49. a. 2.00 N49. a. 2.00 N
b. 6.04 Nb. 6.04 N
51. a. 051. a. 0
b. 33.9 Nb. 33.9 N
53. 50. m 53. 50. m
55. a. 1.78 m/s55. a. 1.78 m/s22
b. 0.367b. 0.367
c. 9.36 Nc. 9.36 N
d. 2.67 m/sd. 2.67 m/s
57. 510 N57. 510 N
59. 32.2 N59. 32.2 N
61. 1.4161. 1.41oo
63. a. 20. m/s63. a. 20. m/s22 to the rt. to the rt.
b. Fb. F11 = 40. N = 40. N
FF22 = 60. N = 60. N
FF33 = 80. N = 80. N
c. mc. m22 by m by m11 = 140 N = 140 N
mm11 by m by m22 = -140 N = -140 N
mm33 by m by m22 = 80. N = 80. N
mm22 by m by m33 = -80. N = -80. N
65. a. between the bottom block 65. a. between the bottom block and the table.and the table.
b. 0.47b. 0.47
