forces chapter 6 pages: 116-147. force a force is a push or pull upon an object resulting from the...
TRANSCRIPT
ForcesForces
Chapter 6Chapter 6
Pages: 116-147Pages: 116-147
ForceForce
A force is a A force is a pushpush or or pullpull upon an object resulting upon an object resulting
from the object's interaction from the object's interaction with another object. with another object.
Contact ForcesContact ForcesLong-Range Forces Long-Range Forces
Contact ForcesContact ForcesContact forcesContact forces are types are types of forces in which the two of forces in which the two interacting objects are interacting objects are physically in contact with physically in contact with each other. each other.
FrictionFriction
TensionTension
Air
Air
Resi
stance
Resi
stance
Long-Range ForcesLong-Range ForcesLong-Range Forces are types of Long-Range Forces are types of
forces in which the two interacting forces in which the two interacting objects are not in physical contact objects are not in physical contact
with each other, but are able to with each other, but are able to exert a push or pull despite the exert a push or pull despite the
physical separation. physical separation.
Grav
ity
Magne
tism
Electri
cal
ForceForce
ForceForceFF for use in equations. for use in equations.
NewtonNewton is the unit for Force. is the unit for Force.
NN abbreviation for Newton. abbreviation for Newton.
Net ForceNet Force can can accelerateaccelerate..
N = kg m/sN = kg m/s22
ForceForce is a is a VectorVector QuantityQuantity
MagnitudeMagnitudeDirectionDirection
4000lb
Read Pages 118-119Read Pages 118-119Answer Question 1 in Answer Question 1 in
NotebookNotebook
HomeworkHomework
Page: 124Page: 124
Questions: 7-11Questions: 7-11
Free Body DiagramsFree Body Diagrams
The purpose of a free-body force diagram is to assist you in trying to determine the net force acting on a body.
Net ForceNet Force
The purpose of a free-body force diagram is to assist you in trying to determine the net force acting on a body.
Free Body DiagramsFree Body Diagrams
The net force is the vector sum of all the individual forces acting on a system.
Fnet = F1 ± F2 ± F3 ± F4 …
Constructing Constructing “free-body force diagram” “free-body force diagram”
1.1. Identify the object(s) you Identify the object(s) you will draw a diagram for. will draw a diagram for.
Constructing Constructing “free-body force diagram” “free-body force diagram”
2.2. Identify all the forces acting Identify all the forces acting directlydirectly on the object and on the object and the object exerting them.the object exerting them.
GravityGravityTable Table
Constructing Constructing “free-body force diagram” “free-body force diagram”
3.3.Draw a dot to represent Draw a dot to represent the object of interest. the object of interest.
Constructing Constructing “free-body force diagram” “free-body force diagram”
4. 4. Draw a vector to Draw a vector to represent each force.represent each force.
Gravity
Table
Constructing Constructing “free-body force diagram” “free-body force diagram”
5. 5. If the object is If the object is stationary or is moving stationary or is moving at a constant velocity, at a constant velocity, the vectors should the vectors should graphically add up to graphically add up to zero. zero.
Constructing Constructing “free-body force diagram” “free-body force diagram”
5. 5. If the object is If the object is accelerating, the sum of accelerating, the sum of the vectors should the vectors should produce a vector in the produce a vector in the same direction as the same direction as the acceleration. acceleration.
Constructing Constructing “free-body force diagram” “free-body force diagram”
Gravity
Floor
Standing on Standing on FloorFloor
FFfloorfloor=F=Fgravitygravity
Constructing Constructing “free-body force diagram” “free-body force diagram”
Gravity
Muscle
JumpingJumping FFmusclemuscle>F>Fgravitygravity
Constructing Constructing “free-body force diagram” “free-body force diagram”
Gravity
In the AirIn the Air FFgravitygravity
Types of MotionTypes of Motion
No MotionNo Motion
Gravity
RoadBrakesMotor
FFRoadRoad=F=FGravityGravity
FFMotorMotor=F=FBrakesBrakesNo Net No Net ForceForce
Types of MotionTypes of Motion
Constant VelocityConstant Velocity
Gravity
RoadFrictionMotor
FFRoadRoad=F=FGravityGravity
FFMotorMotor=F=FFrictionFrictionNo Net No Net ForceForce
Types of MotionTypes of Motion
Speeding UpSpeeding Up
Gravity
RoadFrictionMotor
FFRoadRoad=F=FGravityGravity
FFMotorMotor>F>FFrictionFriction Net ForceNet Force
Types of MotionTypes of Motion
Slowing DownSlowing Down
Gravity
RoadFrictionMotor
FFRoadRoad=F=FGravityGravity
FFMotorMotor<F<FFrictionFriction Net ForceNet Force
Finding Net ForceFinding Net Force
FG=4000N
FR=4000NFF=400NFM=400N
VerticalVerticalFFnetnet = F = FRR - F - FGG
FFnetnet=4000N–=4000N–4000N4000N
FFnetnet= 0N= 0N
HorizontalHorizontalFFnetnet = F = FFF - F - FMM
FFnetnet=400N–400N=400N–400N
FFnetnet= 0N= 0N
Finding Net ForceFinding Net Force
FG=4000N
FR=4000NFF=400NFM=400N
Not Moving Not Moving OrOr
Constant Velocity Constant Velocity
Finding Net ForceFinding Net Force
FG=4000N
FR=4000NFF=100NFM=400N
VerticalVerticalFFnetnet = F = FRR - F - FGG
FFnetnet=4000N–=4000N–4000N4000N
FFnetnet= 0N= 0N
HorizontalHorizontalFFnetnet = F = FFF - F - FMM
FFnetnet=100N–400N=100N–400N
FFnetnet= -300N= -300N
Finding Net ForceFinding Net Force
FG=4000N
FR=4000NFF=100NFM=400N
AcceleratingAcceleratingto the Left. to the Left.
Newton’s Second Law of Newton’s Second Law of MotionMotion
Law Law
ofof
AccelerationAcceleration
Newton’s Second Law of Newton’s Second Law of MotionMotion
The acceleration of an object as produced by a net force is directly proportional to the
magnitude of the net force, in the same direction as the net
force, and inversely proportional to the mass of the
object.
Newton’s Second Law of Newton’s Second Law of MotionMotion
FFff = = 40N40N
FFPP = = 41N41N
FFNetNet = = 1N1N
AccelerateAcceleratess
Newton’s Second Law of Newton’s Second Law of MotionMotion
FFNetNet = ma = maFFmm
aa
Newton’s First Law of MotionNewton’s First Law of Motion
Law Law
ofof
InertiaInertia
InertiaInertia
Inertia is the resistance an object has to a change in its
state of motion.
MassMass
Newton’s First Law of MotionNewton’s First Law of Motion
Seat
Seat
Belt!!!!
Belt!!!!
Newton’s First Law of Newton’s First Law of MotionMotion
An object at rest tends to An object at rest tends to stay at rest and an object in stay at rest and an object in motion tends to stay in motion tends to stay in motion with the same motion with the same speed and in the same speed and in the same direction direction unless acted upon unless acted upon by an unbalanced forceby an unbalanced force..
Mass vs. WeightMass vs. Weight
MassMass is the amount of stuff is the amount of stuff you are made up of. (kg or you are made up of. (kg or slugs) Does not change!!!!slugs) Does not change!!!!
Mass vs. WeightMass vs. WeightWeightWeight depends on how much depends on how much gravity is acting on you at the gravity is acting on you at the moment; you'd weigh less on moment; you'd weigh less on
the moon than on Earth. the moon than on Earth.
(newtons or pounds)(newtons or pounds)
Mass vs. WeightMass vs. Weight
WeightWeight
Mass vs. WeightMass vs. Weight
MasMasss
WeightWeight
To calculate weight use the To calculate weight use the acceleration due to gravity acceleration due to gravity (9.8m/s(9.8m/s22). This will be called g.). This will be called g.
F=ma Fg=mg
Weight ProblemsWeight Problems
Mr. Clune has a mass of Mr. Clune has a mass of 110kg. How much does he 110kg. How much does he weight?weight?Given: m=110kgGiven: m=110kg
g=9.8m/sg=9.8m/s22
Find: FFind: Fgg=?=?
Equation: FEquation: Fgg=mg=mg
=(110kg)(9.8m/s=(110kg)(9.8m/s22))
FFgg=1078N=1078N
F=ma ProblemsF=ma ProblemsA boy pulls a sled that has a mass of A boy pulls a sled that has a mass of
5kg across the snow. The sled 5kg across the snow. The sled accelerates at a rate of 0.5m/saccelerates at a rate of 0.5m/s22. What . What
is the net force of on the sled?is the net force of on the sled?
a=0.5m/sa=0.5m/s22
FFnetnet
Equation: FEquation: Fnetnet=mg=mg
=(5kg)(0.5m/s=(5kg)(0.5m/s22))
Given: m=5kgGiven: m=5kg
a=0.5m/a=0.5m/ss22
Find: FFind: Fnetnet=?=?
FFnetnet=2.5N=2.5N
F=ma ProblemsF=ma Problems
A rock with a mass of 10kg fell A rock with a mass of 10kg fell off a cliff. At a specific time off a cliff. At a specific time
during its’ fall it had an during its’ fall it had an acceleration of 3m/sacceleration of 3m/s22, due to , due to
air resistance. What is the force air resistance. What is the force of air on this rock at this time?of air on this rock at this time?
++
FF
gg
FFairair
aaFFnetnet
FFnetnet== FFair air + + FFg g FFnetnet= =
ma ma FFgg= = mg mg
FFairair== FFnet net - F- Fg g
FFairair== mama - - mgmgFFairair== m(am(a – – g)g) FFairair==
10kg{(-3m/s10kg{(-3m/s22)–(-9.8m/s)–(-9.8m/s22)})}FFairair==
10kg{(-3m/s10kg{(-3m/s22)+(9.8m/s)+(9.8m/s22)})}FFairair== 10kg(6.8m/s10kg(6.8m/s22))
FFairair== 68N68N
HomeworkHomework
Page: 147Questions: 22, 27,29
Due: 10/25/06
FactorsFactors that determine that determine Friction Friction
Wei
ght
Wei
ght Moving
MovingSta
tionary
Sta
tionarySurface
Surface
Friction ForcesFriction Forces
Fg
FT
FPFf
FN
FFNN – Normal Force: This force – Normal Force: This force which will affect frictional which will affect frictional resistance is the component of resistance is the component of applied force which acts applied force which acts perpendicular or "normal" to perpendicular or "normal" to the surfaces which are in the surfaces which are in contact and is typically contact and is typically referred to as the normal force.referred to as the normal force.
Friction ForcesFriction Forces
FFTT – Surface Force: This – Surface Force: This force opposite the normal force opposite the normal force which is equal to this force which is equal to this force. force.
Friction ForcesFriction Forces
FFPP – Push or Pull Force: This – Push or Pull Force: This force is pushing or pulling force is pushing or pulling the object. the object.
Friction ForcesFriction Forces
FFff – Friction Force: – Friction Force: Frictional Frictional resistance to the relative resistance to the relative motion of two solid objects. motion of two solid objects.
Friction ForcesFriction Forces
FFfsfs – – StaticStatic Friction Force: Friction Force: Static frictional forces are Static frictional forces are non-moving forcesnon-moving forces between between two surfaces. It will increase two surfaces. It will increase to prevent any relative motion to prevent any relative motion up until some limit where up until some limit where motion occurs. motion occurs.
Friction ForcesFriction Forces
FFfkfk – – KineticKinetic Friction Force: Friction Force: The force between two The force between two surfaces that are surfaces that are movingmoving with respect to one another, with respect to one another, the frictional resistance is the frictional resistance is almost constant over a wide almost constant over a wide range of low speeds. range of low speeds.
Friction ForcesFriction Forces
μμ – – Coefficient of Coefficient of Friction: Friction: The The ratioratio of the of the force of friction (Fforce of friction (Fff)) between two bodies and between two bodies and the the force pressing them force pressing them together (Ftogether (FNN))..
Friction ForcesFriction Forces
Coefficient Coefficient of Friction of Friction
Ff
FN
μs= Ffs
FN
Coefficient Coefficient of Friction of Friction
Ff
FN
μk= Ffk
FN
Friction ProblemFriction Problem
A refrigerator of total weight 400N A refrigerator of total weight 400N is pushed at a constant speed is pushed at a constant speed across a room by pushing across a room by pushing horizontally on one side with a horizontally on one side with a force of 160N. What is the force of 160N. What is the coefficient of kinetic friction?coefficient of kinetic friction?
FfkFN
Ffk = 160NFN = 400Nμk = ?
μk=
Ffk
FN
μk=
140N400N
μk= 0.35
If the coefficient of static friction between If the coefficient of static friction between the floor and the refrigerator was 0.6, the floor and the refrigerator was 0.6, how much force would be needed to how much force would be needed to start the refrigerator moving?start the refrigerator moving?
Ffs = ?
FN = 400Nμs = 0.6 μs=
Ffs
FN
= μk Ffs FN
= (0.6) Ffs(400N)
= μk Ffs FN
= 240N Ffs
HomeworkHomeworkPage: 133
Questions: 14,15 Page: 145
Questions: 33-35Due: 11/2/06
Newton’s Third LawNewton’s Third Law
"For every action, "For every action, there is an equal there is an equal
and opposite and opposite reaction." reaction."
While driving, Anna Litical While driving, Anna Litical observed a bug striking the observed a bug striking the
windshield of her car. Obviously, windshield of her car. Obviously, a case of Newton's third law of a case of Newton's third law of
motion. The bug hit the motion. The bug hit the windshield and the windshield hit windshield and the windshield hit the bug. Which of the two forces the bug. Which of the two forces is greater: the force on the bug is greater: the force on the bug or the force on the windshield? or the force on the windshield?
Rockets are unable to accelerate in Rockets are unable to accelerate in space because ...space because ... There is no air in space for the There is no air in space for the rockets to push off of. rockets to push off of.
There is no gravity is in space. There is no gravity is in space.
There is no air resistance in There is no air resistance in space. space.
... nonsense! Rockets do ... nonsense! Rockets do accelerate in space. accelerate in space.
A gun recoils when it is fired. The recoil is the result of action-reaction force pairs. As the gases from the gunpowder explosion expand, the gun pushes the bullet forwards and the bullet pushes the gun backwards. The acceleration of the recoiling gun is ...
a.greater than the acceleration of the bullet. b.smaller than the acceleration of the bullet. c.the same size as the acceleration of the bullet.
In the top picture, a physics student is pulling upon a rope
which is attached to a wall. In the bottom picture, the physics
student is pulling upon a rope which is held by the Strongman.
In each case, the force scale reads 500 Newtons. The physics
student is pulling…
with more force when the rope is attached to the wall.
with more force when the rope is attached to the Strongman. the same force in each case.