Post on 12-Jan-2016
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WESBURY COLLEGE OF SCIENCE
2013• GR 12 PHYSICAL SCIENCES
INTERVENTION FOR SCIENCE LEARNERS
• SCIENCE DEPARTMENT PROJECTFOUNDATIONS OF LEARNING
2013-09-01
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KNOWLEDGE AREASKNOWLEDGE AREAS
•MECHANICS
•CHEMICAL CHANGE
•WAVES, LIGHT, SOUND
•MATTER AND MATERIALS
•ELECTRICITY AND MAGNETISM
•CHEMICAL SYSTEMS
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KNOWLEDGE AREAMECHANICS
THEMES• FORCE, MOMENTUM AND IMPULS
(GR 11 MECHANICS)
• MOMENTUM (GR 12 MECHANICS)
• VERTICAL PROJECTILE MOTION (GR 12 MECHANICS)
• FRAMES OF REFERENCE (GR 12 MECHANICS)
• WORK, POWER AND ENERGY (GR 12 MECHANICS)
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KNOWLEDLEDGE AREAMECHANICS
THEME
• FORCE,
• MOMENTUM AND
• IMPULSE
GR 11 MECHANICS
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FORCE, MOMENTUM AND IMPULSEFORCE, MOMENTUM AND IMPULSEFORCE
TWO TYPES OF FORCES-PUSHING AND PULLING FORCE
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FORCE, MOMENTUM AND IMPULSEFORCE, MOMENTUM AND IMPULSEFORCE
CONTACT AND NON-CONTACT FORCES
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FORCE, MOMENTUM AND IMPULSEFORCE, MOMENTUM AND IMPULSEINTERACTION BETWEEN TWO BODIES
TYPES OF FORCES
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FORCE, MOMENTUM AND IMPULSEFORCE, MOMENTUM AND IMPULSEFREE BODY DIAGRAM
FREE BODY DIAGRAMS
•OBJECT REPRESENT A DOT
•FORCES ARE DRAWN AS ARROWS POINTING AWAY
FROM THE
DOT
• LENGTH OF ARROW REPRESENTS SIZE OF FORCE
• POINT OF ARROW INDICATES THE DIRECTION OF THE
FORCE
FORCE DIAGRAM
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FORCE, MOMENTUM AND IMPULSEFORCE, MOMENTUM AND IMPULSEFORCES WORK IN PAIRS –
NEWTON’S THIRD LAW OF MOTION
NEWTONS THIRD LAW OF MOTION
WHEN A BODY (A) EXERTS A FORCE ON A SECOND
BODY (B),
THE SECOND BODY (B) EXERTS A FORCE EQUAL
IN MAGNITUDE,
BUT OPPOSITE IN DIRECTION ON THE FIRST BODY
(A)
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FORCE, MOMENTUM AND IMPULSEFORCE, MOMENTUM AND IMPULSEFORCES WORK IN PAIRS
THE FORCE OF THE GROUND ON YOUR FOOT PUSHES YOU FORWARD
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FORCE, MOMENTUM AND IMPULSFORCE, MOMENTUM AND IMPULSFORCES WORK IN PAIRS
NEWTON’S THIRD LAW OF MOTION EXPLAINS THE MOVEMENT OF THE BALLOON ROCKET
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FORCE, MOMENTUM AND IMPULSEFORCE, MOMENTUM AND IMPULSEFORCES WORK IN PAIRS
NEWTON’S THIRD LAW OF MOTION EXPLAINS THE MOVEMENT OF THE BALLOON ROCKET
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FORCE, MOMENTUM AND IMPULSEFORCE, MOMENTUM AND IMPULSEFORCES WORK IN PAIRS
NEWTON’S THIRD LAW BOOK ON TABLE
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FORCE, MOMENTUM AND IMPULSEFORCE, MOMENTUM AND IMPULSEFORCES WORK IN PAIRS
ANALYSE THE SCIENTIFIC CORRECTNESS OF THE FOLLOWING STATEMENT ABOUT
A HORSE PULLING A CART:
“WHEN A HORSE PULLS A CART, THE CART PULLS THE
HORSE WITH AN EQUAL BUT OPPOSITE FORCE, ……..
CONSEQUENTLY THE FORCES CANCEL EACH OTHER OUT
AND THE CART IS UNABLE TO MOVE”
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FORCE, MOMENTUM AND IMPULSEFORCE, MOMENTUM AND IMPULSE
“… WHEN A HORSE PULLS A CART, THE CART PULLS THE HORSE WITH AN EQUAL BUT OPPOSITE FORCE, …”
ACCORDING TO NEWTON’S THIRD LAW THIS PART OF THE STATEMENT TRUE!!!!!!!!THE CART PULLS THE HORSE WITH AN EQUAL BUT OPPOSITE FORCE THAN WHAT THE HORSE IS PULLING THE CART.
CONSEQUENTLY THE FORCES CANCEL EACH OTHER OUT AND THE CART IS UNABLE TO MOVE”
THIS PART OF THE STATEMENT IS NOT TRUE!!!!!!!!!!!!THE TWO FORCES ACT ON DIFFERENT OBJECTS AND CAN THEREFOR NOT CANCEL EACH OTHER OUT.
ONLY THE FORCES THAT ACT IN ON THE CART – 1 APPLIED FORCE OF THE HORSE 2 FRICTION OF CARTWILL DETEMINE IF THE CART WILL MOVE.
FORCES WORK IN PAIRS
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FORCE, MOMENTUM AND IMPULSEFORCE, MOMENTUM AND IMPULSENEWTONS LAW OF MOTION (ESA)
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FORCE, MOMENTUM AND IMPULSEFORCE, MOMENTUM AND IMPULSEUNDERSTANDING OF NEWTON’S
THIRD LAW OF MOTION
• THE TWO FORCES WORK SIMULTANEOUSLY AND
HAVE THE SAME MAGNITUDE
• THE TWO FORCES HAVE OPPOSITE DIRECTIONS
• THE TWO FORCES ARE THE SAME - BOTH FRICTIONAL OR NORMAL FORCES
• IF TWO FORCES ACT ON DIFFERENT OBJECTS AND CAN THEREFORE NOT CANCEL EACH OTHER OUT
• ONLY FORCES ACTING ON THE SAME OBJECT CAN CANCEL EACH OTHER OUT
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FORCE, MOMENTUM AND IMPULSEFORCE, MOMENTUM AND IMPULSEMOMENTUM – AMOUNT OF MOTION
ANY MOVING OBJECT HAS MOMENTUM
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FORCE, MOMENTUM AND IMPULSEFORCE, MOMENTUM AND IMPULSEWHAT IS LINEAR MOMENTUM?
LINEAR MOMENTUM (MOMENTUM IN A STRAIGHT LINE) CAN BE DEFINED AS THE PRODUCT OF MASS
AND VELOCITY
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FORCE, MOMENTUM AND IMPULSEFORCE, MOMENTUM AND IMPULSECHANGE IN MOMENTUM
A NET FORCE ON AN OBJECT CAUSES A CHANGE IN MOMENTUM
- A TACKLE IN RUGBY CHANGES THE MOMENTUM OF THE OPPONENT
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FORCE, MOMENTUM AND IMPULSEFORCE, MOMENTUM AND IMPULSE
NEWTONS SECOND LAW OF MOTION IN TERMS OF MOMENTUM
THE NET (OR RESULTANT) FORCE EXERTED ON AN OBJECT IS EQUAL TO THE RATE OF CHANGE OF
MOMENTUM
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FORCE, MOMENTUM AND IMPULSEFORCE, MOMENTUM AND IMPULSECHANGE IN MOMENTUM
THROWING AN EGG
TO STOP THE EGG, THE MOMENTUM OF THE EGG MUST BE CHANGED TO ZERO
THE CONTACT TIME IS THE ONLY SOLUTION TO ENSURE THAT THE EGG EXPERIENCE AS SMALL A
FORCE AS POSSIBLE
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FORCE, MOMENTUM AND IMPULSEFORCE, MOMENTUM AND IMPULSECHANGE IN MOMENTUM
CATCH A WATER BALLOON
TO STOP THE WATER BALLOON, THE MOMENTUM MUST BE CHANGED TO ZERO
THE CONTACT TIME IS THE ONLY SOLUTION TO ENSURE THAT BALLOON EXPERIENCE AS SMALL A
FORCE A POSSIBLE
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FORCE, MOMENTUM AND IMPULSEFORCE, MOMENTUM AND IMPULSECHANGE IN MOMENTUM
CRICKET PLAYER CATCHING A BALL
THE CONTACT TIME IS THE ONLY SOLUTION TO ENSURE THAT CRICKET PLAYER EXPERIENCE A
SMALL FORCE
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FORCE, MOMENTUM AND IMPULSEFORCE, MOMENTUM AND IMPULSECHANGE IN MOMENTUM
A BATSMAN HITTING A CRICKET BALL
THE MAGNITUDE OF THE NET FORCE, AS WELL AS THE CONTACT TIME ,
WILL THE DETERMINE THE SUCCESS OF THE SHOT
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FORCE, MOMENTUM AND IMPULSEFORCE, MOMENTUM AND IMPULSECHANGE IN MOMENTUM
SUMMARY
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FORCE, MOMENTUM AND IMPULSEFORCE, MOMENTUM AND IMPULSEIMPULSE
THE PRODUCT OF THE NET FORCE AND THE CONTACT TIME IS CALLED THE IMPULSE (N.s) OF
THE FORCE
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FORCE, MOMENTUM AND IMPULSEFORCE, MOMENTUM AND IMPULSETHE CONCEPT OF IMPULSE
AND SAFETY CONSIDERATIONS IN EVERYDAY LIFEAIRBAGS
AIRBAGS INCREASES THE CONTACT TIME AND THE PASSENGER EXPERIENCE A SMALLER FORCE
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FORCE, MOMENTUM AND IMPULSEFORCE, MOMENTUM AND IMPULSETHE CONCEPT OF IMPULSE
AND SAFETY CONSIDERATIONS IN EVERYDAY LIFE
AIRBAGS
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FORCE, MOMENTUM AND IMPULSEFORCE, MOMENTUM AND IMPULSETHE CONCEPT OF IMPULSE
AND SAFETY CONSIDERATIONS IN EVERYDAY LIFECRUMPLE ZONES
CRUMPLE ZONES INCREASES THE CONTACT TIME AND THE PASSENGER EXPERIENCE A SMALLER
FORCE
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FORCE, MOMENTUM AND IMPULSEFORCE, MOMENTUM AND IMPULSETHE CONCEPT OF IMPULSE
AND SAFETY CONSIDERATIONS IN EVERYDAY LIFEARRESTOR BEDS
ARRESTER BEDS INCREASES THE CONTACT TIME FOR A RUNAWAY TRUCK TO BE STOPPED
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FORCE, MOMENTUM AND IMPULSEFORCE, MOMENTUM AND IMPULSE
WESBURY COLLEGE OF SCIENCE
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KNOWLEDLEDGE AREAMECHANICS
THEME
MOMENTUM
GR 12 MECHANICS
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CONSERVATION OF MOMENTUMCONSERVATION OF MOMENTUM
WHEN DOES MOMENTUM CHANGE?
MOMENTUM CHANGES WHEN A NET FORCE ACTS ON AN OBJECT!
WHEN IS MOMENTUM CONSERVED?
WHEN THE NET FORCE THAT ACTS ON AN OBJECT IS ZERO, THE OBJECT DOES NOT EXPERIENCE AN
ACCELERATION THEREFOR NO CHANGE IN VELOCITY.
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DURING A COLLISION TWO VEHICLES EXPERIENCE EQUAL BUT OPPOSITE FORCES
CONSERVATION OF MOMENTUMCONSERVATION OF MOMENTUM
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CONSERVATION OF MOMENTUMCONSERVATION OF MOMENTUM
THE CONTACT TIME DURING WHICH THE FORCES
ACT ON THE TWO VEHICLES IS/ARE THE SAME
THE VEHICLES EXPERIENCE THE SAME IMPULSE
BUT IN OPPOSITE DIRECTIONS
FA = -FB and tA= tB
FAtA = -FBtB
FAtA + FBtB = 0
CONSERVATION OF MOMENTUM
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THE TOTAL LINEAR MOMENTUM IN A CLOSED SYSTEM IS CONSERVED IN MAGNITUDE AND
DIRECTION
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CONSERVATION OF MOMENTUMCONSERVATION OF MOMENTUM
THE TOTAL LINEAR MOMENTUM IN A CLOSED SYSTEM IS CONSERVED IN MAGNITUDE AND
DIRECTION
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CONSERVATION OF MOMENTUMCONSERVATION OF MOMENTUM
COLLISIONS AND EXPLOSIONS
MOMENTUM STAY CONSERVED IN A CLOSED SYSTEM
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ELASTIC AND INELASTIC COLLISIONS
COLLISIONS ARE OFTEN CLASSIFIED ACCORDING TO THE CHANGE IN TOTAL KINETIC ENERGY
ELASTIC COLLISIONS
TOTAL KINETIC ENERGY OF THE SYSTEM BEFORE THE COLLISION IS EQUAL TO THE TOTAL KINETIC ENERGY AFTER THE COLLISION
INELASTIC COLLISIONS
TOTAL KINETIC ENERGY OF THE SYSTEM IS NOT THE SAME BEFORE AND AFTER THE COLLISION
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ELASTIC AND INELASTIC COLLISIONSELASTIC COLLISIONS
TOTAL KINETIC ENERGY BEFORE A COLLISION =
TOTAL KINETIC ENERGY AFTER A COLLISION
Ek BEFORE COLLISION = Ek AFTER COLLISION
½ mv2 = ½ mv2
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ELASTIC AND INELASTIC COLLISIONSELASTIC COLLISIONSNEWTON’S CRADLE
TOTAL KINETIC ENERGY BEFORE A COLLISION =
TOTAL KINETIC ENERGY AFTER A COLLISION
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ELASTIC AND INELASTIC COLLISIONSELASTIC COLLISIONSNEWTON’S CRADLE
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ELASTIC AND INELASTIC COLLISIONSELASTIC COLLISIONS
GIANT NEWTON’S CRADLE
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MECHANICSMECHANICSNNEWTONS CRADLE – PENDULUM WAVES
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ELASTIC AND INELASTIC COLLISIONSSUMMARY
MOMENTUM WILL ALWAYS BE CONSERVED
DURING COLLISIONS
KINETIC ENERGY WILL ONLY BE
CONSERVED DURING ELASTIC COLLISIONS
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MECHANICSMECHANICSMOMENTUM VIDEO
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MECHANICSMECHANICS
WESBURY COLLEGE OF SCIENCE LEARNERSMODULE 1
p45-46 p47
p48 AKT 6. VRAE 1-4
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KNOWLEDLEDGE AREAMECHANICS
• NEWTON’S SECOND LAW OF MOTION
• NEWTON’S FIRST LAW OF MOTION
GR 11 MECHANICS
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NEWTON’S SECOND LAW OF NEWTON’S SECOND LAW OF MOTIONMOTION
MATHEMATICAL EXPRESSION OF
NEWTON’S SECOND LAW OF MOTION
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NEWTON’S SECOND LAW OF NEWTON’S SECOND LAW OF MOTIONMOTION
WHEN A RESULTANT FORCE ACTS ON A BODY, THE BODY ACCELARATES
THE ACCELARATION IS DIRECTLY PROPORTIONAL TO THE NET FORCE AND INVERSELY PROPORTIONAL
TO THE MASS OF THE BODY
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NEWTON’S FIRST LAW OF NEWTON’S FIRST LAW OF MOTIONMOTION
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NEWTON’S FIRST LAW OF NEWTON’S FIRST LAW OF MOTIONMOTION
APPLICATIONS OF NEWTON’S FIRST LAW
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KNOWLEDLEDGE AREAMECHANICS
THEME
VERTICAL PROJECTILE MOTION
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PROJECTILE MOTIONPROJECTILE MOTION
DIFFERENT TYPES OF PROJECTILE MOTION
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PROJECTILE MOTIONPROJECTILE MOTIONFREE-BODY DIAGRAM FOR A PROJECTILE MOTION
A PROJECTILE HAS ONLY ONE FORCE ACTING UPON IT - THE FORCE OF GRAVITY
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PROJECTILE MOTIONPROJECTILE MOTIONFREE FALL FROM REST
AN OBJECT THAT FALLS FREE FROM REST IS THE SIMPLEST FORM OF PROJECTILE
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PROJECTILE MOTIONPROJECTILE MOTIONFREE FALL FROM REST
THE MOTION EQUATIONS CAN BE ADAPTED FOR FREE FALL AS FOLLOWS
vf = vi + g t
y = vi t + ½ g t2
vf2 = vi
2 + 2 g y
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PROJECTILE MOTIONPROJECTILE MOTIONFREE FALL FROM REST
USEFUL TIPS FOR FREE FALL MOTION EQUATIONS
• THE INITIAL VELOCITY OF A FALLING BODY IS
ZERO
• ALWAYS WRITE THE COMPLETE EQUATION
FIRST
AND SHOW ALL SUBSTITUTIONS, EVEN ZERO
VALUES
• PLACE A UNIT AFTER EVERY FINAL ANSWER
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PROJECTILE MOTIONPROJECTILE MOTIONFREE FALL FROM REST
(DISPLACEMENT) POSITION-TIME GRAPH FOR A FREE FALLING OBJECT
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PROJECTILE MOTIONPROJECTILE MOTIONFREE FALL FROM REST
VELOCITY-TIME GRAPH FOR A FREE FALLING OBJECT
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PROJECTILE MOTIONPROJECTILE MOTIONFREE FALL FROM REST
ACCELERATION-TIME GRAPH FOR A FREE FALLING OBJECT
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PROJECTILE MOTIONPROJECTILE MOTIONVERTICAL PROJECTILE MOTION
ANY OBJECT THAT IS THROWN, KICKED OR SHOT PERPENDICULARLY INTO THE AIR, IS A VERTICAL
PROJECTILE
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PROJECTILE MOTIONPROJECTILE MOTIONVERTICAL PROJECTILE MOTION
USEFUL TIPS
• PROJECTILES FALL FREE AT 9,8 m.s-2
• PROJECTILES EXPERIENCE A CONSTANT DOWNWARD ACCELERATION (9,8 m.s-2) REGARDLESS WHETHER THEY MOVE UPWARDS OR DOWNWARDS• THE VELOCITY OF A PROJECTILE AT ITS FULCRUM IS ZERO
• THE TIME FOR THE UPWARD MOTION OF A PROJECTILE FROM THE STARTING POINT, IS THE SAME AS THE TIME OF THE DOWNWARD MOTION TO THE SAME POINT
• Vi UPWARD MOTION = Vf DOWNWARD MOTION
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PROJECTILE MOTIONPROJECTILE MOTIONVERTICAL PROJECTILE MOTION
GRAPHIC REPRESENTATION
(DISPLACEMENT) POSITION-TIME GRAPH FOR VERTICAL PROJECTILE MOTION
TIME(s)
POSITION (m)
0 0
1 15
2 20
3 15
4 0
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PROJECTILE MOTIONPROJECTILE MOTIONVERTICAL PROJECTILE MOTION
GRAPHIC REPRESENTATION
VELOCITY-TIME GRAPH FOR VERTICAL PROJECTILE MOTION
TIME (s)
VELOCITY(m∙s–1)
0 + 20
1 + 10
2 0
3 - 10
4 - 20
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PROJECTILE MOTIONPROJECTILE MOTIONVERTICAL PROJECTILE MOTION
GRAPHIC REPRESENTATION
ACCELERATION-TIME GRAPH FOR VERTICAL PROJECTILE MOTION
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PROJECTILE MOTIONPROJECTILE MOTIONVERTICAL PROJECTILE MOTION
SKETCH GRAPHS
UPWARD THEN DOWNWARD MOTION
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PROJECTILE MOTIONPROJECTILE MOTIONVERTICAL PROJECTILE MOTION
SKETCH GRAPHS
DOWNWARD THEN UPWARD MOTION
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PROJECTILE MOTIONPROJECTILE MOTIONVERTICAL PROJECTILE MOTION-SKETCH GRAPHS
BOUNCING BALL
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KNOWLEDLEDGE AREAMECHANICS
THEME
FRAMES OF REFERENCE
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WHAT IS A FRAMES OF REFERENCE?WHAT IS A FRAMES OF REFERENCE?
AS THE TREE DOES NOT MOVE, THE CAR MUST HAVE MOVED FROM ONE PLACE TO ANOTHER.
THEREFORE, HERE THE TREE IS CONSIDERED AS THE FRAME OF REFERENCE
IN THIS PICTURE THE CAR IS TO THE RIGHT OF
THE TREE.
AFTER 2 SECONDS, THE CAR IS TO THE LEFT OF
THE TREE.
7474
IN FIG.1, IS DIE KAR AAN DIE REGTERKANT VAN DIE BOOM!
IN FIG.2, NA 2 SEKONDES, IS DIE KAR AAN DIE LINKERKANT VAN DIE BOOM!
DIE BOOM BEWEEG NIE, DIE KAR MOES VAN EEN PLEK NA ‘N ANDER PLEK BEWEEG HET!
DUS KAN DIE BOOM AS DIE VERWYSINGSRAAMWERK GENEEM WORD.
WHAT IS A FRAME OF REFERENCE?WHAT IS A FRAME OF REFERENCE?
7575
MAN STAAN STIL IN BUS
DIE BUS BEWEEG 120 km.h-1 NOORD.DIE MAN IN DIE BUS STAAN STIL, MAAR BEWEEG OOK TEEN 120 km.h-1 NOORDVIR DIE KIND WAT SIT, STAAN DIE MAN STILVIR DIE VROU OP DIE SYPAADJIE BEWEEG DIE MAN TEEN ‘N 120 km.h-1 NOORD
FRAMES OF REFERENCEFRAMES OF REFERENCE
7676
FRAMES OF REFERENCEFRAMES OF REFERENCE
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FRAMES OF REFERENCEFRAMES OF REFERENCE
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FRAMES OF REFERENCEFRAMES OF REFERENCE
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KNOWLEDGE AREAMECHANICS
THEMES• FORCE, MOMENTUM AND IMPULS
(GR 11 MECHANICS)
• MOMENTUM (GR 12 MECHANICS)
• VERTICAL PROJECTILE MOTION (GR 12 MECHANICS)
• FRAMES OF REFERENCE (GR 12 MECHANICS)
• WORK, POWER AND ENERGY (GR 12 MECHANICS)
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KNOWLEDLEDGE AREAMECHANICS
THEME
WORK, POWER AND ENERGY
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WORKWORKTHE CONCEPT “WORK” IN EVERY DAY LIFE
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WORKWORKTHE CONCEPT “WORK” IN PHYSICS
IN PHYSICS THE CONCEPT WORK RELATES TO MOTION
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WORKWORKTHE CONCEPT “WORK” IN PHYSICS
WORK (W) IS DONE WHEN A FORCE (F) CAUSES AN OBJECT TO UNDERGO DISPLACEMENT (x)
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WORKWORKWHEN IS WORK DONE?
ONLY THE HORIZONTAL COMPONENT OF THE FORCE DOES WORK
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WORKWORKWHEN IS WORK DONE?
A FORCE THAT IS PERPENDICULAR TO THE
DISPLACEMENT DOES NO WORK
THE FORCE DOES NOT HAVE A COMPONENT IN THE DIRECTION OF THE DISPLACEMENT
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WORKWORKMATHEMATICAL EXPRESSION OF WORK
Wnet = Fnet x Cosθ
Wnet = ΣW (of each individual force that is exerted on the system )
Fnet = the size of the net force
Δx = size of the displacement
θ = angle between the force Fnet and the displacement Δx
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WORKWORKMATHEMATICAL EXPRESSION OF WORK
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WORKWORKEXAMPLES OF WORK
THE WORK DONE BY THE FORCE “F” ON THE LAWNMOWER IS F x Cos
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WORKWORKEXAMPLES OF WORK
A PERSON HOLDING A SUITCASE IS DOING NO WORK ON THE SUITCASE BECAUSE THERE IS NO
MOTION
x = 0 W = 0
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WORKWORKEXAMPLES OF WORK
WHEN F IS EXERTED PERPENDICULAR TO Δx, THEN Cos Θ = Cos 90º = 0,
AND THEN THE FORCE IS NOT DOING WORK ON THE SUITCASE
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WORKWORKEXAMPLES OF WORK
WORK WILL BE DONE IF A PERSON CARRY A SUITCASE UP A STAIRCASE BECAUSE AND CosΘ WILL BE BETWEEN 0 AND 1
W = FΔx CosΘ
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WORKWORKEXAMPLES OF WORK
WORK WILL BE DONE BY “f” ON THE SUITCASE THAT IS DISPLACED OVER A FLOOR WITH Δx. BUT “f” IS PARALLEL AND IN THE OPPOSITE
DIRECTION AS Δx, SO: CosΘ = Cos 180º = -1
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WORKWORKEXAMPLES OF WORK
FORCE F THAT THE ELECTRIC MOTOR EXERTS ON THE SUITCASE IS DOING WORK.
BUT F IS PARALLEL AND IN THE OPPOSITE DIRECTION TO Δy,
SO: CosΘ = Cos 180º = -1
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ENERGYENERGY
THIS FORCE MUST HAVE SOME FORM OF
ENERGY
ENERGY IS REQUIRED TO DO WORK!!!!
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ENERGYENERGY
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ENERGYENERGY
WHEN WORK IS DONE, OBJECTS EXCHANGE ENERGY
THE OBJECT ON WHICH WORK IS DONE GAINS ENERGY, WHILE THE OBJECT THAT DOES WORK
LOSES ENERGY
ENERGY IS REQUIRED TO DO WORK
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ENERGYENERGY
ENERGY CANNOT BE DESTROYED OR CREATED BUT CAN ONLY BE TRANSFERRED FROM ONE TO THE
OTHER
LAW OF CONSERVATION OF ENERGY
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ENERGYENERGYWHAT IS POTENTIAL ENERGY?
POTENTIAL ENERGY IS THE ENERGY THAT AN OBJECT POSSESSES AS RESULT OF ITS POSITION
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ENERGYENERGYWHAT IS GRAVITATIONAL POTENTIAL ENERGY?
IS THE ENERGY THAT AN OBJECT WITH A MASS (m) POSESSES AS RESULT OF ITS POSITION (h) RELATIVE TO THE SURFACE OF THE EARTH
U = Ep = mgh
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ENERGYENERGYWHAT IS KINETIC ENERGY?
THE ENERGY RESULTING FROM MOTION
K = Ek = ½ mv2
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ENERGYENERGYWHAT IS MECHANICAL ENERGY?
THE ENERGY A OBJECT RECEIVES WHEN WORK IS DONE ON IT IS CALLED MECHANICAL ENERGY,
AND CONSISTS OF…
POTENTIAL ENERGY AND KINETIC ENERGY
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ENERGYENERGYMECHANICAL ENERGY
IN TERMS OF POTENTIAL AND KINETIC ENERGY
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ENERGYENERGYMECHANICAL ENERGY
IN TERMS OF POTENTIAL AND KINETIC ENERGY
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ENERGYENERGYLAW OF THE CONSERVATION OF MECHANICAL
ENERGY
THE TOTAL MECHANICAL ENERGY OF A MOVING OBJECT IN A CLOSED SYSTEM STAYS CONSTANT
IF NO WORK IS DONE BY EXTERNAL FORCES
MECHANICAL ENERGY (i) = MECHANICAL ENERGY (f)
(Ep + Ek)i = (Ep + Ek)f
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ENERGYENERGYLAW OF THE CONSERVATION OF MECHANICAL
ENERGY
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ENERGYENERGYLAW OF CONSERVATION OF MECHANICAL ENERGY
WHAT IS A CLOSED SYSTEM?WHAT IS A CLOSED SYSTEM?
NO EXTERNAL FORCES (LIKE FRICTION) HAS AN EFFECT ON THE SYSTEM.
WHAT IS AN EXTERNAL FORCE?WHAT IS AN EXTERNAL FORCE?
- NET APPLIED FORCE - FRICTIONAL FORCE - ATMOSPHERIC RESISTANCE - NORMAL FORCE
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ENERGYENERGY
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ENERGYENERGYWHAT IS THE WORK-KINETIC ENERGY THEOREM?
THE NETTO WORK DONE ON AN OBJECT IS EQUAL TO THE CHANGE OF THE KINETIC ENERGY OF THE OBJECTS.
OR
THE WORK DONE ON AN OBJECT BY A NET FORCE IS EQUAL TO THE CHANGE IN THE KINETIC ENERGY OF THE OBJECT.
OR
WHEN AN EXTERNAL NET FORCE DOES WORK ON AN OBJECT, THE KINETIC ENERGY OF THE OBJECT CHANGES FROM AN
INITIAL AN VALUE EKI, TO A FINAL VALUE, EKF. THE DIFFERENCE BETWEEN THESE VALUES IS EQUAL TO THE
WORK DONE.
Wnet = Δ K = ΔEk = Ekf – Eki
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ENERGYENERGYWHAT IS THE WORK-KINETIC ENERGY THEOREM?
Wnet = Δ K = ΔEk = Ekf – Eki
but
Wnet = Fnet Δx Cosθ
therefore
FnetΔx Cosθ = ΔK
= Ekf – Eki
Fnet Δx Cosθ= ½mvf2 - ½mvi
2
REMEMBER Wnet = ΣW
(OF EACH INDIVIDUAL
FORCE EXERTED ON THE
SYSTEM)
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MECHANICSMECHANICSDIFFERENT FORCES ACTING ON A BODY MOVING UP
A SLOPE
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MECHANICSMECHANICSWORK DONE ON A OBJECT MOVING DOWN A
FRICTIONLESS SURFACE
Wnet = WFg// + WFN + Ww┴
= Fg//xCos+ FNxCos + Fg┴ xCos
= mgSin30°xCos0° + 0 + 0
Wnet = mgSin30°xCos0°
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MECHANICSMECHANICSWORK DONE ON A OBJECT MOVING UP A
FRICTIONLESS SURFACE
Wnet = WFg// + WFN + WFg┴
= Fg//xCos+ FNxCos + Fg┴ xCos
= mgSin30°xCos180° + 0 + 0
Wnet = mgSin30° x Cos180°
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MECHANICSMECHANICSWORK DONE ON A OBJECT MOVING DOWN
A SURFACE WITH FRICTION
Wnet = WFg// + Wf + WFN + WFg┴
= Fg//xCos+ f xCos FNxCos + Fg┴
xCos
= mgSin30°xCos0° + fxCos180° + 0 + 0
Wnet = mgSin30°xCos0° + fxCos180°
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MECHANICSMECHANICS
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MECHANICSMECHANICSWORK DONE ON A OBJECT MOVING UP
A SURFACE WITH FRICTION
Wnet = WFg// + Wf + WFN + WFg┴
= Fg//xCos+ f xCos FNxCos + Fg┴
xCos
= mgSin30°xCos180° + fxCos180° + 0 + 0
Wnet = mgSin30°xCos180° + fxCos180°
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WORK AND ENERGYWORK AND ENERGYSUMMARY SUMMARY
………………. IF THERE ARE . IF THERE ARE NONO FRICTIONAL FORCES FRICTIONAL FORCES:
USE THE LAW OF CONSERVATION OF MECHANICAL ENERGY:ME(i) = ME(f)
(Ep + Ek)i = (Ep + Ek)fOR
USE THE WORK ENERGY PRINCIPLE:Wnet = ΔK
= Ekf – Eki = ½mvf
2 - ½mvi2
……………….. IF THERE .. IF THERE AREARE FRICTIONAL FORCES FRICTIONAL FORCES
USE THE WORK ENERGY PRINCIPLE:Wnet = ΔK
= Ekf – Eki = ½mvf2 - ½mvi2
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ENERGYENERGY
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MECHANICSMECHANICSPOWER
POWER IS THE RATE AT WHICH WORK IS DONE OR
ENERGY IS USED
WHAT IS POWER?
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MECHANICSMECHANICSPOWER
IF A FORCE THAT IS EXERTED ON AN OBJECT MOVES THE OBJECT AT A CONSTANT VELOCITY, WE CAN
CALCULATE THE INSTANTANEOUS POWER OR
AVERAGE POWER BY USING:
POWER, FORCE AND VELOCITY
INSTANTANEOUS POWER OR AVERAGE POWER
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ENDENDGR 12GR 12
MECHANICSMECHANICS