Chapter 6Chapter 6 Forces Forces

Introduction to Introduction to DynamicsDynamics

Sec 6.1Sec 6.1Force and MotionForce and Motion

►ObjectivesObjectives Define a force and differentiate between Define a force and differentiate between

contact forces and long-range forcescontact forces and long-range forces Recognize the significance of Newton’s Recognize the significance of Newton’s

second law of motion and use it to solve second law of motion and use it to solve motion problemsmotion problems

Explain the meaning of Newton’s first law Explain the meaning of Newton’s first law and describe an object in equilibriumand describe an object in equilibrium

ForceForce

= a push = a push or a pullor a pull

FORCE= a push or a pull

1 N = 1 kg·m/s2

Metric Units for Force = Newtons (N)

= the force needed to give a 1 kg mass an acceleration of 1 m/s2

Note: The English unit for force is the pound: 1 lb equals 4.48 Newtons

Force & Mass are Different

• Force is a push or pull that can change motion.• Measured in newtons (N) or pounds (lbs)

• Weight is a force caused by gravity.• Measured in newtons or pounds, because it is a force.

• Mass is the amount of matter in an object.• Measured in grams and kilograms

Your mass is the same everywhere in the universe, but your weight can be different.

Four Fundamental Forces of Nature

GRAVITATIONAL FORCE

• An attractive force between two bodies

• The weakest of all forces

ELECTROMAGNETIC FORCE

• Charged particles at rest exert electric forces on each other

• Charged particles in motion exert magnetic forces on each other

STRONG NUCLEAR FORCE

• Holds particles of the nucleus together

• The strongest of all forces

WEAK NUCLEAR FORCE

• Form of an electromagnetic force

•Happens when some nuclei radioactively decay

The Forces of NatureThe Forces of NatureI. I. Gravitational:•attraction b/tw masses•Ex: tides, weight

II. II. Electromagnetic:•attraction or repulsion b/tw charges•Ex: friction, tension, adhesion, lift, electrostatic , drag, buoyant, magnetic

III. III. Weak Nuclear:•helps to explain atomic collisionsIV. IV. Strong Nuclear:•binds atomic nuclei

Two Categories of Forces…Two Categories of Forces…

►Contact ForceContact Force Acts on an object only by touching itActs on an object only by touching it

►Long-Range Force (aka “Field Force”)Long-Range Force (aka “Field Force”) Exerted without contactExerted without contact

►MagnetsMagnets►GravityGravity

Agent: a specific, identifiable, immediate cause of a force

Examples of ForcesExamples of Forces► FFf f - - Friction (opposes sliding) - - Friction (opposes sliding)

► FFA A - - Applied Force (an external push or pull) - - Applied Force (an external push or pull)

► FFN N - - Normal (perpendicular to a surface) - - Normal (perpendicular to a surface)

► FFsp sp - - Spring (push or pull of a spring)- - Spring (push or pull of a spring)

► FFT T - - Tension (spring, rope, cable)- - Tension (spring, rope, cable)

► FFthrustthrust - - Thrust (rockets, planes, cars) - - Thrust (rockets, planes, cars)

► FFgg - - Weight (force due to gravity) - - Weight (force due to gravity)

Which of the four types of force are each of these?Which of the four types of force are each of these?

…….all of them but one is electromagnetic!.all of them but one is electromagnetic!

Weight Force, FWeight Force, Fgg

aka gravitational forceaka gravitational force

object

Fg (or FW )

ON EARTH:…•The weight force acts on all objects •The direction of the weight force is always towards the center of the earth •The magnitude of the weight force is always equal to the mass of the object (kg) multiplied by acceleration due to gravity

Fg = mg

W = mg or FW = mg or Fgg = mg = mg

The magnitude of an object’s The magnitude of an object’s weight force is always equal its weight force is always equal its mass times the acceleration mass times the acceleration it it

would have if it were in free-fallwould have if it were in free-fall..

FFgg = mg = mg

All Forces have an All Forces have an AgentAgent and an and an ObjectObject

Agent:Agent: the cause of a force the cause of a force

(what does the pushing or (what does the pushing or pulling)pulling)

Object:Object: the ‘victim’ of a force the ‘victim’ of a force

(what gets pushed or pulled)(what gets pushed or pulled)

Weight Force, FWeight Force, Fgg

object

Fg (or FW )

ON EARTH:…•The agent of the weight force is always the earth.

Fg = mg

Agents and Objects of Agents and Objects of Forces Forces

Example: A car is towed by a tow truck: Example: A car is towed by a tow truck:

AGENT: AGENT: The tow truck The tow truck

OBJECT: OBJECT: The carThe car

Conventional Notation:Conventional Notation:

FFTYPE (AGENT, OBJECT) TYPE (AGENT, OBJECT) FFA (T,C)A (T,C)

Forces are VectorsForces are Vectors Forces have magnitude and direction.Forces have magnitude and direction. We can represent forces by drawing We can represent forces by drawing

arrowsarrows Example: Example:

This diagram This diagram shows all of the shows all of the forces acting on forces acting on an an objectobject::

Drawing Force VectorsDrawing Force Vectors

►The The directiondirection of the arrow represents the of the arrow represents the directiondirection of the force. of the force.

►The The lengthlength of the arrow represents the of the arrow represents the magnitude magnitude of the force. of the force.

►We always draw force vectors as ‘pulls’ We always draw force vectors as ‘pulls’ on objects, not pushes (arrow starts at on objects, not pushes (arrow starts at object)object)

Free Body DiagramsFree Body Diagrams

A picture of a ‘body’ with all the force vectors acting upon it

represented graphically is called a

FREE BODY DIAGRAM

Free Body DiagramsFree Body DiagramsRules for drawing free body

diagrams:

1. Draw the object as a dot2. Only draw forces acting on that object3. Draw all forces as ‘pulls’

(arrow point away from the dot)4. Draw and label every force acting on the

object.5. Length of each arrow must reflect

magnitude(stronger forces longer arrows!)(equal forces same arrow length.)

Try itTry it......

Draw a picture of your book sitting

on the desk. Identify all the

forces acting on it.

Free Body Free Body Diagrams...Diagrams...

Book

FN (normal force)

Fg or FW (weight force)

Normal means perpendicular; not ordinary or regular!

The “Normal” ForceThe “Normal” Force

Book

FN (normal force)

Fg or FW (weight force)

Normal means perpendicular; not ordinary or regular!

Normal Force, FNormal Force, FNN

• The direction of the normal force is always perpendicular to a surface (normal)

• The magnitude of the normal force varies depending on the situation.

• The agent of the normal force is always the SURFACE.

Draw free body diagrams for the Draw free body diagrams for the followingfollowing

►An egg is free-falling from a nest in a An egg is free-falling from a nest in a tree. Neglect air resistance. tree. Neglect air resistance.

►A skydiver is descending with a A skydiver is descending with a constant velocity. Consider air constant velocity. Consider air resistance.resistance.

►Your physics book is sliding across the Your physics book is sliding across the desk at constant speed (no desk at constant speed (no acceleration)acceleration)

Physics

FN

W or Fw

surface

FAFf

Fw or W = Weight Force

FN = Normal Force

FA = Applied Force

Ff = Friction Force

Direction Matters!

Net Force, FNet Force, Fnetnet or or ΣΣFF

•The net force on an object is the SUM OF ALL FORCES acting on an

object

•This is NOT just another force like FN or Fg !

•It must be determined by analysis.

FFnet net = F= F11 + F + F22 + F + F33 … etc. … etc.

Net Force, FNet Force, Fnetnet or or ΣΣFF

• When the net force on an object is zero, then the object is said to be in equilibrium, and the acceleration of the object must be zero.

If Fnet = 0 , then a = 0

FFnet net (Net Force) = (Net Force) = ΣΣFF

= the sum of all forces acting on an object = the sum of all forces acting on an object

Physics

FN

W or Fw

surface

FAFf

Fw or W = Weight Force

FN = Normal Force

FA = Applied Force

Ff = Friction Force

Direction Matters!

What is F again? (aka Fnet)

• F is called:

–the sum of the forces

–the net force

–the total force

• F = F1 + F2

• Remember though that– “Left” is probably negative– “Down” is probably negative

F1F2

or

or

You assign the coordinate system!

Sample Problems

1.What is the total force?

F = F1 + F2

(Let’s make left be the negative direction, so the 12N

force to the left is really -12N.)

F = -12N + 8N

F = -4N

m12N 8N

Sample Problems

2.What is the total force?

F = F1 + F2

(Let’s make down be the negative direction; so the

115N force is really -115N.)

F = 158N + (-115N)

F = 43N

m

F2 = 115N

F1 = 158N

3 Possible Situations

F1 > F2

1. F = F1 - F2

2. mass accelerates left

mF1

F2

3 Possible Situations

1.F1 < F2

2.F = F2 - F1

3.mass accelerates right

mF1

F2

3 Possible Situations

1.F1 = F2

2.F = 0

3.mass stays at a constant velocity

mF1

F2Does this block have a larger F2

because its moving to the

right?

No!The block was already

moving. So, if the force left is equal to the force right then the

block has no way to speed up or slow down.

Balanced Forces = Equilibrium =Constant Velocity

ProduceNO

Acceleration

Produce Acceleration

Free Body Free Body Diagrams...Diagrams...What forces are acting on askier as she races down a hill?

The Answer...The Answer...FN

Ff (an)d Fd

Fg

The Answer...The Answer...FN

Ff (and FAIR)

Fg

Hmmm…What is Fnet ?

The Answer...The Answer...FN

Ff and Fd

W

Vector Resolution!

When does Fnet = 0 ?

Sir Isaac NewtonSir Isaac Newton (1642-1727)(1642-1727)

► English physicist and English physicist and mathematicianmathematician

► Before the age of 30:Before the age of 30: Formulated basic Formulated basic

laws of mechanicslaws of mechanics Discovered the Discovered the

universal law of universal law of gravitationgravitation

Invented CalculusInvented Calculus► In 1687, published the In 1687, published the

Principia. Principia. possibly the single most possibly the single most

important book in the important book in the history of science!history of science!

                               

Aristotle and Newton had differentideas about forces and motion.

Aristotle's idea: For anobject to move at a constantspeed, a constant forcemust be applied.

Newton's idea: An objectmoving at a constant speedwill continue at that speedwithout additional forcebeing applied.

Newton’s Second LawNewton’s Second Law

An unbalanced force will cause acceleration, but mass will resist acceleration.

F = maF = ma

mFa

ExampleExample

►A race car has a mass of 710 kg. It A race car has a mass of 710 kg. It starts from rest and travels 40.0 m in starts from rest and travels 40.0 m in 3.0 s. The car is uniformly accelerated 3.0 s. The car is uniformly accelerated during the entire time. What net force during the entire time. What net force is exerted on it?is exerted on it?

Newton’s Second Law

F = maor

Fnet = ma

mFa

mFneta

Newton’s Second Law

First let’s clarify the variables and units

F or Fnet = Net Force (sum of forces) measured in N

a = acceleration measured in m/s2

m = mass measured in kg

F = ma

Newton’s Second Law

F creates the acceleration on a mass.

F = ma

mF

So to find the acceleration of a mass, m, you need to know

the forces.

a

Newton’s Second Law

Sample #1:

How much net force does it take to make a 1.0kg block accelerate at 1.0m/s2?

F = maF = (1.0kg)(1.0m/s2)F = 1.0 kg·m/s2

F = 1.0N1 N = 1 kg·m/s2

m = 1.0kg a = 1.0m/s2

F = ?

Newton’s Second Law

1. A monkey pulls on a banana on a tree with a force of 25N and the tree resists with a force of 25N. What is the total force on the banana?

Newton’s Second Law

2. Two kids pull on a toy. The bigger girl pulls with 24.0N to the right. The smaller boy pulls 12.0N to the left. If neither lets go of the toy, then what will their acceleration be if the total mass of the boy, girl, and toy is 60.0kg?

Two kids pull on a toy. The bigger girl pulls with 24.0N to the right. The smaller boy pulls 12.0N to the left. If neither lets go of the toy, then what will their acceleration be if the total mass of the boy, girl, and toy is 60.0kg?

Draw a diagram

60.0kg12.0N 24.0N

Write out variables

F1 = 24.0N

F2 = 12.0N

m = 60.0kg

a = ?

Pick equations and Solve

F=F1 + F2

F=24.0N + (-12.0N)

F=12.0N

a = ?

F= ma

a = F

m

a = 12.

60.0kg

a = .200m/s2

Example Problems Example Problems

1.1.What is the weight of a 75 kg person on What is the weight of a 75 kg person on earth?earth?

2.2.What is their weight in an elevator?What is their weight in an elevator?

3.3.What is their weight in a falling elevator?What is their weight in a falling elevator?

4.4.What is the mass of a person that weighs What is the mass of a person that weighs 865 N on earth?865 N on earth?

5.5.What is the weight of a 75 kg person on What is the weight of a 75 kg person on the moon where g = 1.6 m/sthe moon where g = 1.6 m/s2 2 ??

Sec. 6.2Sec. 6.2Using Newton’s LawsUsing Newton’s Laws

► ObjectivesObjectives Describe how the weight and the mass of an Describe how the weight and the mass of an

object are relatedobject are related Differentiate between the gravitational force Differentiate between the gravitational force

weight and what is experienced as apparent weight and what is experienced as apparent weightweight

Define the friction force and distinguish between Define the friction force and distinguish between static and kinetic frictionstatic and kinetic friction

Mass and WeightMass and Weight

►The weight force, FThe weight force, Fgg , is used to find , is used to find the downward force of an object.the downward force of an object.

►Both the weight force and acceleration Both the weight force and acceleration due to gravity are downward. due to gravity are downward. (but the (but the netnet force on an object is force on an object is notnot always equal to always equal to its its weightweight force!) force!)

Weight Force = FWeight Force = Fgg = mg = mg

FFgg = mg = mg

The magnitude of an object’s weight The magnitude of an object’s weight force of is always equal its mass force of is always equal its mass times the acceleration times the acceleration it would it would

have if it were in free-fallhave if it were in free-fall..

Example Problems Example Problems

1.1.What is the weight of a 75 kg person on What is the weight of a 75 kg person on earth?earth?

2.2.What is their weight in an elevator?What is their weight in an elevator?

3.3.What is their weight in a falling elevator?What is their weight in a falling elevator?

4.4.What is the mass of a person that weighs What is the mass of a person that weighs 865 N on earth?865 N on earth?

5.5.What is the weight of a 75 kg person on What is the weight of a 75 kg person on the moon where g = 1.6 m/sthe moon where g = 1.6 m/s2 2 ??

Scale ProblemsScale Problems

The reading on a scale The reading on a scale is the magnitude of the is the magnitude of the

force of the scale on force of the scale on the person or object the person or object

standing on the scale!standing on the scale!

FFNN (Scale , Me)(Scale , Me)

Scale ProblemsScale Problems

When a problems asks When a problems asks you “what is the you “what is the

reading on the scale”, reading on the scale”, it is asking you to it is asking you to

determine the force of determine the force of the scale on the object.the scale on the object.

F F (S , O)(S , O)

Hmmm… will that ever differ from Hmmm… will that ever differ from the weight force of the object ? the weight force of the object ? (Yes, it can differ!)(Yes, it can differ!)

Example Problems Example Problems A 75 kg person stands on a scale A 75 kg person stands on a scale

which is in an elevator, accelerating which is in an elevator, accelerating upwards at 2.0 m/supwards at 2.0 m/s22. What is the reading . What is the reading on the scale?on the scale?

FF(S,P)(S,P) = ? = ?

m = 75 kgm = 75 kg

a = a = ++ 2.0 m/s 2.0 m/s22

g = g = -- 9.8 m/s 9.8 m/s22

FFg (E,P)g (E,P) = = -- 735 N 735 N

FFnetnet = ma = ma

FFnetnet = = ΣΣF F (“(“the sum of all forces”) the sum of all forces”)

FFnetnet = F = F(S,P)(S,P) + F + Fg(E,P)g(E,P)

FF(S,P)(S,P) = F = Fnetnet - F - Fg(E,P)g(E,P)

= ma – (= ma – (-- 735 N) 735 N) = = ++ 885 N 885 N

FFnetnet = 150 N = 150 N

FFNN (S,P)(S,P)

FFg (E, P)g (E, P)

Example Problems Example Problems

A 50.0 kg bucket is pulled by a rope. The A 50.0 kg bucket is pulled by a rope. The rope is guaranteed not to break if the rope is guaranteed not to break if the tension force is less than 500.0 N. The tension force is less than 500.0 N. The bucket is lifted from rest, and after being bucket is lifted from rest, and after being lifted 3.0 meters, it is travelling at 3.0 m/s. lifted 3.0 meters, it is travelling at 3.0 m/s. Is the rope in danger of breaking? Is the rope in danger of breaking?

Example Problem # 2 Example Problem # 2 A 50.0 kg bucket is pulled by a rope. The rope is A 50.0 kg bucket is pulled by a rope. The rope is

guaranteed not to break if the tension force is less than guaranteed not to break if the tension force is less than 500.0 N. The bucket is lifted from rest, and after being 500.0 N. The bucket is lifted from rest, and after being lifted 3.0 meters, it is travelling at 3.0 m/s. Is the rope in lifted 3.0 meters, it is travelling at 3.0 m/s. Is the rope in danger of breaking?danger of breaking?FFT (R,B)T (R,B) = ? = ?

m = 50.0 kgm = 50.0 kg

a = ? a = ?

vv0 0 = 0= 0

v = 3.0 m/sv = 3.0 m/s

dd0 0 = 0 m= 0 m

d = 3.0 md = 3.0 m

FFnetnet = ma = (50.0kg) (1.5 m/s = ma = (50.0kg) (1.5 m/s22) = 75 ) = 75 NN

FFnetnet = = ΣΣF F (“(“the sum of all forces”) the sum of all forces”)

FFnetnet = F = FT (R,B)T (R,B) + F + Fg(E,B)g(E,B)

FFT (R,B)T (R,B) = F = Fnetnet - F - Fg (E,B)g (E,B)

= 75 – (= 75 – (-- 490 N) 490 N) = = ++ 565 N 565 N

g = g = -- 9.8 m/s 9.8 m/s22

FFg (E,P)g (E,P) = = -- 490 N 490 N

F F (R , B)(R , B)

FFg (E, B)g (E, B)vv22 = V = V0022 + 2a(d- + 2a(d-

dd00))

Newton’s First Law:The Law of Inertia

Unless acted upon by an unbalanced force,

objects at rest will stay at rest and objects in

motion will stay in motion.

Newton’s First Law

Unless acted upon by an unbalanced force, objects at rest will stay at rest and

objects in motion will stay in motion.

Unless acted upon by an unbalanced force, objects at rest will stay at rest and

objects in motion will stay in motion.

Newton’s First Law

Unless acted upon by an outside force,

objects at rest will stay at rest and objects in

motion will stay in motion.

 

Newton’s First Law

Newton’s First Law of MotionNewton’s First Law of Motion

►““An object that is at rest will remain at An object that is at rest will remain at rest or an object that is moving will rest or an object that is moving will continue to move in a straight line with continue to move in a straight line with constant speed, if and only if the net constant speed, if and only if the net force acting on that object is zero.”force acting on that object is zero.”

Newton’s First Con’tNewton’s First Con’t

► Inertia—the tendency of an object to Inertia—the tendency of an object to resist change.resist change.

►Equilibrium—object at rest or moving Equilibrium—object at rest or moving at a constant velocityat a constant velocity

Finally…Misconceptions about Finally…Misconceptions about forcesforces

►When a ball has been throw, the When a ball has been throw, the force of your hand remains on it. force of your hand remains on it. NO!NO!

►A force is needed to keep an A force is needed to keep an object moving. NO!object moving. NO!

►Inertia is a force. NO!Inertia is a force. NO!►The quantity The quantity mama is a force. NO! is a force. NO!

Friction is a ForceFriction is a Force►Friction is a force Friction is a force

that resists motion.that resists motion.

►……it is due to it is due to microscopic microscopic roughness on all roughness on all surfaces.surfaces.

► … … it slows down all it slows down all moving objects.moving objects.

FrictionFriction

►Static friction forceStatic friction force The force that opposes the start of The force that opposes the start of

relative motion between the two surfaces relative motion between the two surfaces in contactin contact

►Friction force when object isn’t in motionFriction force when object isn’t in motion

►Kinetic Friction ForceKinetic Friction Force The force that opposes relative motion The force that opposes relative motion

between surfaces in contactbetween surfaces in contact►Friction force when object is in motionFriction force when object is in motion

Calculating FrictionCalculating Friction

Kinetic Friction ForceKinetic Friction Force

(F(Ff f ,,kinetickinetic) = () = (µµkkFFNN))

Static Friction ForceStatic Friction Force

((FFf f ,,staticstatic) < or = () < or = (µµssFFNN))

Typical Coefficients of Typical Coefficients of FrictionFriction

Surface µs µk

Rubber on concrete 0.80 0.65

Rubber on wet concrete 0.60 0.40

Wood on wood 0.50 0.20

Steel on steel (dry) 0.78 0.58

Steel on steel (with oil) 0.15 0.06

Teflon on steel 0.04 0.04

Example Problems Example Problems

Balanced Forces:Balanced Forces:

You push a 25 kg wooden box across a You push a 25 kg wooden box across a wood floor at constant speed. How much wood floor at constant speed. How much force do you exert on the box? (force do you exert on the box? (μμkk = 0.20) = 0.20)

Example Problems Example Problems

Unbalanced Forces:Unbalanced Forces:

If you push the same 25 kg wooden box If you push the same 25 kg wooden box across a wood floor with double the force, across a wood floor with double the force, what is the acceleration of the box?what is the acceleration of the box?

Terminal VelocityTerminal Velocity

►The constant velocity that is reached The constant velocity that is reached when the drag force equals the force of when the drag force equals the force of gravitygravity

►Objects can only fall so fast due to their Objects can only fall so fast due to their size and shape and density of the size and shape and density of the air/fluidair/fluid Ping-pong ball – 9 m/sPing-pong ball – 9 m/s Basketball – 20 m/sBasketball – 20 m/s Baseball – 42 m/sBaseball – 42 m/s Skydiver: Skydiver: >>62 m/s w/o chute62 m/s w/o chute

5 m/s w/ chute5 m/s w/ chute

THE ENDTHE END

Demo TimeDemo Time

►Demo: Hanging WeightsDemo: Hanging Weights►Question: Where will the string break if Question: Where will the string break if

I pull on the bottom string very quickly?I pull on the bottom string very quickly?

Demo TimeDemo Time

►Demo: Hanging WeightsDemo: Hanging Weights►Analysis: Why did the string break Analysis: Why did the string break

where it did?where it did?

The string does not The string does not stretch here, stretch here, because the large because the large mass does not move. mass does not move. A large mass has A large mass has lots of inertia. The lots of inertia. The tension force in the tension force in the bottom of the string bottom of the string does not accelerate does not accelerate the large mass.the large mass.

Demo TimeDemo Time

►Demo: Paper cup and MarbleDemo: Paper cup and Marble►Question: What will happen when Question: What will happen when

the card is flicked hard?the card is flicked hard?

Demo QuestionsDemo Questions

1.1. Explain why the bottom string broke Explain why the bottom string broke in the demoin the demo

2.2. Explain why the ball fell down in the Explain why the ball fell down in the cup demo.cup demo.

Demo Questions ContinuedDemo Questions Continued

4.4. Why do groceries slide to the side when Why do groceries slide to the side when you turn your car?you turn your car?

5.5. Why is it a bad idea to cut off a Hummer Why is it a bad idea to cut off a Hummer and then slow down in your Mini Cooper?and then slow down in your Mini Cooper?

6.6. If you have fuzzy dice hanging from your If you have fuzzy dice hanging from your rearview mirror, why does it swing when rearview mirror, why does it swing when you stop and start moving?you stop and start moving?

Periodic MotionPeriodic Motion

► Pendulums, springs, stringsPendulums, springs, strings► Simple Harmonic MotionSimple Harmonic Motion

Motion that returns an object to its equilibrium Motion that returns an object to its equilibrium position as a result of a restoring force that is position as a result of a restoring force that is directly proportional to the object’s displacementdirectly proportional to the object’s displacement

► Period (T)Period (T) Time needed to repeat one complete cycle of Time needed to repeat one complete cycle of

motionmotion

► AmplitudeAmplitude Maximum distance the object moves from Maximum distance the object moves from

equilibriumequilibrium

Amplitude, Frequency, Amplitude, Frequency, PeriodPeriod

The Amplitude is the displacement.The Frequency is the number of cycles/sec.The Period is the time for one cycle T = 1/f

Period of a PendulumPeriod of a Pendulum

T = Lg

2T = period in seconds2 = 6.28

L = the lengthg = accel of gravity

ProblemsProblems

►Pg. 136 Pg. 136 ►17-1917-19

Sec. 6.3Sec. 6.3Interaction ForcesInteraction Forces

►ObjectivesObjectives Explain the meaning of interaction pairs of Explain the meaning of interaction pairs of

forces and how they are related by forces and how they are related by Newton’s third lawNewton’s third law

List the four fundamental forces and List the four fundamental forces and illustrate the environment in which each illustrate the environment in which each can be observed.can be observed.

Explain the tension in ropes and strings in Explain the tension in ropes and strings in terms of Newton’s third lawterms of Newton’s third law

Interaction forcesInteraction forces

►Two forces that are in opposite Two forces that are in opposite directions and have equal magnitudedirections and have equal magnitude

►Newton’s Third Law—all forces come in Newton’s Third Law—all forces come in pairspairs

►FFA on B A on B = -F = -FB on AB on A