ForcesForcesCh. 6Ch. 6

Milbank High SchoolMilbank High School

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

The Forces of NatureThe Forces of NatureI. I. Gravitational:•attraction bet. masses•tides, gravity, weightII. II. Electromagnetic:•friction •tension•adhesion •lift•electrostatic •drag•buoyant •magnetic

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

ForceForce

► How do forces influence motion?How do forces influence motion?► ForceForce– a push or pull exerted – a push or pull exerted

on an object having magnitude on an object having magnitude and directionand direction

► SystemSystem—object that —object that experiences the forceexperiences the force

► EnvironmentEnvironment—world around —world around the system that exerts the force the system that exerts the force

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 ForceLong-Range Force Exerted without contactExerted without contact

►MagnetsMagnets►GravityGravity

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

Types of ForcesTypes of Forces►FFf f - - Friction (opposes sliding) - - Friction (opposes sliding)

►FFN N - - Normal (surface) - - Normal (surface)

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

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

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

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

Representing Representing ForcesForces......►Forces are vectorsForces are vectors

Forces are drawn as arrows (vectors)Forces are drawn as arrows (vectors) forces add like vectors.forces add like vectors. the sum of all the forces is called the the sum of all the forces is called the

net forcenet force..

►A picture of a body with arrows A picture of a body with arrows drawn representing all the forces drawn representing all the forces acting upon it is called a acting upon it is called a FREE FREE BODY DIAGRAMBODY DIAGRAM..

Free Body DiagramsFree Body Diagrams

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

T (table)

W (weight)

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

The Answer...The Answer...FN

d & f

W

The Answer...The Answer...FN

f and d

W

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.

►A car is coasting to the right and A car is coasting to the right and slowing down.slowing down.

Newton’s Second LawNewton’s Second Law

►F = maF = ma►a = Fa = Fnetnet / m / m

►Expressed in Newtons (N)Expressed in Newtons (N) Force required to give 1kg mass a 1m/sForce required to give 1kg mass a 1m/s22

accelerationacceleration

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 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 the hand that threw it force of the hand that threw it remains on it.remains on it.

►A force is needed to keep an object A force is needed to keep an object movingmoving

►Inertia is a forceInertia is a force►Air does not exert a forceAir does not exert a force►The quantity The quantity mama is a force is a force

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

Describe simple harmonic motion and explain Describe simple harmonic motion and explain how the acceleration due to gravity influences how the acceleration due to gravity influences such motion.such motion.

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 net force and acceleration Both the net force and acceleration are downward.are downward.

FFgg = mg = mg

Example ProblemsExample Problems

►Pg. 128Pg. 128►Practice Problem 12.Practice Problem 12.

Pg. 129Pg. 129

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 with object isn’t in motionFriction force with 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

FFf f ,,kinetickinetic = = µµkkFFnn

Static Friction ForceStatic Friction Force

00< F< Ff, static f, static << µ µssFFNN

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

Steet on steel (with oil) 0.15 0.06

Teflon on steel 0.04 0.04

Example Problems Example Problems

►Pg. 131-133Pg. 131-133 Balanced Friction ForcesBalanced Friction Forces Unbalanced Friction ForcesUnbalanced Friction Forces

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

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