What has more mass, a 50N What has more mass, a 50N block on the moon, or a 50N block on the moon, or a 50N

block on the Earth?block on the Earth?

Ear

th

Moon

Sam

e

0% 0%0%

1.1. EarthEarth

2.2. MoonMoon

3.3. Same Same

0 of 20

Sidekick Saves HeroSidekick Saves Hero► Because of your physics background, you have been Because of your physics background, you have been

able to get a job with a company devising stunts for an able to get a job with a company devising stunts for an upcoming adventure movie being shot in Minnesota. In upcoming adventure movie being shot in Minnesota. In the script, the hero has been fighting the villain on the the script, the hero has been fighting the villain on the top of the locomotive of a train going down a straight top of the locomotive of a train going down a straight horizontal track at 20 mph. He has just snuck on the horizontal track at 20 mph. He has just snuck on the train as it passed over a lake so he is wearing his train as it passed over a lake so he is wearing his rubber wet suit. During the fight, the hero slips and rubber wet suit. During the fight, the hero slips and hangs by his fingers on the top edge of the front of the hangs by his fingers on the top edge of the front of the locomotive. The locomotive has a smooth steel vertical locomotive. The locomotive has a smooth steel vertical front face. Now the villain stomps on the hero's fingers front face. Now the villain stomps on the hero's fingers so he will be forced to let go and slip down the front of so he will be forced to let go and slip down the front of the locomotive and be crushed under its wheels. the locomotive and be crushed under its wheels. Meanwhile, the hero's partner is at the controls of the Meanwhile, the hero's partner is at the controls of the locomotive trying to stop the train.. locomotive trying to stop the train..

Sidekick Saves HeroSidekick Saves Hero► To add to the suspense, the brakes have been To add to the suspense, the brakes have been

locked by the villain. It will take her 10 seconds to locked by the villain. It will take her 10 seconds to open the lock. To her horror, she sees the hero's open the lock. To her horror, she sees the hero's fingers give way before she can get the lock off. fingers give way before she can get the lock off. Since she is the brains of the outfit, she immediately Since she is the brains of the outfit, she immediately opens the throttle causing the train to accelerate opens the throttle causing the train to accelerate forward. This causes the hero to stay on the front forward. This causes the hero to stay on the front face of the locomotive without slipping down giving face of the locomotive without slipping down giving her time to save the hero's life. The movie company her time to save the hero's life. The movie company wants to know what minimum acceleration is wants to know what minimum acceleration is necessary to perform this stunt. The hero weighs necessary to perform this stunt. The hero weighs 180 lbs in his wet suit. The locomotive weighs 100 180 lbs in his wet suit. The locomotive weighs 100 tons. You look in a book giving the properties of tons. You look in a book giving the properties of materials and find that the coefficient of kinetic materials and find that the coefficient of kinetic friction for rubber on steel is 0.50 and its coefficient friction for rubber on steel is 0.50 and its coefficient of static friction is 0.60of static friction is 0.60

Factors that affect FrictionFactors that affect Friction►The magnitude of the friction force The magnitude of the friction force

acting on an object depends on two acting on an object depends on two things.things.

►Contact MaterialContact Material: The type of material : The type of material the object is made of and the type of the object is made of and the type of material it is sliding on makes a material it is sliding on makes a difference.difference.

►Example #1: A waxed snowboard will Example #1: A waxed snowboard will experience little friction against experience little friction against powdery snow.powdery snow.

►Example #2: A waxed snowboard will Example #2: A waxed snowboard will experience quite some friction against a experience quite some friction against a paved asphalt road. paved asphalt road.

Factors that affect FrictionFactors that affect Friction►Normal forceNormal force: How hard the surface : How hard the surface

is pushing on the object will affect is pushing on the object will affect how much friction acts on the object.how much friction acts on the object.

►Example #1: A heavy crate sliding Example #1: A heavy crate sliding will experience more friction than a will experience more friction than a light crate sliding across the same light crate sliding across the same surface.surface.

►Example #2: A crate sliding down a Example #2: A crate sliding down a 10° incline will experience more 10° incline will experience more friction than the same crate sliding friction than the same crate sliding down a 30° incline. down a 30° incline.

Lesson #36Lesson #36Topic: Coefficient of FrictionTopic: Coefficient of Friction

Objectives: Objectives: (After this class I will be able to)(After this class I will be able to)

1.1. State the variables that affect the force of State the variables that affect the force of friction.friction.

2.2. Define coefficient of friction.Define coefficient of friction.3.3. Solve problems involving coefficient of Solve problems involving coefficient of

friction.friction.

12/7/07

Assignment: “Coefficient of friction practice” due Monday

Project: From yesterday’s lab, discuss what factors affected the force of friction.

Friction force depends on…Friction force depends on…

Mas

s an

d norm

al fo

rce

Conta

ct m

ater

ial a

nd...

Conta

ct m

ater

ial a

nd ...

Conta

ct m

ater

ial a

nd ...

0%

90%

10%0%

1.1. Mass and normal Mass and normal forceforce

2.2. Contact material Contact material and massand mass

3.3. Contact material Contact material and weightand weight

4.4. Contact material Contact material and normal forceand normal force

Coefficient of FrictionCoefficient of Friction► For every combination of two contact For every combination of two contact

materials there is a specific ratio between materials there is a specific ratio between friction force and normal force.friction force and normal force.

► This ratio is called coefficient of friction.This ratio is called coefficient of friction.► Examples:Examples:

SurfaceSurface CoefficientCoefficient

Wood on woodWood on wood μμ = 0.4 = 0.4

Ice on iceIce on ice μμ = .01 = .01

Steel on steelSteel on steel μμ = .07 = .07

Rubber on Rubber on concreteconcrete

μμ = 1.0 = 1.0

Coefficient of FrictionCoefficient of Friction► This ratio between forces can be expressed This ratio between forces can be expressed

by:by:

or or ► The first step to these problems is to solve The first step to these problems is to solve

for Ffor Fn n first.first.

Example: What is the coefficient of friction Example: What is the coefficient of friction between Fred and the table if I push him between Fred and the table if I push him along at constant speed with a force of along at constant speed with a force of 10N? Fred has a mass of 5kg.10N? Fred has a mass of 5kg.

n

f

F

F nf FF

Coefficient of FrictionCoefficient of Friction1.1. I push downward with a force of 100N and to I push downward with a force of 100N and to

the right with a force of 50N onto a 1kg book at the right with a force of 50N onto a 1kg book at rest on a table. The book moves at constant rest on a table. The book moves at constant speed.speed.Find the coefficient of friction between the book Find the coefficient of friction between the book and the table.and the table.

Coefficient of FrictionCoefficient of Friction2.2. I lift upward with a force of 5N onto a 1kg I lift upward with a force of 5N onto a 1kg

book at rest on a table. With what book at rest on a table. With what additional force do I need to press onto additional force do I need to press onto the side of the book to make the book the side of the book to make the book move at constant speed? move at constant speed? μμ = 0.2 = 0.2

Coefficient of FrictionCoefficient of Friction3.3. If I push on a 1kg book with a force of If I push on a 1kg book with a force of

100N, I can hold it at rest against the 100N, I can hold it at rest against the wall. wall.

Find the coefficient of friction between Find the coefficient of friction between the book and the wall.the book and the wall.

Coefficient of Friction (3pt Coefficient of Friction (3pt Bonus)Bonus)

►As I take Fred (5kg) for his morning As I take Fred (5kg) for his morning walk I pull on his leash with a force of walk I pull on his leash with a force of 20N at an angle of 50° to the 20N at an angle of 50° to the horizontal. Find the acceleration of horizontal. Find the acceleration of Fred if the coefficient of friction Fred if the coefficient of friction between Fred and the floor is 0.2. between Fred and the floor is 0.2. Fa

Lesson #37Lesson #37Topic: Coefficient of Kinetic vs. Static FrictionTopic: Coefficient of Kinetic vs. Static Friction

Objectives: Objectives: (After this class I will be able to)(After this class I will be able to)

1.1. Explain the difference between Explain the difference between static and kinetic friction.static and kinetic friction.

12/10/07

Assignment: “Static and Kinetic friction” due Tomorrow

Warm Up: If you were sliding a crate of bricks across the floor, is it harder to get the crate to start to slide of to keep sliding?

Kinetic vs. StaticKinetic vs. Static►KineticKinetic: Means motion, so force of : Means motion, so force of

Kinetic Friction would be the friction Kinetic Friction would be the friction force acting on an object in motion.force acting on an object in motion.

►StaticStatic: Means at rest, so the force of : Means at rest, so the force of Static Friction would be the friction Static Friction would be the friction force acting on an object at rest.force acting on an object at rest.

►The force of friction acting on an The force of friction acting on an object decreases when an object object decreases when an object begins to move.begins to move.

Static FrictionStatic Friction►Static friction can range from 0N to Static friction can range from 0N to

some maximum amount. some maximum amount. ►Example: The amount of force needed Example: The amount of force needed

to get Fred to move is 10N. What is to get Fred to move is 10N. What is the friction force acting on Fred the friction force acting on Fred when…when…a. There is no force acting on Fred?a. There is no force acting on Fred?b. I push on him with 1N of force?b. I push on him with 1N of force?c. I push on him with 8N of force?c. I push on him with 8N of force?d. I push on him with 10N of force?d. I push on him with 10N of force?

Kinetic FrictionKinetic Friction► After I reach 10N of force, Fred will After I reach 10N of force, Fred will

accelerate, and the force of friction acting accelerate, and the force of friction acting on him will decrease. on him will decrease.

► The force of friction acting on Fred while he The force of friction acting on Fred while he is in motion is a constant, fixed number that is in motion is a constant, fixed number that is less than the maximum force of static is less than the maximum force of static friction.friction.

► Example: Fred moves at a constant speed Example: Fred moves at a constant speed when I push on him with 9N of force. What when I push on him with 9N of force. What is the force of friction acting on him when…is the force of friction acting on him when…a. I push with 9N of force?a. I push with 9N of force?b. I push with 10N of force?b. I push with 10N of force?c. I push with 100N of force?c. I push with 100N of force?

Coefficients of Kinetic and Static Coefficients of Kinetic and Static FrictionFriction

► Since the force of friction is different for Since the force of friction is different for Kinetic and Static, then the coefficients of Kinetic and Static, then the coefficients of friction are different as well.friction are different as well.

► If a problem refers to a moving object, then If a problem refers to a moving object, then you use or solve for you use or solve for μμk.k.

► If a problem refers to an object at rest, then If a problem refers to an object at rest, then you use or solve for you use or solve for μμs.s.

► μμs s > > μμkk

ExampleExample

►Fred begins to move when I push on Fred begins to move when I push on him with 10N of force, and moves at a him with 10N of force, and moves at a constant speed when I push on him constant speed when I push on him with 9N of force. Fred has a mass of with 9N of force. Fred has a mass of 5kg. 5kg.

►What are the coefficients of static and What are the coefficients of static and kinetic friction between Fred and the kinetic friction between Fred and the table?table?

ProjectProject

►Solve for the coefficient of static friction Solve for the coefficient of static friction and the coefficient of kinetic friction and the coefficient of kinetic friction found between a wood block and an found between a wood block and an aluminum track.aluminum track.

►Do the same for a felt block.Do the same for a felt block.►Assignment: p133 #33-37 due Assignment: p133 #33-37 due

tomorrow.tomorrow.►Bonus: p132 Challenge ProblemBonus: p132 Challenge Problem►Complete “Sidekick Saves Hero”Complete “Sidekick Saves Hero”

Construction SiteConstruction Site► You are passing a construction site on the way You are passing a construction site on the way

to physics class, and stop to watch for awhile. to physics class, and stop to watch for awhile. The construction workers appear to be going on The construction workers appear to be going on coffee break, and have left a large concrete coffee break, and have left a large concrete block resting at the top of a wooden ramp. As block resting at the top of a wooden ramp. As soon as their backs are turned, the block begins soon as their backs are turned, the block begins to slide down the ramp. You quickly clock the to slide down the ramp. You quickly clock the time for the block to reach the bottom of the time for the block to reach the bottom of the ramp at 10 seconds. You wonder how long the ramp at 10 seconds. You wonder how long the ramp is. You estimate that the ramp is at an ramp is. You estimate that the ramp is at an angle of about 20angle of about 20°° to the horizontal. In your to the horizontal. In your physics book you find that the coefficient of physics book you find that the coefficient of kinetic friction between concrete and wood is kinetic friction between concrete and wood is 0.35. 0.35.

Lesson #38Lesson #38Topic: Coefficient of Friction and InclinesTopic: Coefficient of Friction and Inclines

Objectives: Objectives: (After this class I will be able to)(After this class I will be able to)

1.1. Solve for the correct coefficient of friction Solve for the correct coefficient of friction depending on the motion or non-motion of depending on the motion or non-motion of an object on an inclined plane.an object on an inclined plane.

12/7/06

Assignment: p135 #38 - 41, 44 due tomorrow

Warm Up: A wood block on an inclined plane will just start to slide when the plane is raised to make an angle of 30° to the horizontal. What is the coefficient of static friction between the block and the plane?

Warm Up SolutionWarm Up Solutiony

x

Fn

Fwy

Fwx

Ff

The coefficient of static friction The coefficient of static friction is _________ than kinetic friction.is _________ than kinetic friction.

1.1. Always biggerAlways bigger

2.2. Always smallerAlways smaller

3.3. The sameThe same

4.4. Sometimes Sometimes biggerbigger

Find the acceleration of the Find the acceleration of the block if block if μμkk = 0.3 = 0.3y

x

Fn

Fwy

Fwx

Ff

30°

The acceleration is 2m/sThe acceleration is 2m/s22. . What is What is μμkk? ? y

x

Fn

Fwy

Fwx

Ff

30°

There is a 30N applied force up the ramp. There is a 30N applied force up the ramp. Which way does the 3kg block move? With Which way does the 3kg block move? With

what acceleration? what acceleration? μμkk=0.3=0.3y

x30°

Fa

Construction SiteConstruction Site

►Find out how long the ramp is at the Find out how long the ramp is at the construction site. construction site.

►Show all work. Show all work.

►Stuffed Poodle quizStuffed Poodle quiz

Lesson #39Lesson #39Topic: Exam 5 ReviewTopic: Exam 5 Review

Objectives: Objectives: (After this class I will be able to)(After this class I will be able to)

1.1. Practice solving physics problemsPractice solving physics problems

2.2. Complete and check Exam 5 ReviewComplete and check Exam 5 Review

3.3. Plan a tutoring time (if needed)Plan a tutoring time (if needed)

4.4. Complete a bonus problem Complete a bonus problem opportunityopportunity

12/1/06

Assignment: Exam 5 Review due ! Study for Exam 5.

Warm Up: A cart is set at the top of an incline and released from rest. Its final velocity as it reaches the bottom of the incline is 6m/s and it takes 2 seconds to reach the bottom of the incline. What is the angle of the incline? (assume there is no friction)

Warm Up SolutionWarm Up Solutiony

x

Fn

Fwy

Fwx

What affects friction?What affects friction?

25% 25%25%25%1.1. Normal force Normal force and weightand weight

2.2. Surface material Surface material and surface areaand surface area

3.3. Normal force Normal force and surface and surface materialmaterial

4.4. Normal force Normal force and surface areaand surface area

0 of 5

As the angle of an incline As the angle of an incline increases, normal force will…increases, normal force will…

25% 25%25%25%1.1. IncreaseIncrease

2.2. DecreaseDecrease

3.3. Remain the Remain the samesame

4.4. Equal FEqual Fwxwx

0 of 5

FFwx wx is at a maximum when the is at a maximum when the angle of the incline is …angle of the incline is …

25% 25%25%25%1.1. 0°0°

2.2. 45°45°

3.3. 90°90°

4.4. 180°180°

0 of 5

Coefficient of friction isCoefficient of friction is

25% 25%25%25%1.1. A ratio between Ff and A ratio between Ff and Fn that is the same for Fn that is the same for all objectsall objects

2.2. A ratio between Ff and A ratio between Ff and Fn that is specific to Fn that is specific to that materialthat material

3.3. The amount of friction The amount of friction any object will any object will encounterencounter

4.4. The amount of friction The amount of friction a specific material will a specific material will encounterencounter

0 of 5

Static friction …Static friction …

25% 25%25%25%1.1. Is the friction Is the friction acting on an acting on an object at rest.object at rest.

2.2. Can range Can range between zero and between zero and the maximum the maximum friction forcefriction force

3.3. Both 1 and 2Both 1 and 2

4.4. Neither 1 nor 2Neither 1 nor 2

0 of 5

PracticePractice

1.1. A flatbed truck is carrying a heavy A flatbed truck is carrying a heavy crate. The coefficient of static friction crate. The coefficient of static friction between the crate and the bed of the between the crate and the bed of the truck is 0.65. What is the maximum truck is 0.65. What is the maximum rate at which the driver can rate at which the driver can decelerate and still avoid having the decelerate and still avoid having the crate slide against the cab?crate slide against the cab?

PracticePractice

1.1. A 20 kg box slides down a ramp with A 20 kg box slides down a ramp with constant speed. The force of kinetic constant speed. The force of kinetic friction between the box and the friction between the box and the ramp is 60N. What is the angle of the ramp is 60N. What is the angle of the incline?incline?

There is a 30N applied force up the ramp. There is a 30N applied force up the ramp. Which way does the 3kg block move? With Which way does the 3kg block move? With

what acceleration? what acceleration? μμkk=0.3=0.3y

x30°

Fa

BonusBonus

►A roller coaster reaches the top of the A roller coaster reaches the top of the steepest hill with a speed of 6 km/h. It steepest hill with a speed of 6 km/h. It then descends the hill at a 45° angle. then descends the hill at a 45° angle. The hill is 45 m long. What is the The hill is 45 m long. What is the speed of the coaster at the bottom of speed of the coaster at the bottom of the hill? Assume μthe hill? Assume μkk = .12. = .12.