Lesson #33Lesson #33Topic: Topic: Net force in two dimensions

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

1.1. Examine Normal force and observe Examine Normal force and observe different situations involving normal force.different situations involving normal force.

2.2. Solve net force problems involving inclined Solve net force problems involving inclined planes.planes.

11/28/06

Assignment: Transparency 4-4 due tomorrow

Project: Describe a situation where the Normal Force acting on an object does not equal the Weight of the object.

Exploring Normal ForceExploring Normal Force Normal force is a force that acts Normal force is a force that acts

perpendicular to the surface in contact perpendicular to the surface in contact with an object.with an object.

Normal force is Normal force is NOTNOT the opposite of the opposite of weight.weight.

Normal force will vary depending on the Normal force will vary depending on the situation of the object.situation of the object.

Normal Force ExamplesNormal Force Examples1.1. If I push on a 1kg book with a force of 100N, I can hold If I push on a 1kg book with a force of 100N, I can hold

it at rest against the wall. it at rest against the wall. a. Find the weight of the book. a. Find the weight of the book. b. Find the normal force acting on the book. b. Find the normal force acting on the book. c. Find the force of friction acting on the book.c. Find the force of friction acting on the book.

Normal Force ExamplesNormal Force Examples2.2. I push downward with a force of 100N onto a 1kg book I push downward with a force of 100N onto a 1kg book

at rest on a table. The book remains at rest.at rest on a table. The book remains at rest.a. What is the net force acting on the book?a. What is the net force acting on the book?b. What is the normal force acting on the book?b. What is the normal force acting on the book?

Normal Force ExamplesNormal Force Examples3.3. I lift upward with a force of 5N onto a 1kg book at rest I lift upward with a force of 5N onto a 1kg book at rest

on a table. The book remains at rest. on a table. The book remains at rest. a. What is the net force acting on the book?a. What is the net force acting on the book?b. What is the normal force acting on the book?b. What is the normal force acting on the book?

Distribution CompanyDistribution Company You and a few friends have decided to open a small You and a few friends have decided to open a small

business called Wee Deliver. The business will guarantee business called Wee Deliver. The business will guarantee to deliver any box between 5 lbs and 500 lbs to any to deliver any box between 5 lbs and 500 lbs to any location in the Twin City area by the next day. At your location in the Twin City area by the next day. At your distribution center, boxes slide down a ramp between the distribution center, boxes slide down a ramp between the delivery area and the sorting area. In designing the delivery area and the sorting area. In designing the distribution center, you must determine the angle this ramp distribution center, you must determine the angle this ramp should have with the horizontal so that a 500-lb box takes should have with the horizontal so that a 500-lb box takes 5.0 seconds to slide down the ramp starting from rest at 5.0 seconds to slide down the ramp starting from rest at the top. When the box arrives at the bottom of the ramp, the top. When the box arrives at the bottom of the ramp, its speed should not be too large or the contents of the box its speed should not be too large or the contents of the box might be damaged. You decide that this speed should be might be damaged. You decide that this speed should be 3m/s. Using the latest technology, your ramp will have a 3m/s. Using the latest technology, your ramp will have a very slippery surface so you make the approximation that very slippery surface so you make the approximation that the frictional force between the ramp and the box can be the frictional force between the ramp and the box can be neglected. neglected.

ProjectProject Place a cart on a track.Place a cart on a track. What happens to the cart when the plane What happens to the cart when the plane

is inclined?is inclined? Draw the cart and inclined plane.Draw the cart and inclined plane. Draw all of the forces acting on the cart.Draw all of the forces acting on the cart. Why does the cart accelerate?Why does the cart accelerate? Explain in terms of forces acting on the Explain in terms of forces acting on the

cart. cart. Assignment: “Concept Devel 5-3 & 5-4” Assignment: “Concept Devel 5-3 & 5-4”

due Tuesdaydue Tuesday

Inclined PlanesInclined Planes To best analyze this problem. We will make a new To best analyze this problem. We will make a new

coordinate system.coordinate system. With the x axis parallel to the contact surfaceWith the x axis parallel to the contact surface And the y axis perpendicular to the contact surfaceAnd the y axis perpendicular to the contact surface

y

x

Inclined PlanesInclined Planes The only forces acting on the block are its The only forces acting on the block are its

weight and the normal force.weight and the normal force. Since these two forces do not cancel out, the Since these two forces do not cancel out, the

block will accelerate down the incline.block will accelerate down the incline.

y

x

Inclined PlanesInclined Planes To find the net force acting on the block, we need to To find the net force acting on the block, we need to

split forces into purely x and y components.split forces into purely x and y components. FFnn is completely in the y direction. is completely in the y direction. FFww needs to be split into a F needs to be split into a Fwxwx and a F and a Fwywy

y

x

Inclined PlanesInclined Planes The block is not accelerating in the y direction so FThe block is not accelerating in the y direction so Fnetynety=0N=0N Therefore FTherefore Fn n = F= Fwywy

The force that is causing the block to accelerate in the x direction The force that is causing the block to accelerate in the x direction is Fis Fwx.wx.

Therefore FTherefore Fnet net = F= Fwxwx

y

x

Fn

Fwy

Fwx

Inclined Planes (Math)Inclined Planes (Math) So what does FSo what does Fwxwx equal? equal?

y

x

Fn

Fwy

Fwx

Can you tell me the acceleration of the cart?

Practice ProblemsPractice Problems1.1. What is the acceleration of a cart rolling What is the acceleration of a cart rolling

down a plane inclined at 30down a plane inclined at 30°?°?y

x

What is the normal force acting on a 2kg What is the normal force acting on a 2kg cart rolling down a cart rolling down a plane inclined at plane inclined at 3030°?°?

The

sam

e as

Fw

The

sam

e as

Fwx

The

sam

e as

Fwy

The

sam

e as

Ff

0% 0%0%0%

1.1. The same as FThe same as Fww

2.2. The same as FThe same as Fwxwx

3.3. The same as FThe same as Fwywy

4.4. The same as FThe same as Fff

0 of 20

Practice ProblemsPractice Problems2.2. What is the normal force acting on a 2kg What is the normal force acting on a 2kg

cart rolling down a cart rolling down a plane inclined at 30plane inclined at 30°?°?y

x

Practice ProblemsPractice Problems3.3. A cart is accelerating down an incline at a A cart is accelerating down an incline at a

rate of 2m/srate of 2m/s22. What is the angle of the . What is the angle of the incline?incline?

y

x

Distribution CompanyDistribution Company Complete this problem showing all work Complete this problem showing all work

and explaining your process into finding and explaining your process into finding the angle of the incline. the angle of the incline.

Lesson #34Lesson #34Topic: Incline planes and frictionTopic: Incline planes and friction

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

1.1. Solve net force problems involving inclines Solve net force problems involving inclines and frictionand friction

11/30/06

Assignment: New Wikispace post!!!

Project: Observe a wood block at rest on an inclined plane. Discuss with a partner why the block remains at rest? Is there a net force acting on the block? Why or why not?

Force of FrictionForce of Friction The force due to friction always opposes the The force due to friction always opposes the

direction that the object wants to move. direction that the object wants to move. Even if the object is not yet moving.Even if the object is not yet moving. Example: I push on the side of a 5kg block with a Example: I push on the side of a 5kg block with a

force of 10N and the block remains at rest. What force of 10N and the block remains at rest. What is the force of friction acting on the block?is the force of friction acting on the block?

Fa = 10NFf = 10N

Force of FrictionForce of Friction Example #2: I push on the side of a 5kg block Example #2: I push on the side of a 5kg block

with a force of 10N and the block accelerates with a force of 10N and the block accelerates at a rate of 1m/sat a rate of 1m/s22. What is the force of friction . What is the force of friction acting on the block?acting on the block?

Fa = 10NFf = ?

Warm UpWarm UpA 5kg block is at rest on an inclined plane angled at 30° to A 5kg block is at rest on an inclined plane angled at 30° to

the horizontal. What is the force of friction acting on the the horizontal. What is the force of friction acting on the block?block?

Draw a picture of the incline and blockDraw a picture of the incline and block Draw all of the forces acting on the blockDraw all of the forces acting on the block Split forces up into pure x and y componentsSplit forces up into pure x and y components

y

x

Warm Up SolutionWarm Up Solutiony

x

Fn

Fwy

Fwx

Ff

A block is at rest on an inclined plane angled at 30° to the horizontal. What is the force of friction acting on the block?

0 of 20

40N

25N

68.

3N

It’s

the

sam

e as

Fwx

0% 0%0%0%

1.1. 40N40N

2.2. 25N25N

3.3. 68.3N68.3N

4.4. It’s the same as It’s the same as FFwxwx

A 2kg block slides down a plane inclined A 2kg block slides down a plane inclined at 20at 20° at° at constant speed. What is constant speed. What is the force of friction acting on the the force of friction acting on the block? block?

20N

10N

18.

8N

6.8

4N

0% 0%0%0%

1.1. 20N20N

2.2. 10N10N

3.3. 18.8N18.8N

4.4. 6.84N6.84N

0 of 20

Practice ProblemsPractice Problems1.1. A 2kg block slides down a plane inclined A 2kg block slides down a plane inclined

at 30at 30° with a friction force of 7N acting on ° with a friction force of 7N acting on the block. What is the acceleration of the the block. What is the acceleration of the block?block?

y

x

Practice ProblemsPractice Problems2.2. An applied force pulls a 2kg block up a An applied force pulls a 2kg block up a

plane inclined at plane inclined at 3030° at a constant speed. ° at a constant speed. If the force of friction is still 7N, what is the If the force of friction is still 7N, what is the magnitude of the applied force? magnitude of the applied force?

y

x

Lesson #35Lesson #35Topic: Lab: Friction ForceTopic: Lab: Friction Force

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

1.1. Compare the force due to friction acting on Compare the force due to friction acting on objects of different weight, size, and objects of different weight, size, and material.material.

2.2. Determine what variables affect the force of Determine what variables affect the force of friction friction

12/1/06

Assignment: Lab Report Due at the end of the period Thursday.

Lab Task: Make predictions about the force due to friction and then collect data to compare calculations to your predictions.

Lab Question: What variables affect friction force?