ch. 4 pptnoics.ppt [Read-Only] - VMPS Staff...

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Forces and the Laws of MotionChapter 4

Table of Contents

Section 1 Changes in Motion

Section 2 Newton's First Law

Section 3 Newton's Second and ThirdLaws

Section 4 Everyday Forces

Section 1 Changes in MotionChapter 4

Objectives Describe how force affects the motion

of an object.

Interpret and construct free bodydiagrams.

Chapter 4

Force A force is a push or

a pull.

Forces can causeaccelerations.

The SI unit:newton, N.

Forces can actthrough contact orat a distance.


Image:http://www.mcasco.com/images/newton2.gif

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Chapter 4

Force Diagrams Effect of a force depends on both magnitude

and direction. Thus, force is a vectorquantity.

Force diagrams: Diagrams that show forcevectors as arrows.

Free-body diagrams (FBD): Forcediagrams that show only the forces acting ona single object.


Chapter 4

Force Diagrams, continued

In a force diagram,vector arrowsrepresent all theforces acting in asituation.

Section 1 Changes inMotion

A free-body diagramshows only the forcesacting on the object ofinterest—in this case,the car.

Force Diagram Free-Body Diagram

Section 2 Newton’s First LawChapter 4

Objectives Explain the relationship between the

motion of an object and the net externalforce acting on the object.

Determine the net external force on anobject.

Calculate the force required to bring anobject into equilibrium.

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Chapter 4

An object at rest remains at rest, and anobject in motion continues in motionwith constant velocity, unless the objectexperiences a net external force.

When the net force is zero, theacceleration is zero.

Section 2 Newton’s First Law

Newtons’s 1st Law (p. 130)

Chapter 4

Net Force The net force is the vector sum of all

forces acting on an object.


Chapter 4

Inertia Inertia: the tendency of an object to

resist being moved or, if the object ismoving, to resist a change in speed ordirection.

Newton’s first law is often referred toas the law of inertia

Mass is a measure of inertia.


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Chapter 4Inertia and the Operation of a Seat

Belt


Chapter 4

Equilibrium Equilibrium: the net force on an object is

zero.

At rest/moving with constant velocity.

Newton’s first law describes objects inequilibrium.


Practice Problems:

P. 133: #1) A man is pulling on his dogwith a force of 70.0 N directed at an angle of+30.0 degrees to the horizontal. Find the xand y components of this force.

Answer: Fx=60.6 N; Fy=25N

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Practice Problems:

P. 133: #2) A crate is pulled to the right with aforce of 82.0 N, to the left with a force of 115 N,upward with a force of 565 N, and downwardwith a force of 246 N

a. Find the net external force in the x direction.b. Find the net external force in the y direction.c. Find the magnitude and direction of the net external

force.

Answer: a. -33N; b. 329 N; c. 330 N at 96degrees counterclockwise from the positive x-axis.

Practice Problems: P. 133: #3) A gust of wind blows an apple from a

tree. As the apple falls, the force of gravity on theapple is 9.25 N downward, and the force of the windon the apple is 1.05 N to the right. Find themagnitude and direction of the net external force onthe apple.

Answer: 9.31 N at 6.48 degrees counterclockwisefrom straight down.

Chapter 4

Sample Problem p. 132

Determining Net Force Derek leaves his physics book on top of a drafting

table that is inclined at a 35° angle. The free-bodydiagram below shows the forces acting on the book.Find the net force acting on the book.

Section 2 Newton’s FirstLaw

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Section 3 Newton’s Second andThird LawsChapter 4

Objectives Describe an object’s acceleration in terms of

its mass and the net force acting on it.

Predict the direction and magnitude of theacceleration caused by a known net force.

Identify action-reaction pairs.

Chapter 4Newton’s Second

Law The acceleration ofan object is directlyproportional to thenet force acting onthe object andinverselyproportional to theobject’s mass.

ΣF = manet force = mass ×

acceleration

Section 3 Newton’s Second andThird Laws

Image:http://www.lersus.de/res/modules/enu/physics/1/res/files/newtons2law_0__0_.gif

Newton’s 2nd Law Movie

Chapter 4Newton’s Third Law

For every action, there is an equal and oppositereaction.

Forces exist in pairs-action & reaction Action/Reaction forces are equal & opposite, but

don’t necessarily cancel each other out.


Image:http://exploration.grc.nasa.gov

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Chapter 4Newton’s Third Law


Sample Problem Two groups of canoeists meet in the middle of a lake. After

a brief visit, a person in canoe 1 pushes on canoe 2 with aforce of 46 N to separate the canoes. If the mass of canoe1 and its occupants is m1=150 kg, and the mass of canoe 2and its occupants is m2=250 kg, find the acceleration thepush gives to each canoe.

Answer: canoe 2 = 0.18 m/s2; canoe 1 = -0.31 m/s2

Image: http://blog.rv.net/wp-content/canoe-coe-2488-11-5-canoes.jpg

Section 4 EverydayForces

Chapter 4

Image: http://universeadventure.org/fundamentals/popups/gravit-dtrh-massvweight.htm

Section 4 EverydayForces

Chapter 4

Objectives

Explain the difference between mass andweight.

Find the direction and magnitude of normalforces.

Describe air resistance as a form of friction.

Use coefficients of friction to calculatefrictional force.

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Chapter 4Weight


gravitational force (Fg): vector.

The magnitude of this force (Fg) is a scalarquantity called weight.

Calculating weight at any location: Fg = mag ag = free-fall acceleration at that location

Calculating weight on Earth's surface: ag = g = 9.81 m/s2

Fg = mg = m(9.81 m/s2)

Chapter 4

Normal Force


Normal force acts perpendicular to surface. The normal force is not always opposite in

direction to the force due to gravity.

– In the absence of otherforces, the normal force isequal and opposite to thecomponent of gravitationalforce that is perpendicular tothe contact surface.

– In this example, Fn = mg cosθ.

Chapter 4Friction


Static friction resists theinitiation of sliding motionbetween two surfaces that are incontact and at rest.

Kinetic friction:force thatopposes the movement of twosurfaces that are in contact andare sliding over each other.

Kinetic friction is always lessthan the maximum staticfriction. Drawn opposite tomotion direction.

Image: http://www.inlineonline.co.uk/

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Chapter 4

The Coefficient of Friction


the coefficient of friction, µ, expresses thedependence of frictional forces on the particularsurfaces in contact

Coefficient of kinetic friction:

Coefficient of static friction:

Image: http://seniorjournal.com/images

Chapter 4 Section 4 Everyday Forces

Chapter 4

Sample Problem p. 146

Overcoming Friction A student attaches a rope to a 20.0 kg box of

books.He pulls with a force of 90.0 N at an angle of30.0° with the horizontal. The coefficient of kineticfriction between the box and the sidewalk is 0.500.Find the acceleration of the box.


Answer:a=0.12 m/s2 tothe right

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Chapter 4

Practice Problems 4C p. 145

Coefficients of friction1. A 25 kg chair initially at rest on a horizontal floor requires

a 365 N horizontal force to set it in motion. Once thechair is in motion, a 327 N horizontal force keeps itmoving at a contant velocity.

a. Fine the coefficient of static friction between the chair and the floor.b. Find the coeficient of kinetic friction between the chair and the floor.


Answers: a.1.5

b. 1.3

Image:http://cache.gizmodo.com/assets/resources/2006/11/Tank%20Chair.jpg

Chapter 4

Practice Problems 4C p. 145Coefficients of friction3. A museum curator moves artifacts into place on many

different display surfaces. Use the values in Table 4-2 onp. 144 to find Fs, max and Fk for the following situations:

a. Moving a 145 kg aluminum sculpture across a horizontal steel platformb. Pulling a 15 kg steel sword across a horizontal steel shieldc. Pushing a 250 kg wood bed on a wood floord. Sliding a 0.55 kg glass amulet on a glass display case


Answers:

a. 8.7 x 102 N; 6.7 x102N

b. 110 N, 84 N

c. 1000 n, 500 N

d. 5N, 2N

Chapter 4

Practice Problems 4D p. 147Overcoming friction1. A student moves a box of books down the hall by pulling

on a rope attached to the box. The students pulls with aforce of 185 N at an angle of 25.0° above the horizontal.The box has a mass of 35.0 kg and the coefficient ofkinetic friction between the box and the floor is 0.27.Find the acceleration of the box.


Answer: 2.7 m/s2 in thepositive x direction

http://weblog.greenpeace.org/oceandefenders/archive/060207myez_nadia-rope_ADavies_sm.jpg

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Chapter 4

Practice Problems 4D p. 147Overcoming friction2. The student in item 1 moves the box up a ramp inclined at

12° with the horizontal. If the box starts from rest at thebottom of the ramp and is pulled at an angle of 25.0° withrespect to the incline and with the same 185 N force,what is the acceleration up the ramp? Assume thecoefficient of kinetic friction is 0.27.


Answer: 0.77 m/s2 up theramp

http://weblog.greenpeace.org/oceandefenders/archive/060207myez_nadia-rope_ADavies_sm.jpg

Chapter 4

Practice Problems 4D p. 147Overcoming friction3. A 75 kg box slides down a 25.0º ramp with an acceleration of

3.60 m/s2.a. Find the coefficient of kinetic friction between the box and the ramp.b. What acceleration would a 175 kg box have on this ramp?


Answer: a. 0.061

b. 3.61 m/s2

down the ramp

Image:http://www.spawinch.com/assets/felger6.jpg

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