HP_04_win

8/8/2019 HP_04_win

1/85

Copyright by Holt, Rinehart and Winston. All rights reserved.

ResourcesChapter menu

To View the presentation as a slideshow with effectsselect View on the menu bar and click on Slide Show.

To advance through the presentation, click the right-arrow

key or the space bar.

From the resources slide, click on any resource to see a

presentation for that resource.

From the Chapter menu screen click on any lesson to go

directly to that lessons presentation.

You may exit the slide show at any time by pressing

the Esc key.

How to Use This Presentation

8/8/2019 HP_04_win

2/85

8/8/2019 HP_04_win

3/85



Forces and the Laws of MotionChapter4

Table of Contents

Section 1 Changes in Motion

Section 2 Newton's First Law

Section 3 Newton's Second and Third Laws

Section 4 Everyday Forces

8/8/2019 HP_04_win

4/85



Section 1 Changes in MotionChapter4

Objectives

Describe how force affects the motion of an object.

Interpret and construct free body diagrams.

8/8/2019 HP_04_win

5/85



Chapter4

Force


8/8/2019 HP_04_win

6/85



Chapter4

Force

A force is an action exerted on an object which may

change the objects state of rest or motion.

Forces can cause accelerations.

The SI unit of force is the newton, N.

Forces can act through contact or at a distance.


8/8/2019 HP_04_win

7/85



Chapter4

Comparing Contact and Field Forces


8/8/2019 HP_04_win

8/85



Chapter4

Force Diagrams

The effect of a force depends on both magnitude

and direction.Thus, force is a vectorquantity.

Diagrams that show force vectors as arrows arecalled force diagrams.

Force diagrams that show only the forces acting on a

single object are called free-body diagrams.


8/8/2019 HP_04_win

9/85



Chapter4

Force Diagrams, continued

In a force diagram, vector

arrows represent all the

forces acting in a

situation.


A free-body diagram showsonly the forces acting onthe object of interestinthis case, the car.

Force Diagram Free-Body Diagram

8/8/2019 HP_04_win

10/85



Chapter4

Drawing a Free-Body Diagram


8/8/2019 HP_04_win

11/85



Section 2 Newtons First LawChapter4

Objectives

Explain the relationship between the motion of an

object and the net external force acting on the object.

Determine the net external force on an object.

Calculate the force required to bring an object into

equilibrium.

8/8/2019 HP_04_win

12/85



Chapter4

Newtons First Law

An object at rest remains at rest, and an object in

motion continues in motion with constant velocity

(that is, constant speed in a straight line) unless the

object experiences a net external force.

In other words, when the net external force on an

object is zero, the objects acceleration (or the

change in the objects velocity) is zero.

Section 2 Newtons First Law

8/8/2019 HP_04_win

13/85



Chapter4

Net Force

Newton's first law refers to the net force on an

object.The net force is the vector sum of all forces

acting on an object.

The net force on an object can be found by using themethods for finding resultant vectors.


Although several forces are

acting on this car, the vector sumof the forces is zero. Thus, the

net force is zero, and the car

moves at a constant velocity.

8/8/2019 HP_04_win

14/85



Chapter4

Sample Problem

Determining Net Force

Derekleaves his physics book on top of a drafting

table that is inclined at a 35 angle. The free-body

diagram below shows the forces acting on the book.

Find the net force acting on the book.


8/8/2019 HP_04_win

15/85



Chapter4

Sample Problem, continued


1. Define the problem, and identify the variables.

Given:

Fgravity-on-book= Fg= 22 NFfriction = Ff= 11 N

Ftable-on-book= Ft= 18 N

Unknown:Fnet= ?

8/8/2019 HP_04_win

16/85



Chapter4



2. Select a coordinate system, and apply it to the

free-body diagram.

Tip: To simplify the prob

lem, a

lwayschoose the coordinate system in

which as many forces as possible lie

on the x- and y-axes.

Choose thex-axis parallel to and the y-axis perpendicular to

the incline of the table, as shown in (a). This coordinate

system is the most convenient because only one force needs

to be resolved intoxand ycomponents.

8/8/2019 HP_04_win

17/85



Chapter4



3. Find the xand ycomponents of all vectors.

Add both components to the free-body diagram, as shown in (c).

cos U !Fg,x

Fg

Fg,x ! Fgcos U

Fg,x ! (22 N)(cos 55r)

Fg,x ! 13 N

sin U !Fg,y

Fg

Fg,y ! Fgsin U

Fg,x ! (22 N)(sin 55r)

Fg,x ! 18 N

Draw a sketch, as shown in (b), to help find

the components of the vectorFg. The angle U

is equal to 180r 90r 35r = 55r.

8/8/2019 HP_04_win

18/85



Chapter4



For the ydirection:

7Fy= Ft Fg,y7Fy= 18 N 18 N

7Fy= 0 N

4. Find the net force in both the xand ydirections.

Diagram (d) shows another free-body

diagram of the book, now with forcesacting only along thex- and y-axes.

For the xdirection:

7Fx= Fg,x Ff7Fx= 13 N 11 N

7Fx= 2 N

8/8/2019 HP_04_win

19/85



Chapter4



5. Find the net force.Add the net forces in thexand ydirections together asvectors to find the total net force. In this case, Fnet = 2 N in

the +xdirection, as shown in (e). Thus, the book acceleratesdown the incline.

8/8/2019 HP_04_win

20/85



Chapter4

Inertia

Inertia is the tendency of an object to resist beingmoved or, if the object is moving, to resist a changein speed or direction.

Newtons first law is often referred to as the law ofinertia because it states that in the absence of a netforce, a body will preserve its state of motion.

Mass is a measure of inertia.


8/8/2019 HP_04_win

21/85



Chapter4

Mass and Inertia


8/8/2019 HP_04_win

22/85



Chapter4

Inertia and the Operation of a Seat Belt

While inertia causespassengers in a car tocontinue moving forward asthe car slows down, inertia

also causes seat belts to lockinto place.

The illustration shows howone type of shoulder harnessoperates.

When the car suddenly slowsdown, inertia causes the largemass under the seat tocontinue moving, whichactivates the lock on thesafety belt.


8/8/2019 HP_04_win

23/85



Chapter4

Equilibrium

Equilibrium is the state in which the net force on anobject is zero.

Objects that are eitherat rest or moving withconstant velocity are said to be in equilibrium.

Newtons first law describes objects in equilibrium.

Tip: To determine whether a body is in equilibrium, find the netforce. If the net force is zero, the body is in equilibrium. If thereis a net force, a second force equaland opposite to this netforce willput the body in equilibrium.


8/8/2019 HP_04_win

24/85



Section 3 Newtons Second and

Third LawsChapter4

Objectives

Describe an objects 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.

8/8/2019 HP_04_win

25/85



Chapter4

Newtons Second Law

The acceleration of an object is directlyproportional to the net force acting on theobject and inversely proportional to the

objects mass.

7F = ma

net force = mass v acceleration


Third Laws

7F represents the vector sum of allexternal forces

acting on the object, or the net force.

8/8/2019 HP_04_win

26/85



Chapter4

Newtons Second Law


Third Laws

8/8/2019 HP_04_win

27/85



Chapter4

Newtons Third Law If two objects interact, the magnitude of the force

exerted on object 1 by object 2 is equal to themagnitude of the force simultaneously exerted on

object 2 by object 1, and these two forces areopposite in direction.

In other words, for every action, there is anequal and opposite reaction.

Because the forces coexist, either force can becalled the action or the reaction.


Third Laws

8/8/2019 HP_04_win

28/85



Chapter4

Action and Reaction Forces

Action-reaction pairs do not imply that the net

force on either object is zero.

The action-reaction forces are equal and opposite,

but either object may still have a net force on it.


Third Laws

Consider driving a nail into wood with

a hammer. The force that the nail

exerts on the hammer is equaland

opposite to the force that the hammerexerts on the nail. But there is a net

force acting on the nail, which drives

the nail into the wood.

8/8/2019 HP_04_win

29/85



Chapter4

Newtons Third Law


Third Laws

8/8/2019 HP_04_win

30/85



Section 4 Everyday ForcesChapter4

Objectives

Explain the difference between mass and weight.

Find the direction and magnitude of normal forces.

Describe air resistance as a form of friction.

Use coefficients of friction to calculate frictional force.

8/8/2019 HP_04_win

31/85



Chapter4

Weight


The gravitational force (Fg) exerted on an object

by Earth is a vectorquantity, directed toward the

center of Earth.

The magnitude of this force (Fg) is a scalar

quantity called weight.

Weight changes with the location of an object inthe universe.

8/8/2019 HP_04_win

32/85



Chapter4

Weight, continued


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)

8/8/2019 HP_04_win

33/85



Chapter4

Comparing Mass and Weight


8/8/2019 HP_04_win

34/85



Chapter4

Normal Force


The normal force acts on a surface in a directionperpendicularto the surface.

The normal force is not always opposite indirection to the force due to gravity.

In the absence of other forces, the

normal force is equal and opposite

to the component of gravitational

force that is perpendicular to thecontact surface.

In this example, Fn = mgcos U.

8/8/2019 HP_04_win

35/85



Chapter4

Normal Force


8/8/2019 HP_04_win

36/85



Chapter4

Friction


Static friction is a force that resists the initiation

of sliding motion between two surfaces that are in

contact and at rest.

Kinetic friction is the force that opposes the

movement of two surfaces that are in contact and

are sliding over each other.

Kinetic friction is always less than the maximum

static friction.

8/8/2019 HP_04_win

37/85



Chapter4

Friction


8/8/2019 HP_04_win

38/85



Chapter4

Friction Forces in Free-Body Diagrams


In free-body diagrams, the force of friction is always

parallel to the surface of contact.

The force ofkinetic friction is always opposite the

direction of motion.

To determine the direction of the force ofstatic

friction, use the principle of equilibrium. For anobject in equilibrium, the frictional force must point

in the direction that results in a net force of zero.

8/8/2019 HP_04_win

39/85



Chapter4

The Coefficient of Friction


The quantity that expresses the dependence offrictional forces on the particular surfaces incontact is called the coefficient of friction, Q.

Coefficient of kinetic friction:

Qk !Fk

Fn

Qs !Fs,max

Fn

Coefficient of static friction:

8/8/2019 HP_04_win

40/85



Chapter4 Section 4 Everyday Forces

8/8/2019 HP_04_win

41/85



Chapter4

Sample Problem

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 of

30.0 with the horizontal. The coefficient of kineticfriction between the box and the sidewalk is 0.500.

Find the acceleration of the box.


8/8/2019 HP_04_win

42/85

8/8/2019 HP_04_win

43/85



Chapter4



The diagram on the right shows the

most convenient coordinate system,

because the only force to resolve

into components is Fapplied.

2. Plan

Choose a convenient coordinate system, andfind the xand ycomponents of all forces.

Fapplied,y= (90.0 N)(sin 30.0) = 45.0 N (upward)

Fapplied,x= (90.0 N)(cos 30.0) = 77.9 N (to the right)

8/8/2019 HP_04_win

44/85



Chapter4



Choose an equation or situation:

A. Find the normal force, Fn, by applying the condition of

equilibrium in the vertical direction:

7Fy= 0

B. Calculate the force of kinetic friction on the box:

Fk= QkFn

C.Apply Newtons second law along the horizontal direction tofind the acceleration of the box:

7Fx= max

8/8/2019 HP_04_win

45/85



Chapter4



3. CalculateA. To apply the condition of equilibrium in the vertical direction,

you need to account for all of the forces in the ydirection:

Fg, Fn, and Fapplied,y. You know Fapplied,yand can use the boxs

mass to find Fg.Fapplied,y= 45.0 N

Fg= (20.0 kg)(9.81 m/s2) = 196 N

Next, apply the equilibrium condition,

7Fy= 0, and solve forFn.7Fy= Fn + Fapplied,y Fg= 0

Fn + 45.0 N 196 N = 0

Fn = 45.0 N + 196 N = 151 N

Tip: Remember to

pay attention to the

direction of forces.

In this step, Fg issubtracted from Fnand Fapplied,ybecause Fg is

directed downward.

8/8/2019 HP_04_win

46/85



Chapter4



B. Use the normal force to find the force of kinetic friction.

Fk= QkFn = (0.500)(151 N) = 75.5 N

C. Use Newtons second law to determine the horizontal

acceleration.

ax !applied,x Fk

m !

77.9 75.5

20.0 kg !

2.4

20.0 kg !

2.4 kg y m/s2

20.0 kg

a = 0.12 m/s2 to the right

7Fx ! Fapplied Fk ! max

8/8/2019 HP_04_win

47/85



Chapter4



4. EvaluateThe box accelerates in the direction of the netforce, in accordance with Newtons second law.The normal force is not equal in magnitude to theweight because the ycomponent of the studentspull on the rope helps support the box.

8/8/2019 HP_04_win

48/85



Chapter4

Air Resistance


Air resistance is a form offriction. Whenever an

object moves through a fluid medium, such as air or

water, the fluid provides a resistance to the objects

motion.

For a falling object, when the upward force of air

resistance balances the downward gravitational

force, the net force on the object is zero. The objectcontinues to move downward with a constant

maximum speed, called the terminal speed.

8/8/2019 HP_04_win

49/85



Chapter4

Fundamental Forces


There are four fundamental forces:

Electromagnetic force

Gravitational force

Strong nuclear force

Weak nuclear force

The four fundamental forces are all field forces.

8/8/2019 HP_04_win

50/85



Multiple Choice

Standardized Test PrepChapter4

a !F

m1

a !F

m2

a !F

m1 m2

a !F

(m1)(m2 )

Use the passage below to answer questions 12.

Two blocks of masses m1 and m2are placed in contact with eachother on a smooth, horizontal surface. Block m1 is on the left ofblock m2. A constant horizontal force Fto the right is applied to

m1.

1. What is the acceleration of the two blocks?

A. C.

B. D.

8/8/2019 HP_04_win

51/85



Multiple Choice


a !F

m1

a !F

m2

a !F

m1 m2

a !F

(m1)(m2 )

Use the passage below to answer questions 12.

Two blocks of masses m1 and m2are placed in contact with eachother on a smooth, horizontal surface. Block m1 is on the left ofblock m2. A constant horizontal force Fto the right is applied to

m1.

1. What is the acceleration of the two blocks?

A. C.

B. D.

8/8/2019 HP_04_win

52/85



Multiple Choice, continuedUse the passage below to answer questions 12.

Two blocks of masses m1 and m2 are placed in contact with eachother on a smooth, horizontal surface. Block m1 is on the left ofblock m2. A constant horizontal force Fto the right is applied to

m1.

2. What is the horizontal force acting on m2?

F. m1a

G. m2a

H. (m1 + m2)aJ. m1m2a


8/8/2019 HP_04_win

53/85




Two blocks of masses m1 and m2 are placed in contact with eachother on a smooth, horizontal surface. Block m1 is on the left ofblock m2. A constant horizontal force Fto the right is applied to

m1.

2. What is the horizontal force acting on m2?

F. m1a

G. m2a

H. (m1 + m2)aJ. m1m2a


8/8/2019 HP_04_win

54/85



Multiple Choice, continued


3. A crate is pulled to the right with a force of 82.0 N, to the leftwith a force of 115 N, upward with a force of 565 N, anddownward with a force of 236 N. Find the magnitude anddirection of the net force on the crate.

A. 3.30 N at 96 counterclockwise from the positive x-axis

B. 3.30 N at 6 counterclockwise from the positive x-axis

C. 3.30 x 102at 96 counterclockwise from the positive x-axis

D. 3.30 x 102at 6 counterclockwise from the positive x-axis

8/8/2019 HP_04_win

55/85

8/8/2019 HP_04_win

56/85





4. A ball with a mass ofm is thrown into the air, as shown in thefigure below. What is the force exerted on Earth by the ball?

A.mba

llgdirected down

B.mballgdirected up

C.mearthgdirected down

D.mearthgdirected up

8/8/2019 HP_04_win

57/85





4. A ball with a mass ofm is thrown into the air, as shown in thefigure below. What is the force exerted on Earth by the ball?

A.mba

llgdirected down

B.mballgdirected up

C.mearthgdirected down

D.mearthgdirected up

8/8/2019 HP_04_win

58/85





5. A freight train has a mass of 1.5 x 107 kg. If the locomotivecan exert a constant pull of 7.5 x 105 N, how long would it taketo increase the speed of the train from rest to 85 km/h?(Disregard friction.)

A. 4.7 x 102s

B. 4.7s

C. 5.0 x 10-2s

D. 5.0 x 104s

8/8/2019 HP_04_win

59/85





5. A freight train has a mass of 1.5 x 107 kg. If the locomotivecan exert a constant pull of 7.5 x 105 N, how long would it taketo increase the speed of the train from rest to 85 km/h?(Disregard friction.)

A. 4.7 x 102s

B. 4.7s

C. 5.0 x 10-2s

D. 5.0 x 104s

S d di d P

8/8/2019 HP_04_win

60/85




A truck driver slams on the brakes and skids

to a stop through a displacement (x.


6. If the trucks mass doubles, find the trucks skidding distance interms of(x. (Hint: Increasing the mass increases the normalforce.)

A. (x/4

B. (x

C. 2(xD. 4(x

St d di d T t P

8/8/2019 HP_04_win

61/85







6. If the trucks mass doubles, find the trucks skidding distance interms of(x. (Hint: Increasing the mass increases the normalforce.)

A. (x/4

B. (x

C. 2(xD. 4(x

St d di d T t P

8/8/2019 HP_04_win

62/85







7. If the trucks initial velocity were halved, what would be thetrucks skidding distance?

A. (x/4

B. (x

C. 2(x

D. 4(x

St d di d T t P

8/8/2019 HP_04_win

63/85







7. If the trucks initial velocity were halved, what would be thetrucks skidding distance?

A. (x/4

B. (x

C. 2(x

D. 4(x

St d di d T t P

8/8/2019 HP_04_win

64/85





8. What is the relationship between the forces at point A?

F. Fs=FappliedG. Fk=Fapplied

H. FsFapplied

Use the graph at right toanswer questions 89. Thegraph shows the relationshipbetween the applied force

and the force of friction.

Standardi ed Test Prep4

8/8/2019 HP_04_win

65/85





8. What is the relationship between the forces at point A?

F. Fs=FappliedG. Fk=Fapplied

H. FsFapplied

Use the graph at right toanswer questions 89. Thegraph shows the relationshipbetween the applied force

and the force of friction.

Standardized Test Prep4

8/8/2019 HP_04_win

66/85





9. What is the relationship between the forces at point B?

A. Fs, max=FkB. Fk> Fs, max

C. Fk>FappliedD. Fk

8/8/2019 HP_04_win

67/85





9. What is the relationship between the forces at point B?

A. Fs, max=FkB. Fk> Fs, max

C. Fk>FappliedD. Fk

8/8/2019 HP_04_win

68/85



Short ResponseBase youranswers to questions 1012 on the

information below.

A 3.00 kg ball is dropped from rest from the

roof of a building 176.4 m high.While the ballis falling, a horizontal wind exerts a constant

force of 12.0 N on the ball.

10.How long does the ball take to hit the ground?


Standardized Test PrepCh t 4

8/8/2019 HP_04_win

69/85



Short ResponseBase youranswers to questions 1012 on the

information below.




10.How long does the ball take to hit the ground?Answer: 6.00 s



8/8/2019 HP_04_win

70/85



Short Response, continued


Base youranswers to questions 1012 on the

information below.




11. How far from the building does the ball hit the ground?


8/8/2019 HP_04_win

71/85




information below.




11. How far from the building does the ball hit the ground?Answer: 72.0 m




8/8/2019 HP_04_win

72/85




information below.




12. When the ball hits the ground, what is its speed?




8/8/2019 HP_04_win

73/85




information below.




12. When the ball hits the ground, what is its speed?Answer: 63.6 m/s




8/8/2019 HP_04_win

74/85




passage.A crate rests on the horizontal bed of a pickup truck.

For each situation described below, indicate themotion of the crate relative to the ground, the motion ofthe crate relative to the truck, and whether the cratewill hit the front wall of the truck bed, the back wall, orneither. Disregard friction.

13. Starting at rest, the truck accelerates to the right.




8/8/2019 HP_04_win

75/85




Short Response, continuedBase youranswers to questions 1315 on the



13. Starting at rest, the truck accelerates to the right.

Answer: at rest, moves to the left, hits back wall

Standardized Test PrepChapter 4

8/8/2019 HP_04_win

76/85




Short Response, continuedBase youranswers to questions 1315 on the



14. The crate is at rest relative to the truck while the

truck moves with a constant velocity to the right.


8/8/2019 HP_04_win

77/85






14. The crate is at rest relative to the truck while the

truck moves with a constant velocity to the right.

Answer: moves to the right, at rest, neither




8/8/2019 HP_04_win

78/85






15. The truck in item 14 slows down.




8/8/2019 HP_04_win

79/85






15. The truck in item 14 slows down.

Answer: moves to the right, moves to the right,

hits front wall




8/8/2019 HP_04_win

80/85



16.A student pulls a rope attached to a 10.0 kg wooden

sled and moves the sled across dry snow. The student

pulls with a force of 15.0 N at an angle of 45.0.

IfQk

between the sled and the snow is 0.040, what

is the sleds acceleration? Show your work.

pChapter4

Extended Response


8/8/2019 HP_04_win

81/85



16.A student pulls a rope attached to a 10.0 kg wooden

sled and moves the sled across dry snow. The student

pulls with a force of 15.0 N at an angle of 45.0.

IfQk

between the sled and the snow is 0.040, what

is the sleds acceleration? Show your work.

Answer: 0.71 m/s2

pChapter4

Extended Response


8/8/2019 HP_04_win

82/85



pChapter4

Extended Response,continued

17. You can keep a 3 kg book from dropping by pushing

it horizontally against a wall. Draw force diagrams,

and identify all the forces involved. How do they

combine to result in a zero net force? Will the force

you must supply to hold the book up be different for

different types of walls? Design a series ofexperiments to test your answer. Identify exactly

which measurements will be necessary and what

equipment you will need.


8/8/2019 HP_04_win

83/85



17. You can keep a 3 kg book from dropping by pushing

it horizontally against a wall. Draw force diagrams,

and identify all the forces involved. How do they

combine to result in a zero net force? Will the force

you must supply to hold the book up be different for

different types of walls? Design a series ofexperiments to test your answer. Identify exactly

which measurements will be necessary and what

equipment you will need.

Answer: Plans should involve measuring forces such

as weight, applied force, normal force, and friction.

pChapter4

Extended Response, continued

Chapter 4 Section 1 Changes in Motion

8/8/2019 HP_04_win

84/85



Chapter4

Force Diagrams

g

8/8/2019 HP_04_win

85/85

Date post:	09-Apr-2018
Category:	Documents
Upload:	jlvmrbd777
View:	214 times
Download:	0 times

HP_04_win

Documents