chapter 5. force and motion

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.

Chapter 5. Force and MotionChapter 5. Force and Motion

Chapter Goal: To establish a connection between force and motion.


Student Learning Objectives

• To recognize what does and does not constitute a force.

• To identify the specific forces acting on an object.• To draw an accurate free-body diagram of an object.• To begin the process of understanding the connection

between force and motion.• To begin learning how to explain an observation on

the basis of physical principles.


Changes to Workbook homework due Thursday 9/30 and Tuesday 10/5

• Omit section 5.3 #3-8 • REPLACE omitted section with section 5.7, #24-29,

which is now due Thursday 9/30• Tuesday 10/5, sections 5.5 and 5.6 due


A force is an interaction between two objects

• A force is a push or a pull on an object.

•If I push a book across a table, the book pushes me back (inanimate objects can exert force!)

• A force is a vector. It has both a magnitude and a direction.

•The force (interaction) has the same magnitude for both me and the book. However the direction of the force on me is opposite to the direction of the force on the book.


A force is an interaction between two objects

•A force requires an agent. Another object does the pushing or pulling.

•This is another way of confirming that a force is an interaction between objects. It takes two to tango!

• A force is either a contact force or a long- range force.

•Gravity is the only long-range force we will study this semester.

•All other forces only exist when the two objects are in contact.


Tactics: Drawing force vectors


Combining Forces


Two of three forces exerted on an object are shown. The net force points to the left. What is the missing third force? The magnitudes of (a) and (c) are greater than the magnitudes of (b) and (d).


A Short Catalog of Forces - Gravity

• Gravity is a long-range attractive force between two objects.

•In this class, our emphasis is on the interaction between the Earth and objects on or near its surface (the weight force).


Spring Force


Tension (tension pulls it never pushes)


Tension


Normal Force (That’s perpendicular, as opposed to not weird)


Friction – friction reacts, but never overreacts


Thrust – rockets and airplanes*

*The thrust of a non-jet airplane is due to the air from the prop


Air resistance (Drag)


You’ve just kicked a rock, and it is now sliding across the ground about 2 meters in front of you. Which of these forces is NOT acting on the rock?

A. Gravity, acting downwardB. The normal force, acting upwardC. The force of the kick, acting in the

direction of motionD. Friction, acting opposite the direction of

motionE. All of the above act on the rock


Identifying forces (see tactics box 5.2)


Tactics: Drawing a free-body diagram


EXAMPLE 5.4 An elevator accelerates upward


A log is dragged down an incline, using a rope. Which is the best free-body diagram?

A. B. C. D.


What do forces do?

Can you apply a net force to an object and make it move at a constant speed in a straight line?

It turns out that you can’t (hard to show due to lack of truly frictionless surfaces)


An object pulled with a constant force moves with a constant acceleration

a = cF

where c, the proportionality constant, is the slope of the graph.


What is the proportionality constant?• As the “size” (in this case,

number) of the object increases , the slope decreases.

• The size of the object is it’s mass, a measure of the amount of matter it contains.

• Now we can write the equation as:

a = F/mThe proportionality constant is the

inverse of the mass of the object.


Proportional or Ratio Reasoning

If u = cv, thenu1 = cv1 and u2 = cv2 :

We can deduce information about u or v, without having to know the value of c.

vu

1

2

1

2

vv

uu


EOC # 8

What is the ratio of the masses of object 3 to object 1?

A. 6.25

B. 0.4

C. 0.16

D. 2.5


Non-linear Proportionality

If w is tripled, by how much does u change?

2

1w

u


Non-linear Proportionality

2

2

1

1

2

ww

uu

2

1

2

3

ww

uu

If w is tripled, by how much does u change? Use ratio reasoning to find u2:

2

1w

u where w2 = 3w1

u2 = (1/9) u1

91

31 2

1

2

uu


Stopping distanceA car is moving at an initial speed of v when the brakes

are locked and the car slides to a stop in a distance of d. This procedure is repeated in the same car under the same conditions, only the initial speed is increased. Now the stopping distance is 2d. By what factor has v changed? Assume the same, constant acceleration in each case:

A.Factor of 2 B. Factor of root 2C. Factor of (root 2)/2 D Factor of 4


Three forces act on an object. In which direction does the object accelerate?


An elevator slows to a stop as it goes up. Which is the correct free-body diagram?

chapter 5. force and motion

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