Phys130 Wk2 Newton's 1st Law f14

8/11/2019 Phys130 Wk2 Newton's 1st Law f14

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NEWTONS 1STLAW OF

MOTION -INERTIA09/02/14

PHYSICS 130: ELEMENTARY PHYSICS

CHAPTER 2

Cypress CollegePhysics & Astronomy


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TODAYS AGENDA

Last times Next time question Problem of the Day Aristotle on Motion GalilieosObservations

The Leaning Tower of Pisa Inclined Planes

Newtons First Law of Motion Net Force & The Equilibrium Rule

Support Force Equilibrium of Moving Things

The Moving Earth (and other objects) Review of Key concepts/terms Next Time Question Homework


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Which of the following is no ta

scientific hypothesis?

A. Protons carry an electric charge.

B. Undetectable particles are the cause of some ofnature

s unresolved secrets.

C. Charged particles bend when in a magnetic field.

D. All of the above are scientific hypotheses.

Last times Next Time Quest ion


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Which of the following is no ta

scientific hypothesis?

A. Protons carry an electric charge.

B. Undetectable particles are the cause of some ofnature

s unresolved secrets.

C. Charged particles bend when in a magnetic field.

D. All of the above are scientific hypotheses.

Last times Next Time Question

This statement is not scientific mainly because there is no way to

disprove it. We have not way of knowing what undetectable

particles are out there, and we dont know which secrets are related

to these particles. Secondly, if the particles are undetectable by

definition, how can we make observations on them?


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What will happen when I pull

down hard on the lower

string?

A. The upper string will break

B. The lower string will break

C. The upper string breaks if I pull fast and the

lower string breaks if I pull slowly

D. The lower string breaks if I pull fast and theupper string breaks if I pull slowly

E. This problem is too hard

Problem of the Day


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ARISTOTLE ON MOTION

Aristotle (322 BC) thought aboutmotion in terms of an objectsplace in the universe.

Every object in theuniverse has a properplace determined by acombination of four

elements: earth, water, air,and fire.

Any object not in itsproper place will

strive to get there.


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ARISTOTLE ON MOTION

Natural motion Straight up or straight down for all things on Earth. Beyond Earth, motion is circular. Example: The Sun and Moon continually circle

Earth.

Violent motion Produced by external pushes or pulls on objects. Example: Wind imposes motion on ships.


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ARISTOTLE ON MOTION

Major implication: continuous movementdepends on sustained forces No force = no movement Small force = slow movement

Large force = fast movement No apparent forces acting on earth and earth

belongs in the center of the universe, soclearly earth doesnt move


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GALILEOS OBSERVATIONS

Dropping objects from the Leaning Tower of Pisa

According to Aristotle, heavier

objects should fall faster than lighter

objects If an object is 2 times heavier, then it

should fall 2 times as fast Instead of only

using observation

and logical thinking,

Galileo conductedexperiments to test his ideas.

His tests showed that weight of

dropped objects had no real effect on

the speed at which they fell


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Observing the starting and ending heights of ballson inclines

Trial1

Trial 2: Smaller

slope of 2nd

incline, same

ending height

Galileos logic: the

slope of the

incline slows

down the speed

of the ball,

regardless of how

steep it is. So ifthere is no 2nd

slope, the speed

of the ball will not

slow down.


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Observing the starting and ending heights of ballson inclines

Clearly a slope is needed to speed up or slow down

the ball Steeper downward slope makes speed increase

faster

Steeper upward slope makes speed increaseslower

Logically (and based on observation) a flat surface(no slope) should mean that the speed should not

change

The above statements are true regardless of the

starting speed of the ball


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The use of inclined planes for Galileo

s

experiments helped him to

A.eliminate the acceleration of free fall.

B.discover the concept of energy.C.discover the property called inertia.

D.discover the concept of momentum.

REVIEW


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The use of inclined planes for Galileo

s

experiments helped him to

A.eliminate the acceleration of free fall.

B.discover the concept of energy.C.discover the property called inertia.

D.discover the concept of momentum.

Comment: Note that inertia is a propertyof

matter, not a reason for the behavior of matter.

REVIEW


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NEWTONS FIRST LAW OFMOTION

Every object continues in a state of rest

or of uniform speed in a straight line

unless acted on by a nonzero net force. If an object is already moving, it will keep

moving at the same speed

If an object is not moving, it will stay still

The direction of the object will not change.

What is a nonzero net force?


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NET FORCE (VECTORS)

Vectorquantity

a quantity whose description requires bothmagnitude (how much) and direction (which way)

can be represented by arrows drawn to scale, calledvectors

length of arrow represents magnitude and arrowhead

shows direction

Examples: force, velocity, acceleration

Force vector


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NET FORCE

Net force is the combination of all forces thatchange an objects state of motion.

Units of force are Newtons (N).


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THE EQUILIBRIUM RULE

Example: A spring scale holding up a bag of flour

Two forces act on the bag of flour: Tension force acts upward.

Weight acts downward.

Both are equal in magnitude and opposite indirection.

When added, they cancel to zero. So, the bag of flour remains at rest.

The Tension and the Weight oppose eachother


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The vector sum of forces acting on a non-accelerating object equals zero. In other words, the

net force is zero.

In equation form: F = 0.


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Drawing a f ree-body d iagram helps us todetermine all the forces acting on an object

The diagram below lists all the forces acting on the

plank.

REVIEW


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Find the tension inthe rope on the right

for each picture

REVIEW

REVIEW


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Find the tension inthe rope on the right

for each picture 500

830

1000The total upward

force (the combined

tension of both

ropes) should alwaysequal 1000 N

REVIEW

REVIEW


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A bowling ball is in equilibrium when it

A. is at rest.

B. moves steadily in a straight-line path.

C. Both of the above.

D. None of the above.

REVIEW

REVIEW


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A bowling ball is in equilibrium when it

A. is at rest.

B. moves steadily in a straight-line path.



Explanation: Equilibrium means no change in motion,so there are two options:

If at rest, it continues at rest.

If in motion, it continues at a steady rate in a straight

line.

REVIEW

REVIEW


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The equilibrium rule, F= 0, applies to

A. vector quantities.

B. scalar quantities.



REVIEW

REVIEW


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The equilibrium rule, F= 0, applies to

A. vector quantities.

B. scalar quantities.



Explanat ion: Vector addition accounts for+ andquantities. So, two vectors in

opposite direction can add to zero.

REVIEW

REVIEW


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Find weight of plank

REVIEW

REVIEW


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The entire system (plank

plus man) is at equilibrium,

so the net force should bezero. This means that the

upward forces must be

balanced by the downward

forces.

Since the combined tension

in both ropes is 800 N, thismeans that the combined

weight of the plank and the

man is also 800 N.

Therefore, the weight of the

plank must be 300 N

Find weight of plank

REVIEW

REVIEW


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A 100 N WEIGHT HANGS ON EACH SIDE

The scale reads 0, 100, or 200N?

REVIEW

REVIEW


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A 100 N WEIGHT HANGS ON EACH SIDE

The scale reads 0, 100, or 200N?

REVIEW

REVIEW


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REVIEW

Nellie Newton hangs

from the pulley as

shown. How does the

reading on the scalecompare with her

weight?

REVIEW


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REVIEW

Nellie Newton hangs

from the pulley as

shown. How does the

reading on the scalecompare with her

weight?

The reading on the scale

is exactly half of herweight. This can be

easily seen by drawing a

free-body diagram of

Nellie Newton.


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SUPPORT FORCE

Support force (normal = perpendicularforce) is an upward force on an object that

is opposite to the force of gravity.

Example: A book on a table compressesAtoms in the table, and the compressed

atoms produce the support force.

There is always a Normal force betweenany two objects in contact.


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SUPPORT FORCE

Since Net Force = 0, Gravitydown is balanced by the scale

pushing up. Scale reads the

normal (support) force. Keep in mind that the forces

shown are the forces acting on

the person, not on the scale!

Try drawing the free-bodydiagram for the scale.


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EQUILIBRIUM OF MOVINGOBJECTS

In the previous examples, the net force on a non-moving (static) object was equal to zero. This is

called stat ic equi l ibr ium

When the net force on a moving object is equal tozero, this is called dynam ic equi l ibr ium.


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THE MOVING EARTH (ANDOTHER OBJECTS)

If the earth is moving through space, why doesntthe bird fall diagonally backwards when it jumps off

the branch to catch the worm?

Everything on the earth - includingthe bird, the tree, the worm, and

the atmosphereare already

moving at the same speed as the

earth, which is why we dont feelthe motion. As a result, the bird

will simply fall straight down.

REVIEW


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You are riding in a van at a steady

speed and toss a coin up. Where will

the coin land?

A. behind youB. ahead of youC. back in your handD. There is not enough information.

REVIEW

REVIEW


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Explanat ion: The coin has inertia. It

continues sideways along with the van and

your hand and lands back in your hand.

REVIEW

You are riding in a van at a steady

speed and toss a coin up. Where will

the coin land?

A. behind youB. ahead of youC. back in your handD. There is not enough information.

nswer to the Problem of the Da


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What will happen when I pull

down hard on the lower

string?

A. The upper string will break

B. The lower string will break

C. The upper string breaks if I pull fast and the

lower string breaks if I pull slowly

D. The lower string breaks if I pull fast and theupper string breaks if I pull slowly

E. This problem is too hard

nswer to the Problem of the Da

Next Time Quest ion


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Next Time Quest ion

Next Time Quest ion


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Next Time Quest ion


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THIS WEEKS HOMEWORK

Chapter 2 (due next Tuesday, 9/9) Ranking #2, 4

Exercises #8, 28, 40, 46 Problem #2

Reading Assignment (finish by thisThursday, 9/4)

Textbook chapter 3Linear Motion

Phys130 Wk2 Newton's 1st Law f14

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