PHYSICS CH 2 Conceptual Physics

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PHYSICS CH 2 Conceptual Physics

• V. HASSELL

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Aristotle Galileo

• Aristotle was great philosopher but not such a
  good scientist.
• Aristotles theory of motion is wrong.
• Took 2000 years before Galileo got motion right.

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Aristotle on Motion (350 BC)

• Aristotle attempted to understand motion by
  classifying motion as either
• (a) natural motion
• forces acting at a distance
• (b) or violent motion
• contact forces

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Motion according to Aristotle (II)

• Pushing or pulling an object causes unnatural
  motion (or violent motion).
• If cause is removed (stop pushing) then object
  returns to natural state and stops moving.

Pushed brick slides but then comes to a stop
BRICK
BRICK
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Motion according to Aristotle (I)

• Every object has a natural state.
• In natural motion, Earth elements (stone,
  apple, you, etc.) are drawn to the Earth.
• Heavier objects are more strongly attracted so
  they fall faster (stone falls faster than a
  feather).

Aristotle
Important These are Aristotles ideas, but hes
wrong!
Reality
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Natural Unnatural Motion

• Aristotle thought objects in unnatural motion
  would first stop, then fall.
• He was wrong!

Unnatural
Natural
Reality
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• Large object tend to 'strive harder'.
• He stated that The Earth remains at rest.

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• Geocentric Model - Earth Centered Universe

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Copernicus (1500's)

• "The Earth and planets orbit the Sun.
• He reasoned this from his astronomical
  observations.

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Galileo - one of 1st modern scientist

• Challenged beliefs-
• earth center of universe
• large masses fall faster
• Developed the scientific method

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Galileos Inclines (I)
Downhill Speed increases
Uphill Speed decreases
Flat surface Speed increases, decreases, or
constant?
Questions existence of natural Earth state of
not moving.
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• Friction - a force that resists motion
• e.g. air resistance and sliding on rough surfaces
• Inertia - the resistance of an object to change
  in its state of motion
• Sluggishness
• http//www.youtube.com/watch?vw3Vhz9b6iKg Bow
  and Arrow
• Demo Ball and incline plane

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Galileos Incline Planes

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Galileo Falling

• Pull of gravity (weight) obviously larger for a
  bowling ball than for a baseball.
• YET the two balls fall with the same speed.
• What about a feather?
• (Answer in Chapter 4)

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Isaac Newton (1642-1727)

• a.k.a Law of Inertia
• A body remains at rest or moves in a straight
  line at a constant speed unless acted upon by a
  force.
• His three laws of motion first appeared in his
  book called Principia.

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If I have seen further it is by standing on the
shoulders of giants
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Newtons First Law Examples

• Weight and string
• Card, cup, and coin
• Fixing a Hammer
• Demo - Coins on elbow
• Demo - Lead Brick and Hammer
• Demo - Table setting
• http//www.youtube.com/watch?vT9KPwNeCdSg

Figure 2.4
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Newtons First Law of Motion

• An object at rest remains at rest
• an object in motion remains
• in uniform motion,
• unless a force acts on the object.
• Moving in a straight line with constant speed.

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Newtons First Law of Motion

• Object at rest
• (not moving)

then
No force on object
then
Uniform motion
Note Object at rest is just uniform motion at
zero speed.
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Demo Riding Light Rail
When a moving train stops, you continue moving
forward. When the stopped train starts moving
again, you remain stationary and are thrown
backwards
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Demo Rolling, rolling, rolling
WALL
All moving together
Bottom car stops. Other two continue With same
speed.
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Demo Tablecloth Pull
Flower Vase
Yank quickly
Tablecloth
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Demo Ring Yank
Coin
Ring
Yank quickly
Beaker
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Demo Inertia Balls
Upper String
Which string breaks in each case?
Heavy Cannon Balls
Lower String
Slow pull
Quick yank
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Demo Paper Pull
What happens in each case? Which roll has more
inertia?
Quick yank
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Net Force

• When several forces act on an object, the forces
  add together.
• Sum of forces called net force or total force

3 Newtons
8 Newtons
BRICK
5 Newtons
same as
The Newton is metric unit of force (about 1/5
pound).
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Check Yourself
?
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Image from Isaac Newton's Principia (1687).
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NET FORCE

• A force or a combination of forces produces
  changes in motion (accelerations).

10 N

20 N
10 N

10 N
10 N
0 N

10 N
20 N
10 N
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THE EQUILIBRIUM RULE
Scales pushing up
Examples of Mechanical Equilibrium
Normal up
Computer setting on a table
Weight down
Weighing yourself on a set of scales
Hanging from a tree
Tree pulling up
Weight down
Car parked on an incline
Normal
Friction

Weight down
Weight down
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Equilibrium Rule

• If an object is at rest then the net force must
  be zero. Similarly if in uniform motion.

Zero Newtons (No Force)
3 Newtons
3 Newtons
BRICK
same as
When this happens we say that forces balance.
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Demo UpUpDownDown
Scale reads 7 Newtons
Scale reads 5 Newtons
57102
STICK (2 Newtons)
10 Newton Iron mass
Stick is at rest.
The two upward forces (by people holding the
scales) must always be equal to the two downward
forces (weight of stick and iron mass).
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Support Force

• Solid surfaces exert a force, called a support
  force, on objects pressed against them.

Downward force (weight) balanced by upward force
(support).
100 Newton Gold Brick
100 Newton Support force
How much is the net force on the brick?
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• The Equilibrium Rule

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SUPPORT FORCE

• A table can supply an upward support force also
  known as a normal force.
• When we say normal to we are
  saying at right angles to.

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EQUILIBRIUM OF MOVING THINGS

• Equilibrium is a state of no change.
• If an object moves in a straight line with no
  change in speed, it is in equilibrium.

Examples Driving at constant velocity
Normal up
Air resistance
Air Resistance
Force from road
Weight down
Terminal velocity in parachuting
Weight down
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THE MOVING EARTH

• It is hard to detect the motion of the earth
  because we are moving with it.
• Early science could not predict large enough
  forces to move the earth.
• Can Hewitts bird drop down and catch the worm if
  the Earth moves at 30 km/s?
• Demo - Cart and ball launcher

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Which person below is most highly recognized for
proposing a heliocentric universe?

• (a) Aristotle
• (b) Newton
• (c) Galileo
• (d) Copernicus

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If an object weighs 10 lb, what must the air
resistance force be if the object is falling and
has reached terminal velocity?
(a) 10 lb (b) 32 lb (c) there is no way of
telling without knowing what the value of the
terminal velocity is
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Newton's concept of motion said that the natural
state of an object was

• (a) constant velocity
• (b) constant acceleration
• (c) constant net force

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Demonstrations for Chapter 2

• Ball and incline plane
• Weight and string
• Card, cup, and coin
• Swinging Rocks
• Coins on elbow
• Table setting
• Bottle, hoop, and chalk
• Cart and ball launcher
• Lead Brick and Hammer