Newton’s 1st Law of Motion
Inertia
Ch. 4.5-4.8
Review
• Aristotles concept of natural vs. violent
motion
• Galileo’s concept of motion
• Newton’s 1st Law of motion
• Some review questions . . .
• HW questions review: p.56 (1, 3, 4, 6-8)
Objectives
1. Distinguish among mass, volume, &
weight
2. Convert kilograms to Newtons
3. Calculate a net force
Mass
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Let’s kick some stuff
Drop your pen on the floor and kick it.
Now, kick your desk.
Which one has more mass?
What is Mass?
Mass:
– The measure of inertia in an object.
– Quantity of matter in an object
Mass vs. Volume
• Which has more mass?
(show example)
• Which has greater
inertia?
• Which has a greater
volume?
• Volume:
– A measure of space
(cm3, m3)
• MASS IS NOT
VOLUME!
Mass vs. Weight
• Remind me what mass is?
– The measure of the amount of material in an
object.
• What is weight?
• Weight:
– The measure of gravitational force on an object
• Can an objects weight or mass change
depending on where it is? For example, outer
space? Check your neighbor . . .
Mass vs. Weight
• Will the mass of this satellite be the same in outer space as it is on
earth?
– What about the weight?
• The amount of material in the satellite will always be the same no
matter where it is located --> an object’s mass always stays the
same, no matter where it is!
• Shake your book back and forth. It will feel the same in space.
The mass, or amount of inertia, will not change!
• Weight is different. Less gravity --> less weight
Mass vs. Weight
Does the man’s mass stay the same? What about his weight?
Check your neighbor…
• Review with your neighbor the difference
between mass, volume, and weight.
Quick Review
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Mass is. . .
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–
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Volume is . . .
–
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the amount of matter in an object
not affected by gravity
the amount of space that matter occupies
Weight is. . .
–
the force of gravity on an object.
Hewitt vs. The Anvil
• Why doesn’t this hurt Mr. Hewitt?
Demo with weight and string
Part 1
•
I am going to gradually pull harder on the
bottom string. Based on the questions
below, make a prediction with your
neighbor:
1. Is the string tension greater in the upper or
lower string?
2. Which string is likely to break?
3. Which property, mass or weight, is
important here?
Demo with weight and string
Part 1
1. Upper string has greater tension b/c it
supports the weight of the object plus your
pull. Lower string supports only your
pull.
2. The upper string broke.
3. Weight is more important here.
Demo with weight and string
Part 2
•
I am now going to QUICKLY pull the string
downward. Will anything be different? Make a
prediction with your neighbor based on the
following questions.
1. Which string is more likely to break?
•
The bottom string broke! Demonstrates inertia. The
metal weight resists the sudden downward
acceleration of the lower string.
2. What is more important here, mass or weight?
•
Mass is more important here!
Inertia in Action
• Let’s watch some true geniuses as they are
confronted with the task of dealing with inertia and
snow. . .
Shorter at Night?
• Remember the hammer
demo?
• Demo:
– Same idea with piece of
wood and dowel
• Wood remains in motion
while dowel stops.
• What does this have to do
with you being shorter at
night?
Shorter at Night?
• The bones of your spine
settle together throughout
the day.
• Find a place at home just
out of reach before you go
to bed
• Wake up in the morning
and try to touch it
• Astronauts returning from
orbit are more than an
inch taller!
Newtons and force
• In the U.S., the amount of matter is described by
it’s gravitational pull to earth, or its weight
• The U.S. uses units called pounds.
• The rest of the world uses kilograms.
• What about force?
• The SI (International Unit) unit we use to measure
force is called a Newton (N).
• 1 kg = 9.8 N (2.2 lbs)
• Weight (in newtons) = Mass x Acceleration due to
Gravity
– Weight = mg
Definition of a Newton
Quick Practice
• How many Newtons is a 3-kg bag of bananas?
– 3 kg x 9.8 = 29.4 N
• How many Newtons is Mr. Ward?
– I weigh 170 pounds
– There are 2.2 pounds in every kg
• 170/2.2 = 77.3 kg
• 77.3 kg x 9.8 N = 757.5 N
• I weigh 757 N
More Practice
• Does a 2 kg iron block have twice as much inertia as a 1
kg block of iron? Twice as much mass? Twice as much
volume? Twice as much weight when weighed in the
same location?
• A: Yes to all questions!
• Does a 2 kg bunch of oranges have twice as much
inertia as a 1 kg loaf of bread? Twice as much mass?
Twice as much volume? Twice as much weight when
weighed in the same location?
• A: Yes to all questions EXCEPT volume.
– The density of oranges is much greater than that of bread -->
oranges occupies less volume
Let’s Practice
• Pass out worksheet 4-1
• Please work in pairs. THIS MEANS TWO
PEOPLE ONLY!
Net Force
• Volunteer?
• Pushing chair across floor with 10 N
• Neglecting friction, if I push this chair with
10 N, then the net force, is 10 N.
• Volunteer come up and push in opposite
direction with equal force. What will the net
force be?
• What if student pushed on it in opposite
direction with a force of 4 N? What would
net force be?
Net Force
• Net Force:
– The combination of all forces acting on an object
• Let’s draw this out together.
Equilibrium
• What forces are acting on
the cat while it’s
motionless on the table?
• Gravity AND the support
force of table
• Table pushes up with a
force equal to books weight
• We call this equilibrium.
• Equilibrium:
– The net force equals zero.
Equilibrium
• Hanging from a rope is another
example.
• Rope pulls you up and gravity pulls
you down
• Tension in rope equals your weight
• What if you are hanging from two
ropes? What will the tension of each
rope be?
• 1/2 your weight
• When you step on a
bathroom scale, the
downward force supplied
by your feet and the
upward force supplied by
the floor compress a
calibrated spring. The
compression of the spring
gives your weight. In
effect, the scale measures
the floor’s support force.
Check Question
• If you weigh 50 kg, what will each scale read if
you stand on two scales with your weight equally
divided between them? What happens if you
stand with more of your weight on one foot than
the other?
Vertical vs. Horizontal lines
• Vertical lines can
support your weight
much better than a
horizontal line.
• Why?
• Tension in horizontal
line is much greater
• Look at figure 4.15 in
text
Vertical vs. Horizontal Lines
• For any pair of ropes, scales, or wires supporting
a load, the greater their angle from the vertical, the
larger the tension force is.
• What does this mean?
• The more horizontal a line is, the less it can
support.
• A can support more than B
• Demo with chain and weights (p. 53) (birds on a
wire)
A
B
The Moving Earth
• The earth moves
around the sun at 30
km/s. If this is so,
than how can the bird
catch the worm?
• Everything is moving
at 30 km/s; the bird,
tree, air, worm, grass,
etc!
The Moving Earth
• If the plane is
traveling at a high
speed, then how can
we flip a coin in the
air and catch it?
Shouldn’t it fly over
our head at the speed
of the plane?
Review Questions
1. Your empty hand is not harmed if it bangs
lightly against a wall, but is harmed if it bangs
against the wall while carrying a heavy load.
Why?
A: A heavy load has a lot of mass, and once in
motion has a lot of tendency to remain in
motion. More force is required to stop the mass
so your unfortunate hand is squashed.
Review Questions
2. Does a person diet to lose mass or to lose
weight?
A: A person diets to lose mass. One loses
weight whenever gravity is reduced, like
being on the surface of the moon. Even
with less weight on the moon, an obese
person is still obese.
Review Questions
3. How does a car headrest help to guard
against whiplash in a rear-end collision?
A: In a rear end collision your head tends to
stay put while your body is pushed
forward by the seat. Relative to your
body, your head “whips” back. The
headrest extends the seat up to your head
and keeps your head and body together.
Review Questions
4. A car at a junk yard is compressed until its
volume is less than 1 cubic meter. Has its
mass changed? Has its weight changed?
A: Neither its mass nor its weight has changes
when it has been compressed, because the
same quantity of matter is present. Only
its volume is less.
Worksheet
• Let’s give worksheet 4-2 a try.