
Aristotle, in the 4th century BC,
made the mistake of saying that if
something is moving, it is because a
force is acting.

Galileo said that no force is
required to keep an object moving.


Isaac Newton came along soon afterward and
stated: “If something is at rest (and you don’t
“mess” with it), it will remain at rest. If
something is moving (and you don’t mess with
it), it will keep moving.”
We refer to this as Newton’s 1st Law, or the
Law of Inertia.

So, in the absence of a force:
Something
moving will continue
moving with constant velocity
Something
at rest will stay at rest

Inertia is another word for Mass or Matter.

Zero net force means that all of the forces
acting on an object must be balanced (cancel
out).

If there is no net force, there is no
acceleration (another word for change).
When Mr. Hewitt suspends a heavy ball by a
string and pulls on a second string hanging below
the ball to compare the roles of weight and mass
in determining whether the top or bottom string
breaks, which string breaks when he pulls
slowly?
TOP STRING
Why? The tension is greater in the (UPPER ,
LOWER) string, so the property of (mass,
weight) is more important in determining which
string breaks. Explain more fully here if needed.
Which string breaks when he pulls quickly?
BOTTOM STRING
Why? The property of (Mass , Weight) is more
important. Explain more fully here if needed:
The top string is protected by the massive ball.
When Mr. Hewitt lies on a table with the anvil on his
stomach and has a student hit the anvil with a
sledgehammer, why is he not hurt?
The mass of the anvil creates a tendency
for the anvil to stay where it is. The mass
protects his stomach.
If the head of a hammer is loose, and you wish to
tighten it by banding the handle against the top of
a workbench, why does this work to tighten the
hammerhead?
The mass of the hammerhead has a large
inertia when the handle stops as it hits
the table, the hammerhead wants to keep
going
 tightens the hammerhead to the hand.

Why are you shorter at night
than in the morning?
All day long, your inertia (mass) acts
on your spinal chord and you get
shorter
In the scaffold system
illustration, the key concept is
that because the system is not
accelerating, the net force on the
system must be zero. In other
words, “the ups equal the
downs.”
In the video, Hewitt leaves you at the end with a
question to think about: If Harry weighs 200
pounds, but the rope has dry rot and will only
support 150 pounds, why would tying one rope to
a flagpole make this Harry’s last day on the job?
Harry’s weight of 200 lbs. is divided up by 2 ropes.
If he ties one end off, all of his weight is
redistributed to 1 rope. UH… OOHHH!!!
If you were in outer space and threw a ball, how
much force would be required to keep it moving
through the vacuum of space?
Zero – a body in motion will stay in
motion
Law of Inertia
If you push on something horizontally with 10 N
of force and a friend pushes with 8 N of force in
the opposite direction, what is the net force?
10 N – 8 N = 2 N to the right
-----10N-----> <-----8N----- = 2 N 
What is the tension in the arms of a 150 pound
person who does pull-ups with both arms?

75 lbs. on each arm
What is the net force on any object
in equilibrium?

Zero Newtons
(Mass , Weight) is a measure of the
actual material in a body, depending
only on the number and kind of atoms
that compose it. (Mass , Weight) is a
measure of the gravitational force that
acts on the material and depends on
where the object is located. The
amount of inertia an object has
depends on its (mass , weight).
Two painters are on a scaffold system similar to
the one drawn in the video by Paul Hewitt. If the
board has a weight of 150 N and each painter has a
weight of 800 N, how much tension is in the two
ropes if each painter stands on an opposite end of
the board?
800N + 800N + 150N = 1750 N
Divided by 2 (ropes) = 875 N
The same two painters have rearranged themselves on
the scaffold. Both painters are now standing on the
right half of the scaffold. This has increased the
tension in the right rope to 1300N. What is the tension
in the other rope?



All forces balance out to be zero.
The weight acting down equals the tension
acting up
A body at rest tends to stay at rest = 0 acceleration




Right rope = 1300 N down
Scaffold weight = 150 N down
Total weight down = 1750 N
X (left) = # N on the left of the scaffold
1750 N (total) = 1300 N (right) + 150 N (scaffold) + X( left)
X (left) = 300 N