# Inertia and Newton st Law

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```							                                           2/6/2011

Inertia and Newton’s 1st
Law

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What will we learn?
Inertia and Newton’s 1st Law

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topic(s): inertia, center-of-mass

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2/6/2011

Question
If you were to place dishes on a tablecloth and
tablecloth,
quickly remove the tablecloth what would happen
to the dishes? Why?

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topics: inertia

Inertia
is the tendency of an object to resist changes to its
object s
velocity; it is the object’s resistance to forces
impressed upon it.

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topics: inertia

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2/6/2011

Mass
is a measure of an object’s inertia; it is a scalar
object s
quantity that is independent of the object’s
position.

The SI unit of mass is the kilogram (kg).

5
topics: inertia

A force . . .
• is a single interaction between two objects.
p         p              j
• acts as either a push or a pull on each object.
• is equal in magnitude, but opposite in direction on each object.
• acts in a straight line on each object.
• is a vector.
The SI unit of force is the Newton (N): 1 N = 1 kg m/s2

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2/6/2011

Newton’s 1st Law
Newton first quantified the idea of force, inertia,
q
and equilibrium in his 1st Law of Motion:

1) A body will remain in a state of equilibrium until
acted upon by an applied net external force.

In other words, a body’s velocity cannot change
it.
without an external net force to change it

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topics: Newton’s 1st Law

Mechanical Equilibrium
A body is said to be in a
state of mechanical                     ∑F = 0
equilibrium when the net
force acting on it is zero.               a=0

Static equilibrium is the state when a body is at
rest, while dynamic equilibrium is the state when
velocity.
the body is moving at constant velocity
Therefore, a body is in equilibrium when it is not
accelerating.

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2/6/2011

Exercise
Identify the magnitude and direction of the net
force in each of the following situations. Which
b          in    ilib i ?      l i
boxes are i equilibrium? Explain.

5N
3N             6N
a)                                                          5N
b)

23 N        20 N               43 N             25 N
c)
d)
18 N
9
8N

A force diagram,
(also called “free-body” diagram), is a diagram that
represents each force that acts on a single body.
Fy

The center-of-mass of the
body is represented as a
particle at the origin of the
FdW
diagram.                                                              Fx
Each force vector is drawn
with its tail placed at the
origin.                                           FdS

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2/6/2011

Exercise
An airplane is in level flight at a speed of 100 km/h and
headed east. A strong (westward) headwind imparts a
horizontal d
h i              force of 700 N onto th airplane. Th
t l drag f       f          t the i l       The
airplane’s wings generate an upward lifting force of 9600
N. If the airplane has a mass of 1000 kg,
a) what is the magnitude and direction of the force that
the airplane’s engine must produce in order to
maintain level flight at 100 km/h into the wind?
b) Is the airplane in equilibrium with that force?
Explain.

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Newton’s 2nd Law
Newton quantified the idea of force, inertia, and
acceleration in his 2nd Law of Motion:

2) An object will accelerate in the direction of the
net external force applied to it. That is, it will
change its velocity only in the direction of the
net external force that causes it.

a=
∑F
m

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topics: Newton’s 2nd Law

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2/6/2011

Example
You use a rope to pull a heavy block across a horizontal
rough surface. Friction exerts a force on the block of
10 N west; gravity pulls down on the block with 20 N of
force; the ground pushes up on the block with 3.0 N of
force; the rope pulls with 20.0 N of force eastward at
60° above the horizontal.

a) Draw the block’s free-body diagram.
Newton s
b) Write Newton’s 2nd Law for the block in component
form.
c) What is the block’s acceleration?

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Example
A sailboat finds itself in a strong eastward ocean current
g
that exerts 5000 N of force on it at an angle of 53.1°
north of east. A gust of wind blowing from the south
exerts 4000 N of force on the boat’s sails. Is the sailboat
in equilibrium? If not, what is its acceleration?

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2/6/2011

Points to Remember!
Newton was the first to quantify the concept of force
in his Laws of motion:

1) A body will remain in a state of equilibrium until
acted upon by an applied net external force.

2) An object will accelerate in the direction of the
it.
net external force applied to it

3) The force of interaction between two bodies is
always equal in magnitude and opposite in
direction.
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