# Newton st Law of Motion by sanmelody

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```									Newton’s   1st   Law of Motion

Inertia
Objectives
• Explain the relationship between the motion
of an object and the net external force acting
on the object.
• Determine the net external force on an
object.
• Calculate the force required to bring an
object into equilibrium.
Newton’s 1st Law of Motion States:

• An object at rest tends to stay at rest and an
object in motion tends to stay in motion with
the same speed and in the same direction
unless acted upon by an unbalanced force.
1st Law
1st Law

All objects resist changes in their state
of motion – they tend to "keep on
doing what they're doing."
1st Law
1st Law
Inertia is the resistance an object has
to a change in its state of motion.
1st Law
1st Law
• Galileo, developed the concept of inertia.
• Galileo reasoned that moving objects
eventually stop because of a force called
friction.
1st Law
1st Law
• The tendency of an
object to resist
changes in its state
of motion is
dependent upon its
mass.
1st Law
• Mass is not volume.
• Mass is measured in kilograms.
1st Law
• Mass is not weight.
• It is more fundamental than weight.
• Mass is a measure of the actual material
in an object. It depends only the number
of atoms that compose it.
• Weight is a measure of the gravitational
force that acts on a material and
depends on where the object is located.
1st Law
1st Law
• Mass: the quantity of matter in a body.
• The measure of inertia.
• Weight: the force due to gravity upon a
body.
1st Law
• The SI unit of force is the NEWTON.
• 1 kg = 9.8 N = 2.2 lbs.

1. Imagine a place in the cosmos far from
all gravitational and frictional influences.
Suppose an astronaut in that place
throws a rock. The rock will:
B. continue in motion in the same
direction at constant speed.
• An 2-kg object is moving horizontally with a
speed of 4 m/s.
• How much net force is required to keep the
object moving with the same speed and in the
same direction?
• Mac and Tosh are arguing in the cafeteria.
Mac says that if he throws his jello with a
greater speed it will have a greater inertia.
• Tosh argues that inertia does not depend
upon speed, but rather upon mass.

• With whom do you agree? Why?
• If you were in a weightless environment in
space, would it require a force to set an object
in motion?
• Mr. Wegley spends most Sunday afternoons at
rest on the sofa, watching pro football games
and consuming large quantities of food.

• What effect (if any) does this practice have
upon his inertia? Explain.
• Ben is being chased through the woods by a bull
moose which he was attempting to photograph.
• The enormous mass of the bull moose is
extremely intimidating.
• Yet, if Ben makes a zigzag pattern through the
woods, he will be able to use the large mass of
the moose to his own advantage.
• Explain this in terms of inertia and Newton's first
law of motion.
State of Motion
• Inertia is the tendency of an object to
resist changes in its velocity.
State of Motion
• We learned before that an object which
is not changing its velocity is said to have
an acceleration of 0 m/s2.
• Thus, an alternate definition of inertia
would be:
• Inertia is the tendency of an object to
resist accelerations.
• Several physics teachers
are taking some time off
to play a little mini golf.
The 15th hole has a large
metal rim which putters
must use to guide their
ball towards the hole.
• Mrs. Menken guides her
golf ball around the metal
rim. When the ball leaves
the rim, which path (1, 2,
or 3) will the golf ball
follow?
• A 4.0 kg object is moving across a frictionless
surface with a constant velocity of 2 m/s.
Which one of the following horizontal forces is
necessary to maintain this state of motion?
• 0N
• 0.5 N
• 2.0 N
• 8.0 N
Balanced and Unbalanced Forces
• An object at rest tends to stay at rest and an
object in motion tends to stay in motion with
the same speed and in the same direction
unless acted upon by an unbalanced force.
• What is an unbalanced force?
Balanced and Unbalanced Forces
Balanced and Unbalanced Forces
• Since these two forces are of equal magnitude
and in opposite directions, they balance each
other.
• The book is said to be at equilibrium.
Balanced and Unbalanced Forces
Balanced and Unbalanced Forces
Balanced and Unbalanced Forces

• The book is not at equilibrium and it
subsequently accelerates.
• Unbalanced forces cause accelerations.

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