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Newton’s First Law of Motion
Today’s Objectives
• Distinguish between balanced and net forces
• Describe Newton’s first law of motion
• Explain how friction affects motion
First we need to define the word
FORCE:
• The cause of motion (what causes objects
to move)
• Two types of forces
– Pushes
– Pulls
Slide from www.science-class.net
Forces can be exerted in different
ways….
• A paper clip moved:
• By a magnet
• Pull of the Earth’s
gravity
• Or the force when
you pick it up.
Forces can affect motion in several
ways. They can make objects:
•
•
•
•
•
•
Start moving
Move faster
Move slower
Stop moving
Change direction
Change shape
• Force facts:
• Usually act in pairs
• Act in a particular
direction
• Cannot be seen but
their effects can.
Combining Forces
• More than one force
can act on an object
at a time.
• NET FORCE: the
combination of all
the forces acting on
an object.
• When more than one
force is acting on an
object, the net force
determines the MOTION
of the object.
Can you think of some examples of
balanced and unbalanced forces?
HOW do forces combine to form
the net force?
• If the forces are in
the SAME direction,
they ADD
TOGETHER to form
the net force
• If the two forces are
in the OPPOSITE
direction, then the
net force is the
DIFFERENCE
between the two
forces, AND the
force is in the
direction of the
LARGER force.
Forces may be balanced or
unbalanced
• Balanced forces – all forces acting on an object are
equal
– There is NO MOTION
– Forces of equal strength are acting in
opposite directions, so there is a NET FORCE
of ZERO.
• Unbalanced forces – one or more forces acting on
an object are stronger than others
– There is MOTION
• A NET FORCE
Objects at Rest
• Objects at rest tend to
stay at rest unless acted
upon by a force. [push
or pull]
• Newton described this
tendency as inertia.
• Inertia can be
described as the
tendency of an object to
keep doing whatever’s
it’s doing.
Inertia
• The resistance to a
change in motion
– Ex. A car at a stoplight
will stay still
– When a force (gas to
engine to wheels) moves
it, people inside feel
pushed back into their
seat, when actually their
body is still and the car
moves them into them,
pushing them forward
Mass & Inertia
Which vehicle has more
inertia?
• Mass is the amount of
matter in an object.
• The more MASS an
object has, the more
INERTIA the object
has.
• Bigger objects are
harder to start & stop
Slide from www.science-class.net
What about objects that are
already in motion?
• Newton stated that
objects in motion tend
to stay in motion until
acted upon by a force
(or hits it.)
Newton’s 1st Law
(also known as the law of inertia)
• A moving object moves in a straight line with
constant speed unless a force acts on it.
• The tendency of an object at rest to remain
at rest and an object in motion to remain in
motion unless acted upon by an unbalanced
force.
• Objects do not change their motion unless
a force acts on them
The truck is in motion. What is the force
that causes it to stop?
The push of the stopped car.
The car is at rest. What is the force that
causes it to move?
The push of the truck.
Slide from www.science-class.net
Friction
What is friction? Friction is a force
that acts to resist sliding between two
touching surfaces.
Friction is the reason why you never
see objects moving at a constant
velocity unless a net force is applied.
More about Friction
There are several different forms of
friction, BUT frictional forces will
always try to prevent one object from
sliding on the other object.
It will always slow a moving object.
Other types of friction
Static Friction: the force that prevents an
object from moving when a force is applied.
(like trying to move a refrigerator).
Sliding Friction: slows down an object that
slides. (like pushing a chair across the
room).
Rolling Friction (needed for wheels to turn)
occurs between the ground and the part of
the tire touching the ground.
Galileo (1564-1642)
Galileo understood that an object in
constant motion is as natural as an
object at rest.
If friction could be removed, objects
would continue to move in a straight line
at a constant speed until another force
acted on it.
In your ISN
1. For the following actions, explain
whether the forces involved are
balanced or unbalanced:
1. You push a box until it moves
2. You push a box, but it doesn’t
move.
3. You stop pushing a box and it
slows down.