# How do forces affect motion

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```					How do
forces
affect
motion?
Forces always
work
in pairs.
One force pushes
in one direction,
and the other
pushes with equal
strength in the
opposite direction.
Walking
demonstrates action
and reaction because
one foot pushes on
the ground as the
ground pushes the
body forward.
Earth pushes
forward against
equal reaction
force.
A push or pull is called a
force and forces make
things move faster or
slower, stop, change
direction, or change size
and shape.
Magnetism
A force that acts at a distance and
cannot be seen. The closer the
objects, the greater the magnetic
force.
The needle of a compass moves
because of Earth’s magnetism.
Magnetism
Materials that create this force are
said to be magnetic and are
called magnets.

The closer the objects, the greater
the magnetic force.
Friction:
The force that
opposes motion
between two
surfaces that are
touching.
The rougher the
surfaces are, the
harder they press
together, the
more friction
there will be.
A smooth surface
allows you to go
faster, while a
rough surface
would be
easier
to
stop
on.
Occurs between liquids
and gases as well as
between solids.

Without friction you
would not be able to slow
or stop the motion of
objects.
http://www.school-for-champions.com/science/images/friction_uses-bear.jpg
Friction can be reduced by
using lubricants, for
example motor oil, wax,
grease, by making surfaces
smoother, or by using
rollers.
Lubrication
Lubrication, for example, oil or
grease, reduces the effects of
friction. Without lubrication,
moving parts of machines
would slow down or stop very
quickly.
How does gravity
affect motion?
Gravity is a force that
makes any object
attract, or pull toward,
another object.
Gravity

All objects, large and
small, pull on all
other objects.
Gravity can
make things
slow down or
stop.
• The attraction is not noticeable unless one
of the objects is very large.

Earth is so big it
keeps everything
on the
ground.
• http://svideo.greenville.k12.sc.us/videos/1
1883/chp898138_256k.asf
• Introduction to forces and gravity 3:20

• Some types of forces (for review)
• Types of Force
The changing position of
an object over time when
compared to its
beginning reference
point.
Measure of how far
something travels in a
certain period of time
The course or path that an
object is moving and can
be determined by
terms of “north”, “south”,
“east”, or “west”.
Direction can be
described using these
terms: right, left, up,
down, forward,
backward, and toward
Is the object “above”,
“below”, “beside”, “behind”,
What is its length away from
the other object?
Sir Isaac Newton’s
Laws of Motions
• http://www.slideshare.net/scienceinteractiv
e/ks3-9k-balanced-and-ubalanced-forces/

• http://www.bbc.co.uk/schools/ks2bitesize/s
cience/revision_bites/forces_action3.shtml
Unbalanced forces change the rate and
direction of the motion of objects

Several forces can act on an object at the
same time.

Forces are balanced when they don’t
change the motion of the object.
Unbalanced forces cause changes in
motion.

How can unbalanced forces cause
motion in the picture below?
OBJECT at REST
If an unbalanced force acts on an object at
rest the object will move in the direction of
the force.

Like a golf ball being hit by Ms. Woods

A stronger force (push or pull) will make
it move faster.
(Ms. Woods always hits the sweet spot!)
If an object is moving, an unbalanced force
will change the motion of the object in
different ways depending on how the force
is applied.
The unbalanced force may speed the object
up, slow it down, or make it change
direction.
• If the force is applied in the same
direction as the object is moving, the
object will speed it up.
• If the force is applied in the opposite
direction as the object is moving, the
object will slow it down or stop it.
• If the force is applied to the side of
the moving object, the object will turn.
Newton’s First Law
of Motion
Inertia:

The tendency of an
object to stay in
motion or to stay
still unless acted
upon by an
unbalanced force
Every object
whether it is
moving or at
rest has inertia.
In order to start
something moving
or stop it from
moving, you need
to use force to
overcome its
inertia.
Earth’s gravity is so
strong it can
overcome inertia
and make things
fall toward Earth.
On the earth, invisible
forces, such as gravity
and the drag of air or a
rough surface,
overcome inertia to
slow objects down or
pull them toward the
earth.
Two objects
same amount of
mass attract
each other
equally.
MASS

Amount of
matter
When two objects have
different masses, even
though the force of the
gravity between them
is equal, the object
with the lesser mass
moves more than the
object with greater
mass.
An object’s mass
stays the same
everywhere in
the universe.
An object’s mass
stays the same;
however, its weight
changes according
to the pull of gravity.
Weight is the
measurement of the
amount of force that
gravity exerts on you.
Falling Objects
And the role
Of
Gravity
How do we
interpret a
motion graph?
• A distance-time graph tells us how far an
object has moved with time.
• The steeper the graph, the faster the
motion.
• A horizontal line means the object is not
changing its position - it is stationary.
• A downward sloping line means the object
is returning to the start.
http://www.bbc.co.uk/schools/gcsebitesize/physics/forces/speedvelocityaccelerationfhr
ev2.shtml
Q: Who won the race?
A: Albert won – he reached 100 m
first
Q: Which runner stopped for a rest?
A: Charlie stopped for a rest at 50 m.
Q: How long did he stop for?
A: He stopped for 5 seconds.
Q: How long did Bob take to complete the
race?
A: 14 seconds.
Q: Calculate Albert’s average speed
A: speed (m/s) = distance (m)/time (s)
speed (m/s) = 100m/12s
• Air Resistance
• Liquid Resistance
• Surface Texture
• Surface Area
• Lubrication
Air Resistance:
Molecules of gas
in the air push
against the surface of
moving objects and
slow their
movements.
Texture of the Surface
• The rougher the surface, the
greater the friction. Smooth
surfaces reduce friction. Carpets
have more friction than tile floors.
Soles of shoes have rough
textures to increase friction
between the shoes and the floor
so that it is possible to walk
without slipping.
Amount of Surface Area
• The greater the surface area, the
greater the friction. If tires of a car
or truck are larger, more surface
area of the tire will touch the road
making friction greater. Trucks
have larger tires to make it easier
for them to stop or slow down.
How does surface texture affect the
amount of friction on a moving
object?
• The Magic School Bus Plays Ball (24 min)

http://www.fearofphysics.com/Friction/friction
.html
Experiments with friction on rubber tires and
different conditions of asphalt of the road

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