What is Force?
• A force is a push or pull that causes an object to
move faster or slower, stop, change direction, or
change size or shape.
• Without force, nothing would ever move.
Let’s review information about
forces!
http://www.engineeringinteract.org/resources/parkworldplot/flash/concepts/allabo
utforces.htm
Magnetic Force
Magnetism is the force of pushing or pulling between
magnetic poles. It acts at a distance and cannot be
seen.
Materials that create this force are said to be
magnetic and are called magnets.
When like poles of magnets are near each other, a
repulsive force exists, and the magnets move away
from each other if the force is great enough.
When opposite poles of magnets are near each other,
an attractive force exists and the magnets move
toward each other if the force is great enough.
The closer the objects, the greater the magnetic force.
The magnetic force is greatest at the poles of
magnets.
http://www.engineeringinteract.org/resources/parkworldplot/flash/concepts/magneticforces.htm
Let’s look at what we already know about forces.
What would happen with these pairs of magnets?
Will they attract or repel each other?
Write and draw what you think will happen, and
then we will see if you are correct.
1.
2.
3.
Gravitational Force
• Gravity is a
force that pulls
all objects
toward each
other.
http://www.engineeringinteract.org/resources/parkworldplot/flash/concepts/gravity.htm
•The force of gravity causes two objects of
different weights to fall to the ground at the
same time.
•The force of gravity is a pull that attracts
objects to each other. This attraction is not
noticeable unless one of the objects is very large
(like a planet, moon, or the Sun). The force of
gravity between Earth and anything on it is
extremely noticeable because the mass of Earth
is so large.
•The pull of Earth’s gravity makes any object fall
to the ground. As The Moon goes around
Earth, its gravity pulls on Earth causing water in
the oceans to move toward the Moon.
•Earth’s gravity also pulls on
the Moon. This force of gravity
keeps the Moon moving
around Earth. Similarly, the
pull of the Sun’s gravity keeps
Earth moving around the Sun.
http://www.seed.slb.com/en/scictr/watch/skydiving/galileo_pisa.htm
Which will hit the ground first?
They travel at
the same speed!
The size of the
mass does not
change the rate
of speed that
objects fall.
Which will hit the ground first?
The paper was
slowed by air
getting trapped
under as it fell.
Shape of object
may affect the
speed in which
it falls!
Frictional Forces
• Friction is the force that is applied by a
surface as an object moves across it.
• It is always present when two surfaces rub
together.
• Friction is affected by the nature of the
surface (bumpy, smooth) and upon the
degree to which they are pressed together .
http://www.engineeringinteract.org/resources/parkworldplot/flash/concepts/friction.htm
Friction
Some forces can be seen when a moving object is
touching another object, for example, a toy
sliding across a table and slowing down.
Friction is the force that opposes motion between
two surfaces that are touching.
The rougher the surfaces are, and the harder they
press together, the more friction there will be.
Friction
Friction can be reduced by using lubricants
(like motor oil, wax, or grease), by making
surfaces smoother, or by using rollers.
Friction occurs in liquids and gases as well
as between solids.
Without friction, it would be very hard to slow
or stop the motion of objects.
high
high
low
low
low
low
The effect of friction
can be changed in
the following ways:
• The rougher the surface, the greater the
friction.
• Smooth surfaces reduce friction.
So, carpets have more friction that 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 more surface of an object touches another
object, the friction will be greater. 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.
Lubrication
• Lubricants- a
substance put on a
surface to reduce
friction
We use oil in a car to
reduce friction on the
motor.
Without
lubrication,
moving parts of
machines would
slow down or
stop very
quickly.
Motion
• The motion of an object can be described by
its position, direction of movement, and
speed.
How do we know this flag is in motion?
Speed
• One way to describe motion is speed.
• Speed is a measure of the distance an
object moves in a certain amount of time.
• Speed is a measure of how fast an object is
moving.
• Motion is the change in position of an object
over time when compared with a reference
point.
Position
The position of an object is its location relative to
another object (the reference point).
For example, we can use the words “above”,
“below”, “beside”, “behind”, “ahead of” plus
the distance from the other object.
The distance (length) from the reference point
changes when the object moves.
Point of Reference
• Point of referenceyour point of motion
when you observe a
motion
Starting Position
• Speed, ramp height,
length, and surface
type can affect
motion.
• The higher the
position, the greater
the speed and
distance will be in the
object’s motion.
Balanced and Unbalanced
Forces
• http://www.engineeringinteract.org/resourc
es/parkworldplot/flash/concepts/balanceda
ndun.htm
Balanced Forces
• Balanced forces are
forces equal in size
and opposite in
direction.
• Neither object moves
because it is equal in
both directions.
Unbalanced Force
Unbalanced forces occur when one force is
greater than its opposite force. They
cause the object’s motion to speed up,
slow down, or stop. The object moves in
the direction applied by the greater force.
Unbalanced forces cause a non-moving
object to start moving. They affect the
rate and direction of motion in objects.
Unbalanced Force
Think of the game tug-of-war:
If you are the only one pulling on your side, and on the other
side there are three people pulling, the forces are
unbalanced because it is three pulling against one.
However, if three people were on each side, the forces would
be more balanced, making the rope move less quickly.
The Jeep pulling the car is another example of an unbalanced force
because the Jeep has a bigger and more powerful force.
If the Jeep was pulling another Jeep, the forces would be balanced and
neither would move very far because the force being used is the same
amount.
Rate of motion
Rate of motion is the speed of the object or
how fast or slow the object is moving.
Unbalanced forces can cause the speed or
the rate of motion to change by increasing,
decreasing, or stopping the motion.
Direction
If the total force is unbalanced the forces acting on
an object are not equal in all directions.
Thus, the unbalanced force causes a change in
the direction of the motion as follows:
The object moves away from the unbalanced force
if it is a push, or toward the unbalanced force if it
is a pull. The object will move in the direction of
the greatest force acting on it.
• Speed of an object is the measure of the
distance (change in position) an object
moves in a given amount of time.
• Speed is distance divided by time, so you
can take the value on the Y axis and
divide it by the value on the X axis.
Slower
Speed
The line on
the graph is
a flatter line,
illustrating a
slower
speed
Faster
Speed
Speed Stopped
The line on
the graph is
a steeper
line,
illustrating a
faster speed
The line illustrates a
constant speed from
0-5 seconds. From 5
to 10 sec, the line is
flat with no increase
in position. This
graph illustrates an
object that is stopped
between 5 and 10
seconds.
A change in force or
mass affects the
motion of an object as
follows:
Force
• As the force increases, the speed of an object
increases.
• As the force applied to an object is decreased,
the object will move slower than the object that
was given a greater push or pull.
• If there is no friction (for example, in outer
space), an object that is already moving does
not need a force to keep it moving.
• Because of friction, however, an object slows or
stops eventually.
Mass
• As the mass increases, the speed of an object
decreases if the force remains the same.
• The speed decreases as the object’s mass
increases.
• It is much harder to change the speed of a
heavy object than a light object.
• An object with a small mass is easier to stop or
cause a change in motion than an object with a
large mass.
All About Forces
• http://www.engineeringinteract.org/resourc
es/parkworldplot/flash/concepts/allaboutfor
ces.htm