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.
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.
Magnetic Force
Continued…
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.
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.
Gravity Continued…
•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.
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.
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 type of surface (bumpy,
smooth) and by how hard they are pressed together
Friction Continued…
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.
Friction can be reduced by using lubricants (like motor
oil, wax, or grease), by making surfaces smoother, or
by using rollers.
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 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 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.
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) is the change in position of an
object over time when compared with a reference point.
Point of Reference
 Point of reference is
your point of view
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 Forces
 Balanced forces are
forces equal in size and
opposite in direction.
 Neither object moves
because it is equal in
both directions.
Unbalanced Forces
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.
Examples: Tug-of-war
A truck pulling a car
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.
 The 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.
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.
Changing Forms of Energy
 Energy is most noticeable as it transforms from one
type to another.
 What are some examples of transforming electrical
energy?
 A light bulb
 A hair dryer
Changing forms of Energy
 An example of transforming chemical energy is a car
engine. Chemical potential energy in gasoline is
transformed into kinetic energy of the car as it moves!!
KE and PE
 In many situations, there is a conversion between
potential and kinetic energy.
 The total amount of potential and kinetic energy in a
system is called the mechanical energy
 Mechanical energy = PE + KE
Mechanical Energy
 Mechanical energy is due to the
position and motion of the
object.
 What happens to the mechanical
energy of an apple as it falls from
a tree?
Mechanical Energy
 The mechanical energy does not change because the
loss in potential energy is simply transferred into
kinetic energy.
 The energy in the system remains constant!!
The Law of Conservation of Energy
 The Law of Conservation of Energy states that energy
cannot be created or destroyed.
 Energy is transformed… not destroyed
Energy in Your Body
 Even the energy converted in your body follows the
law of conservation of energy.
 Chemical potential energy is transferred to kinetic
energy that allows your body to move!!