Force and Motion- Sci. 8

Getting to Know Sir Isaac
Newton
 Origin/ Background
 Branch of Science
 How did he change the world for the
better?
 Best known for:
 Why is it important to get know Sir Isaac
Newton through his work? What does this
knowledge tell you about the study of
motion?

The Natural of Force
FOCUS POINTS:
 Explain how balanced and unbalanced
forces are related to motion.
 Describe friction and identify the factors
that determine the friction force between
two surfaces.

Stack several metal washer on top of a
toy car. Place a heavy book on the floor
near the car. Predict what will happen
to both the car and the washers if you
roll the car into the book.
What might be the reason for any difference
between the motion of the car and the washer?

Forces and Motion
 Aristotle's view of motion is "it
requires a force to make an
object move in an unnatural"
manner - or, more simply,
"motion requires force".

 Galileo’s studies of falling and
rolling objects showed that the
natural tendency of an object was
to continue moving at a constant
rate in straight line, and that no
force was needed to sustain its
motion.
 He reasoned that the object would
stop moving due to a frictional
force, which when eliminated
would cause the object to remain
in motion, unless it encountered
an external force.

 Force- as a push or pull acting upon an
object as result of its interaction with
another object.
Carriage being pulled by a
horse or a game of tug- of-
war when two opposing
teams are pulling on the
ropes.

What changes can force do to
motion?
1. Forces can make an object move
starting from the rest position.
2. Force can make a moving object
stop.
3. Force can change the direction
of a moving object.

 Whenever an object moves from a rest
position, its velocity increases; if its
stops, then its velocity decreases.
 When it changes direction, the objects
velocity also changes.
 The direction of motion depends on the
direction of the applied force.

 Force is the quantity which is
measured using a standard metric unit
known as the newton (N).
 One newton is the amount of force
required to give a 1-kg mass an
acceleration of 1 m/s.
 1N + 1kgm/s

A force may not always cause an
object to move, especially if there is
more than one force present.
This single force that acts and
causes a change in the state of
motion of an object is known as the
net force of the resultant force.
The resultant force is the vector sum
of all forces acting in an object.

If no changes in state of
motion are produced, these
forces are said to be
balanced force. If there is a
net force, then this force is
said to be unbalanced force.

 Suppose you and a friend are playing
tug-of-war , and you are both pulling
the opposite ends of the rope. You are
pulling the rope toward you with a force
of 5N while your friend is pulling toward
the opposite direction with the same
force of 5N. What is the net force on
the rope?

Types of Forces
 Forces can be classified into two basic
types based on how are applied or how they
act on the other objects.
 Contact Forces- forces that require physical
contact
 Non-contact force or action-at-a-distance
force or field force- does not require physical
contact to have any effect on the other
object.

Examples of contact forces
 Applied force
 Normal force
 Frictional force
 Air resistance force
 Tensional force
 Spring force

Non- contact force
Gravitational force
Electrical force
Magnetic force

Contact Forces
Applied force (Fapp)
- An applied force is a force applied to an
object by another object or a person.
The direction of the applied force
depends on how the force is applied
Normal Force (N)
- Is the support force extend upon an
object that is in contact with another stable
object. Sometimes referred to as the
pressing force.

 Frictional Force (f)
- The force exerted by the surfaces in
contact with each other and move
relatively being pressed with each other.
The friction force opposes the motion of
an object. The friction force can be
calculated using the equation: f= µN.
Where µ is the coefficient of friction and
N is the normal force

Air Resistance Force (Fair)
- Is a special type of frictional force that acts on
objects as they travel through the air. Like all
frictional forces, the force of air resistance
always opposes the motion of the moving
object. ( Skydiver or downhill skier)
Tensional Force ( T)
Is transmitted through a string, rope, or wire when
it is pulled tight by forces acting at each end.
Spring Force (Fstring)
Is the force exerted by a compressed or stretched
spring on my object that is attached to the string.

Non-contact Force
 Force of Gravity (also known as Weight, W)
- Is the force with which the Earth, moon, or other
massive body attracts an object toward itself.
- This force on an object on Earth is always equal
to the weight of the object as given by the
equation: W=Mg, where g is the acceleration due
to gravity which has a value of 9.8m/s² (on Earth)
and m is the object’s mass (in kg)

 Electrical Force
- The force that exists between
charges. It can either be attractive
or repulsive force.
Magnetic Force
-Force that exists in magnets. It is
the force that makes magnet attract
or repel objects or other magnets

Free- Body Diagram
 Forces can be seen based on how they
acted on an object. Forces acting on an
object can be shown by representing
them as arrows. This representation is
called a Free-Body Diagram (FBD). An
FBD is a diagram showing all the
known and unknown forces acting on
the object indicating their magnitudes
and directions.

Force and Motion- Sci. 8

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