The Nature of Force Chapter 10 section 1

```The Nature of Force
Chapter 10 section 1
What is a force

Force is a push or pull.

Forces are either
balanced or
unbalanced.





Unbalanced Forces
Can cause an object to start moving, stop moving,
or change direction.
Unbalanced forces acting on an object will change
the object’s motion.
When 2 forces act in the same direction they add
together.
When 2 forces act in opposite directions they add
together however one number is positive and one
is negative.
When 2 equal forces act in opposite directions the
object stays put.
What is the net force?

The net force is the
overall force after all
together.

When there is a net
force acting on an
object the forces are
unbalanced.
Balanced Forces

Equal forces acting on
one object in opposite
directions are called
balanced forces.

Balanced forces acting
on an object will not
change the objects
motion.
Newton’s First Law of Motion
An object at rest will remain at rest
and an object moving at a constant
velocity will continue moving at a
constant velocity unless acted upon
by an unbalanced force.
Inertia

INERTIA is the tendency of an object to
resist change in motion.

Example: when the car stops suddenly and
you continue to move forward.
Mass

Mass is the amount of matter in an object.
 SI unit is the kilogram.
 The amount of inertia an object has depends
on its mass. The greater the mass the
greater the inertia.
Force, Mass, and Acceleration
10 - 2
Newton’s
nd
2
Law of Motion
The net force on an object is equal to
the product of its acceleration and its
mass.
Force = Mass x Acceleration

Sometimes people
refer to the equation as
Newton’s 2nd Law.
Units of measurement

One Newton = 1 kg x
m/s/s

Force = ma

Acceleration = f/m
Changes in force and mass


How can you increase
the acceleration of the
wheel barrel?
Increase Force
 Decrease Mass
Friction and Gravity
10 - 3
Friction
Friction is the force that one surface exerts on another
when the two rub against each other.
Friction opposes motion.
Friction acts in a direction opposite to the object’s
direction of motion.
Without friction an object
would continue to move at a
constant speed forever.

The strength of the
force of friction
depends upon two
factors: the types of
surfaces involved and
how hard the surfaces
push together.
Is friction useful?

Depends on the
situation – helps us
shoes and tires.
Different types of friction
Sliding friction – solid
surface sliding over
one another.
 Rolling friction –
object rolls over a
surface.
 Fluid friction – objects
move through a fluid.

Gravity – force that pulls the
object toward the earth.
Free fall
Projectile motion
Air resistance
weight.
Free Fall

When the only force
acting upon the falling
object is gravity the
object is said to free
fall.

As an object falls it
accelerates at a rate of
9.8 m/s/s
Acceleration due to the force
of gravity.

9.8 meters per second squared.
 After 1 sec. = 9.8 m/s/s
 2 sec. =19.6 m/s/s
 3 sec = 29.4 m/s/s

Objects in free fall accelerate at the same
rate regardless of mass.
Projectile Motion

Projectile – an object
thrown horizontally.

Will a projectile land
on the ground at the
same time as an object
dropped straight
down? Yes
Air Resistance

Objects falling through the
air experience some type
of fluid friction called air
resistance.

Air resistance is not the
same for all objects.
Depends on surface area –
the greater the surface area
the greater the air
resistance.

Air resistance increases
with velocity. So as a
falling object speeds up,
the air resistance against it
increases. Eventually the
air resistance equals the
force of gravity. When
forces are balanced there
is no acceleration.
Terminal Velocity

The greatest velocity
an object reaches is
called terminal
velocity.
Weight vs. Mass

Weight is the measure of the force of
gravity on an object.

Mass is the amount of matter in an object.

Weight = mass x acceleration due to gravity.
Universal Gravitation

The law of universal gravitation states that
the force of gravity acts between all objects
in the universe.

Gravitational force depends on mass and
distance between the objects.
Action and Reaction
10-4
Newton’s third Law of Motion

States that if one
object exerts a force
on another object, then
the second object
exerts a force of equal
strength in the
opposite direction on
the first object.
Equal but Opposite

Action and reaction
forces will be equal
and opposite, the same
force acting on a
greater mass results in
a smaller acceleration.
Action – Reaction In Action
Walking – you push on ground and the
ground pushes on you.
 Squid – Water jet

Do Action-Reaction Forces
Cancel?
Newton’s third law refers
to forces on two different
objects.
 Example: Soccerball
 If one player hits the ball –
force is upward. The ball
exerts an equal but
opposite downward force
on the player. The action
and reaction forces are
acting on different objects
and therefore cannot be

Momentum = mass x velocity

A small mass can
produce a large
momentum if it is
moving at a high
velocity ie. Bullet.
Conservation of momentum

The total momentum of any group of
objects remains the same unless outside
forces act on the objects.

Conservation means the conditions before
and after some event.
Two Moving Objects

Two snowboarders
traveling the same
direction bump into
each other.

Momentum is
conserved - The back
person slows down
while the front person
speeds up.
Orbiting Satellites
10 - 5
Rocket Launch

Rockets can rise into
the air because the
gases it expels with a
downward force exert
an equal but opposite
force on the rocket.
What is a satellite?

Any object that travels
around another object
in space.
 Centripetal force is
any force that causes
an object to move in a
circle.

Centripetal force is the
gravitational force that
pulls the satellite
toward the center of
the Earth.
Satellite Motion

Satellites in orbit around Earth continually
fall toward the Earth, but because Earth is
curved they travel around it. A satellite is a
projectile that falls around the Earth rather
than into it.
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