# The Nature of Force Chapter 3 section 4

```The Nature of Force
Chapter 3 section 4
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.
Friction and Gravity
3.5
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.
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.
Weight

Mass is measured in grams
 Weight is a force measured in Newton’s.
 A kilogram of mass on Earth weighs 9.8N
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.
Force, Mass, and Acceleration
4.1
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
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.
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
Changes in force and mass


How can you increase
the acceleration of the
wheel barrel?
Increase Force
 Decrease Mass
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.
4:2 Projectile and Circular
Motions
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
Horizontal Motion
Motion that is parallel to the Earth’s
surface.
 Once the ball is thrown forward its
horizontal velocity is constant if you ignore
air resistance.

Vertical Motion

When you let go of the ball the ball has
horizontal motion but since gravity starts to
pull it downward it has vertical motion.
 Not the ball has constant horizontal velocity
with increasing downward vertical velocity.

Demo: Ball dropping straight down vs. ball
being thrown as a projectile.
Motion along curves

A change in acceleration is a change of
speed or direction.
Centripetal Acceleration

Acceleration towards the center of a curve.

When riding a bike the rider experiences
centripetal acceleration and they lean
toward the inside of the curve.
Free Fall

When an object is influenced only by
gravity it is said to be in free fall.

Space shuttles, satellites are in free fall –
they fall around the Earth
Centrifugal Force

This the force acting away from the center
of a curved or circular path.
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.
```