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Forces
A force is a PUSH or a PULL.
 Described by:
1. Its strength
2. The direction in which it acts
 Measured in: Newtons (N)
 Measured by: Spring Scale
 Represented by: An arrow (direction)

Forces Acting in the Same
Direction
Unbalanced
Motion
Occurs
5N
5N
We can add two forces together to
produce a larger net force.
5N + 5N = 10 N
Forces Acting in the Opposite Direction
With Motion
5N
5N
5N
5N
5N
Unbalanced Motion Occurs
Forces can subtract to produce net force in
the direction of the larger force.
5N + 5N + 5N +5N + 5N = 5 N
Forces Acting in Opposite Direction
Without Motion
No Motion Occurs
5N
5N
Forces may cancel each other and
produce no net force.
5N + 5N = 0 N
Friction – A Force
Friction is the force two surfaces exert on
each other when they rub against each
other.
The strength of the force of friction
depends on two things:
1. How hard the surfaces push
together.
2. The types of surfaces
involved.
Four Types of Friction
Static Friction
The friction that
acts on objects
that are not
moving.
 To move a
stationary object,
you must
overcome static
friction.

Four Types of Friction
Sliding Friction
The friction that
occurs when two
solid surfaces slide
over one another.
 Examples:
Brakes on a bicycle
Sliding on ice
 Less friction than
static friction

Four Types of Friction
Rolling Friction
The friction
between
something that
rolls and the
surface it is rolling
on.
 Examples:
Roller skates
Ball bearings
 Less friction than
static or sliding
friction

Four Types of Friction
Fluid Friction



The friction that
occurs when a solid
object moves through
a fluid. (water, air, oil,
etc)
Examples:
Surfing
When objects rub
against air
Meteors in the
atmosphere
Least of all 4 types of
friction
Gravity
Newton concluded that a force acts to pull
objects straight down toward the center of the
Earth.
 Gravity – a force that pulls objects together
 Gravity acts everywhere, not just on Earth


Gravity = G where G = (9.8 m/s2) (mass of object)
Two Factors That Affect Gravitational
Pull

Two factors that affect the gravitational
attraction between objects
 Mass
 Distance
○ Mass , Force (more mass, more attraction)
○ Distance , Force (more distance, less attraction)
Law of Universal Gravitation

The force of gravity acts between all
objects in the universe.
Weight vs. Mass
What’s the difference?
Weight changes if gravitational pull
changes.
 Mass does not depend on gravitational
pull; it stays the same regardless of
location.
 Weight – a force of gravity on a person
or object at the surface of a planet.
 Mass – the measure of the amount of
matter in an object.

Newton’s 1st Law
Why does an object move?
Why does an object stop moving?
An object in motion will remain in motion
or an object at rest will remain at rest
unless acted upon by an outside force.
 Inertia – the tendency of an object to
resist change in motion
 Newton’s First Law is sometimes called
the law of inertia.
 Ex. Earth going around the Sun

NASCAR Wreck
Newton’s
nd
2
Law
What happens to the acceleration of an object if I increase its mass?
What happens to the acceleration if I increase the force applies to the
object?
The amount of acceleration is dependent upon
the mass of the object and the amount of force
applied to the object.
 Acceleration = net force/mass


F = ma where F = force, m = mass, a = acceleration
Increasing mass decreases acceleration
 Increasing force increases acceleration

Newton’s 3rd Law
What happens when two objects collide?
For every action, there is an equal and
opposite reaction.
 Momentum – the amount of motion:
Momentum is related to the mass and
the velocity of the object
 Momentum = (mass)(velocity)

Football Hits
Momentum

Momentum – a characteristic of a moving object
that is related to the mass and the velocity of the
object
 More mass, more momentum
 More momentum, harder to stop
 Momentum = mass x velocity
 Law of Conservation of Momentum: In the absence of
outside forces, the total momentum of objects that
interact does not change. Friction is an outside force.
 Conservation – The amount of momentum has been
conserved if the amount of momentum is the same after
an event as it was before the event.