Download FORCES

FORCES
FORCES
 PUSH
OR A PULL
 DESCRIBED NOT ONLY
BY HOW STRONG THEY
ARE, BUT ALSO BY THE
DIRECTION IN WHICH
THEY ACT
UNBALANCED
FORCES
2
Forces act in the same
direction----add together
+
 The
=
width of the arrow =
strength of force
UNBALANCED
FORCES
2
Forces act in opposite
directions----add together
+
=
Like adding “+” and “-”
UNBALANCED
FORCES
2
Forces acting in
opposite directions---add
together
+
=
**equal forces in
opposite directions cancel
each other out
UNBALANCED
FORCES
 Net
force: overall
force of an object
after all the forces
are added together
UNBALANCED
FORCES
There is a net force acting on
an object.
 Can caused an object to

 Start
moving
 Stop moving
 Change direction

Will change an object’s motion
 Cause
an object to accelerate
BALANCED
FORCES
 Equal
forces acting on an
object in opposite
directions
 Will NOT change the
object’s motion
 NO
 Net
change in acceleration
force = 0
NEWTON’S FIRST LAW
OF MOTION
 Inertia:
the tendency of
an object to resist change
in its motion
 Example: air hockey
puck
NEWTON’S FIRST LAW
OF MOTION
 An
object at rest will
remain at rest and an
object that is moving at a
constant velocity will
continue moving at a
constant velocity unless
acted upon by an
unbalanced force
NEWTON’S FIRST LAW
OF MOTION
 Also known as the Law
of Inertia
 Explains many
common events
NEWTON’S FIRST LAW
OF MOTION
The amount of inertia an object
has depends on its mass.
 The greater the mass of the
object----the greater the inertia
the object has
 Mass can be defined as a
measure of the inertia of an
object

NEWTON’S 2ND LAW OF
MOTION
 Explains
how force, mass,
and acceleration are related
 Net force on an object is
equal to the product of its
acceleration and its mass
 FORCE = MASS X
ACCELERATION
NEWTON’S 2ND LAW OF
MOTION
Mass measured in kg
 Acceleration measured in m/s2
 Force measured in
(kg x m/s2) or N (Newtons)

N = 1kg x 1m/s2
N = the force needed to move 1
kg of mass at 1 meter per
second per second
NEWTON’S 2ND LAW OF
MOTION
Acceleration = Force
Mass
NEWTON’S 2ND LAW OF
MOTION



A 52 kg water-skier is being pulled
by a speedboat. The force causes
her to accelerate at 2 m/s2.
Calculate the net force that causes
this acceleration.
Force = 52 kg x 2 m/s2
Force = 104 N
NEWTON’S 2ND LAW OF
MOTION

What is the net force on a
1,000kg elevator accelerating at
2 m/s2?

A net force of 825 N is needed
to accelerate a 55kg cart. What
is the acceleration?
NEWTON’S 2ND LAW OF
MOTION




Acceleration and force change
in the same way
force causes
acceleration
Acceleration and mass change in
opposite ways
mass causes
acceleration
Friction & Gravity
 Friction:
the force that
one surface exerts on
another when the two rub
against each other
 Acts in a direction
opposite the object’s
direction of motion
Friction
 Strength
depends on:
 Type of surfaces
involved
 How hard the surfaces
push together
 Rough =
friction
 Smooth =
friction
Type of Friction
 Sliding
friction: solid
surfaces slide over each
other
 Rolling friction: object
rolls over surface
 Fluid friction: object
moves through gas/liquid
Gravity
A
force that pulls objects
toward each other
 Free fall: when the only
force acting on it is
gravity
 Projectile: object thrown
Gravity
A
projectile in a free fall
falls at the same rate
9.8 meters per second
for every second,
velocity increases by 9.8
meters per second
Gravity
 All
objects are supposed
to fall at the same rate
 Don’t
because of air
resistance
 Upward
force
 Type fluid friction
Gravity
 Increase
surface area,
increase air resistance
 Air resistance
increases with velocity---falling objects speed
up so air resistance
increases.
Gravity

Eventually air resistance
equals the force of gravity
 Forces
balanced—no
acceleration
 Continues to fall, velocity no
longer increases

Terminal velocity: greatest
velocity an object reaches
Gravity
 Weight:
measure of force
of gravity on an object
 Mass: measure of
amount of matter in an
object
 Weight = mass x 9.8 m/s2
Gravity
 Law
of Universal
Gravitation: the force of
gravity acts between all
object in the universe
 Depends
on mass and
distance
 Farther
apart the objects,
weaker the force
Action and Reaction
 Forces
are NOT one-sided
 Newton’s 3rd Law: If one
object exerts a force on
another object, then the
second object exerts a force
of equal strength in the
opposite direction o the first
object
Action and Reaction
 Do
action/reaction forces
cancel each other out?
 NO; forces can only be
added together if they are
acting on the same object.
Momentum
 Momentum
= Mass X
Velocity
 Quantity of motion
 kg X m/s is unit for
momentum
 Momentum is described by
its direction as well as its
quantity
Momentum
 Which
has more
momentum: a 3kg
sledgehammer swung at
1.5 m/s or a 4kg
sledgehammer swung at .9
m/s?
 M = mass x velocity