Download Forces and Motion

Forces and
Motion
Why do objects move?
A force acts on them
a FORCE is a push or a
pull, in any direction,that
may cause a change in
an object’s motion
Will a force always cause an object
to move?
NO
Every object has several forces
always acting on them.
Gravity
Air Pressure
Friction
Air Pressure
Friction
Normal
The sum of all the forces acting on
an object is called NET FORCE
If an object is motionless, the
Net Force = 0 , The forces
are said to be Balanced.
With Balanced Forces, the
forces are equal but in
opposite directions
Since the applied force is not
The forces are said to be
stronger than the force of friction
Balanced…
the object does not move
Gravity
Air Pressure
Applied Force
Air Pressure
Friction
Normal
If a force is exerted on an object
causing it to move, this is
called an UNBALANCED
FORCE, the net force is
greater than zero and the
object moves.
If the applied
force is
strong
enough than
to over
The applied
force
is stronger
come the
existing
forces,
objectofwill
friction…
this
is an the
example
an
move
Unbalanced
force
Gravity
Air Pressure
Air Pressure
Friction
Applied Force
Normal
Some Common Forces
Friction
The force exerted when two
objects rub up against each
other.
Friction opposes motion, it
causes objects to slow down
What effects the Friction between
objects?
• Mass of the Object
• Type of Surface
• Amount of surface area
touching
Sliding Friction
Motion
Friction
Rolling Friction
The amount of friction between the two
objects is less than sliding friction
The object is easier to
move
Liquid Friction
A lubricant, such as oil, makes a thin
layer between the two objects.
Which one will move farther if an
equal force is applied
200 kg
1000 kg
Which one will move farther if an
equal force is applied
On ICE
200 kg
On Carpet
200 kg
Which one will move farther if an
equal force is applied
200 kg
200 kg
Gravity
• The force that pulls objects towards
each other
• Every object has gravity, meaning
that all objects are being pulled
toward each other.
• So why are we not being pulled
together right now?
The larger the object is, the more
we are effected by its gravitational
pull.
We are more attracted to the Earth’s
gravity than we are to gravity of the
things in the room
Where will the red ball move
Since the green ball is
bigger, its gravity effects
the red ball more than
the white ball does, so
the red ball moves
towards the green ball
Why do the two objects not hit the
ground at the same time?
Gravity is acting more
on the elephant than
on the feather. And a
heavier object
overcomes the
upward force of wind
resistance (friction)
more easily thus
falling faster than the
light object
What happens if we
can eliminate the wind
resistance?
The two objects hit the ground at
the same time
Gravity is still
acting more on
the elephant
than the feather,
but the inertia of
the elephant
keeps it from
accelerating
faster
All objects will
accelerate the
same when
the force of
gravity is the
only force
action on it.
Motion
When the position of an object is
changing
An object is in motion when the
distance between two objects is
changing
How does a force
effect the motion of
an object?
When a force is applied to an
object it can change its
motion by causing it to
speed up, slow down or
change directions
How can we tell if an
object is in motion?
Use a Reference
Point,
A place or object
used for
comparison to
determine if an
object is moving
Speed
How fast an object is moving
The distance an object travels in
a certain unit of time
Equation for Speed
Speed =
Distance
__________
Time
Speed Problem
A person drives a car for 5 hours and goes
150 miles, how fast was he traveling?
150 miles
Speed = _________
5 hours
= 30 miles per hour
Velocity
Speed in a certain direction
Velocity Problem
A person drives a car from Houston to Dallas it
took 5 hours to go the 300 miles, What was his
velocity
Velocity =
300 miles
_________
5 hours
= 60 mph NORTH
Direction from
Houston to
Dallas
Acceleration
The rate at
which
velocity
changes
When does something accelerate
It speeds up….
It slows down …
Remember that velocity involves direction
Changes direction…
An object may be moving at a
constant speed but if it changes
direction it is said to be
accelerating
N
W
E
S
Say the speed of this ball is 10 mph.
Its Velocity is 10 mph EAST. It
has no speed in any other
direction.
N
W
E
If the ball changes direction, it then
has a velocity of 10mph in the new
direction, which is south here.
S
Velocity = 10 mph East, and 0 mph South
Change of direction =
acceleration
Velocity = 10 mph
South, and 0 mph
East
VELOCITY = SPEED + DIRECTION
• If you chance the speed, does the velocity
change?
Yes
If you change the Direction does the Velocity
change?
Yes
So if you change either the speed or direction
Velocity changes
Is it accelerating
A ball rolling to a stop?
YES… its slowing down or decelerating
Is it accelerating
A car traveling at 30 mph then going up to
60 mph to pass a slower moving car?
Yes… it changed its speed from 30mph to
60mph
The moon orbiting around the Earth
YES! The moon
may be at a
constant speed,
but it is constantly
changing
directions
Graphing Speed
What was the
speed of the
object at Point A
B
A
Distance = 250 meters
Time = 15 seconds
Speed = distance/time
250meters/15 seconds=
B
16.6 m/s
A
Distance = 250 meters
Time = 15 seconds
What was the
speed of the
object at Point B
B
A
Distance = 500 meters
Time = 30 seconds
Speed = distance/time
500meters/30 seconds=
B
16.6 m/s
A
Distance = 500 meters
Time = 30 seconds
A straight line
means the
object has a
constant speed
It is not speeding
up or slowing
down
Object A
Which object
had the
faster speed
Object B
Object A
Object B
500m/30s = 16.6m/s
100m/30s = 3.3m/s
The steeper the slope,
the faster the object
went
Describe the object’s
motion at point A
D
A
C
B
It has stopped
What is the speed of
the object at Point
B?
A
B
D
C
O m/s, a flat line
means the object
has stopped
What is the objects
speed at point C?
5 seconds
50 Meters
A
B
C
50m/5s = 10m/s
D
What is the Average Speed of the
Object?
Average speed = total distance/total time
D
A
C
B
350m/30s = 11.6m/s
S = 16.6 m/s
An upward curved line means
that the object is accelerating,
it keeps getting faster and
faster
S= 8.8 m/s
S= 6.6 m/s
How do we find Acceleration
Final velocity – Initial Velocity/ time
Final velocity – Initial Velocity/ time
16.6 m/s - 0 m/s / 30 s
16.6m/s / 30 s = .55m/s
2
A downward sloped line
means the object is
slowing down
Sir Isaac Newton and the 3 Laws of
Motion
Inertia
The resistance of an object to
changes in motion
Basically Stated… its harder to alter the
motion of a heavier object than a lighter
object
Newton’s First Law
An object in motion will stay in motion
and an object at rest will stay at rest
until acted on by a force.
Basically stated… objects keep moving until
something slows them down (friction) and
objects will stay still until something causes them
to move (force)
Newton’s First Law Applied:
As the wall stops the car… the guy
keeps going, there is nothing to
stop his forward motion
Newton’s First Law Applied:
as the truck stops, there is nothing
keeping the ladder from stopping; it
continues to move forward until
gravity and wind resistance stops it
Newton’s Second Law
Shows the relationship between an
objects mass its acceleration and
the applied force.
Basically stated… it takes a stronger force to
move a heavier object that a lighter object
and a stronger force to get an object to
move faster
Newton’s Second Law Applied:
If an equal force is applied, the less
massive object moves farther
200 kg
1000 kg
Newton’s 2nd Law Applied:
In order to move the more massive
object the same distance at, it will
take a much larger force
200 kg
1000 kg
Newton’s Third Law
For every action, there is an
opposite and equal reaction
Basically stated… apply a force in one
direction, there is the same force being
applied right back at you
Action: Thrust
Reaction: Jet
moves forward
As the swimmer pushes against
the water, the water pushes
back and the swimmer moves
forward
Which Law Applies
If you step off
a boat, the
boat is
moved
backward
THIRD
LAW
Which Law Applies
Second Law;
its harder to
throw a
heavy
object than
a light one
Which Law Applies?
Third Law: the
downward
force causes
the space ship
to go up
Which Law Applies?
nd
2
law: it takes a big force
to mover the heavy object
First Law: The object will not move
until something causes it to move
Would this cart be able
to make the loop on
this roller coaster?
Newton’s 3rd law says
every action has an
equal reaction…
Here is the
action
force…
Remember.. Equal… not
greater…
So the reaction force can
only be this high…
Here is the
action
force…
The cart would only go
this high then fall back