Transcript Forces and Motion

Forces and Motion
Motion
 What
do you think it means to be in
motion? How can we tell if something
is in motion?
Motion
 Motion:
a change in position, measured
by distance and time.
Motion
•
Determining if an object changes
position requires a frame of reference.
The Earth is the most common frame of
reference
Speed
 Speed:
the distance traveled by a moving
object per unit of time
To calculate speed, use the equation
Speed = distance / time
 Speed
only gives distance and time!
Average Speed
 Average
speed: the speed of moving
objects is not always constant
 To calculate average speed you look at
total time and distance
Average speed =
total distance / total time
Average Speed
 1.
What is your average speed if you take
2h to jog 8 miles ?
 Speed= distance/ time
 8m/2h= 4 mph
 2. What is the average speed of a car if it
travels 500 miles in 20 hours?
 Speed= distance/ time
500m/20h = 25 mph
Velocity
 Velocity
is speed in a given direction.
 Velocity
gives distance, time, and the
direction of travel.
 Velocity
changes by changing direction
or changing speed!
Scientists have a special way to
measure a change in velocity:
Acceleration
Acceleration is a change in
velocity.
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Acceleration
 The
change in velocity/the time it takes for
the change to occur
Acceleration =
(final velocity – initial velocity)
time
Velocity is measured in m/s and time in
seconds.
Calculating Acceleration
A roller coaster’s velocity at the top of the
hill is 10 m/s(initial) Two seconds later its
velocity is 26 m/s (final). What is the
acceleration of a roller coaster?
Acceleration= final velocity- initial velocity
time
Accel= 26m/s -10m/s = 16 = 8 m/s/s
2
2
The Way Acceleration Works:
 If
you’re speeding up, you have
positive acceleration.
 If you’re slowing down, you have
negative acceleration.
 If you’re changing speed quickly, you
have high acceleration.
 If you’re changing speed slowly, you
have low acceleration.
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True or false?
1.
2.
3.
If you slow down on your bicycle, you
accelerate.
If you ride your bicycle at a constant
speed, you cannot accelerate.
Changing the speed and changing the
direction of your bicycle are both
examples of acceleration.
Example:
You’re driving along in
your car and suddenly
have to slam on your
brakes for a red light.
Negative or positive
acceleration? High or
low acceleration?
High negative acceleration because you’re
slowing down quickly.
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Momentum

Momentum
 The measure of how difficult it is to stop
a moving object
p = mv
p
m v
p:
m:
v:
momentum (kg ·m/s)
mass (kg)
velocity (m/s)
Calculating Momentum
 A 20,000
kg ship is traveling east at 20
m/s. What is the momentum of the ship?
 P=mv
 20,000 kg X 20 m/s =
 400,000 kg* m/s
Law of conservation of momentum
 According
to the law of conservation of
momentum: the total momentum of
objects that collide will be the same before
and after the collision.
 Momentum
collision.
will not be lost during the
Quick Review
 Velocity:
speed of motion
 Acceleration:
change in velocity
The measure of
how difficult it is to stop a
moving object
 Momentum
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Force
Force: any push or pull
Forces are measured in _____
A _____ is the combination of
all of the forces acting on an
object.
Forces
Force

Balanced Forces


forces acting on
an object that are
opposite in
direction and
equal in size
no change in
velocity
Balanced vs. Unbalanced
Forces
Balanced forces will not cause a
change in the motion of an object; they
have a net force of zero N.
 Examples
of balanced forces are:
 1. ______, 2. _____, 3. _____ .
Balanced vs. Unbalanced forces
 Unbalanced
forces could be best
described as
__________________________ .
 Examples
of unbalanced forces are
___________________________ .
Force Drawing
 Create
a drawing that shows an
example of balanced forces. The
drawing should show the forces
acting on the object and what
happens to the object if the forces
become unbalanced.
Forces
Forces
 You
and a younger brother or
sister are at a neighborhood pool.
Your brother or sister asks why
there are signs that say “NO
RUNNING!” What would be your
answer?
Forces
 _____
, ______ , and ______ are
examples of common lubricants.
 Therefore,
it is easier to slip when there is
water on the floor because the water acts
as a ______ that lessens the friction
between the bottom of your shoe and the
floor.
Pushing pedals, pressing
brakes, and kicking soccer balls
are all examples of forces.
A force is anything that changes the velocity of
an object.
But what’s a change in velocity?
A change in velocity is acceleration, so . . .
A force is anything that gives an object
acceleration.
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Friction
 _____
is a force that opposes motion
between two surfaces that are in contact.
 A _____
is a substance that lessens
friction.
Friction

Friction
 force that opposes motion between 2 surfaces
 depends on the:
• types of surfaces
• force between the surfaces
 A _____ is a substance
that lessens friction.
Friction
 There
are four types of friction.
Friction

Four Types of Friction




Static Friction – force that acts on objects that
are not moving. (Couch Potato)
Sliding Friction - force that opposes the
direction of motion of an object as it slides over
a surface. (Ice skating or bobsledding)
Rolling Friction – friction force that acts on
rolling objects. (Rollerblading)
Fluid Friction – force that opposes the motion of
an object through a fluid. (Planes flying or
submarines traveling)
Friction
 Write
a story
that describes
what a day in
your life would
be like if there
was no
friction.
Gravity
Gravity
 Isaac
Newton discovered the ______
______ of Gravitation.
 Gravity
acts on any objects with mass.
Example: You and your books, pencils,
paper, etc.
Universal Law of Gravitation
 Mass
and gravity have a directly
proportional relationship.
 The
distance between the centers of
masses of the two objects and gravity
have an inversely proportional
relationship.
Differentiate between Mass and
Weight.
 Weight
is dependent upon the
acceleration due to gravity on a
particular planet.
 For
 The
instance: g on earth is 9.8 m/s²
larger the planet, the _____ the
gravitational force.