Transcript Motion and Forces - 7thGradeHillsboro

Motion and Forces
Displacement in Time and Space
Focus Questions
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What is motion?
How can you tell if an object is speeding up or slowing down?
Motion Vocabulary
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Balanced Forces
Unbalanced Forces
Inertia
Gravity
Friction
Force
Mass
Magnitude
v=d/t
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Total Distance
Total time
Motion
Position
Reference Point
Direction
Speed
Average speed
Motion is a change in position of an object
with respect to time.
www.idcide.com
Position - the location of an object.
The change in
position is
measured in the
amount of
distance an object
has moved from
one position
(reference point) to
another.
Cowpens to Downtown Spartanburg: 14 miles
www.idcide.com
www.fierydarts.com
www.telegraphics.com
Examples of units of speed are:
“meters per second” (m/s)
“kilometers per hour” (km/h)
and
“miles per hour” (mph).
Direction - the relationship of the position of a
moving object to another position.
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www.telegraphics.com
Speed - the distance traveled by an object in one unit of time.
Speed is the rate of
change of the
position of an
object, or how long
it takes something
to move a distance.
Speed does not
necessarily mean
that something is
moving fast.
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The average speed of an object tells you the
(average) time at which it covers a given distance.
While the speed of the object may vary during the
total time it is moving, the average speed is the
result of the total distance divided by the total time
taken.
Speed can be calculated by dividing the distance the
object travels by the amount of time it takes to travel
that distance. Speed measurements contain a unit of
distance divided by a unit of time.
Average speed can be calculated using the formula
v = d/t
where:
v is the average speed of the object
d is the distance or length of the path of the object
t is the time taken to cover the path
Calculate the average speed of an object in
motion:
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Snowmobile
http://sunshine.chpc.utah.edu/javalabs/java12/fnm/act1/lab.htm
Distance (in km)
Time (in hrs)
Mangler
500
15.82
Otter Pop
15
0.45
Slider
50
1.41
Snowflake
240
6.38
White Fang
30
0.75
Avg. Speed (in km/hr)
We can measure the distance and time of
an object in motion. This data can be
represented in a data table.
For example:
Time (s)
Distance
(m)
0
0
1
5
2
10
3
15
4
15
5
15
6
10
7
5
This data can then be represented on a
time-distance graph
Distance (m)
Time-Distance Graph
20
15
10
5
0
0
1
2
3
4
Time (s)
5
6
7
This graph can then be used to describe the
position, direction and speed of the motion of
the object.
Distance (m)
Time-Distance Graph
20
15
10
5
0
0
1
2
3
4
5
6
7
Time (s)
http://teacherline.pbs.org/teacherline/resources/activities/race/readings/race.htm
Distance (m)
Time-Distance Graph
20
15
10
5
0
0
1
2
3
4
5
6
Time (s)
Reference Point
Starting Place – Point of Origin
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Position
Relative to the reference point (X-axis), the object at position A is 10
meters away, at position B the object is 15 meters away, and at position C
the object is 10 meters away.
Distance (m)
Time-Distance Graph
B
B
20
A
A
15
10
C
C
5
0
0
1
2
3
4
Time (s)
5
6
7
The direction of the object is described as whether it is “moving away”
from or “moving toward” the reference point. If the object is “moving
away” from the reference point, the line will go up (distance increasing)
as in position A.
If the object is “moving toward” the reference point the line will go
down (distance decreasing) as in position C.
Distance (m)
Time-Distance Graph
20
B
B
15
A
A
10
C
C
5
0
0
1
2
3
4
Time (s)
5
6
7
The slope of the line can tell the relative speed of the object. When the
slope of the line is steep, the speed is faster than if the slope were flatter.
When the slope of the line is flatter, the speed is slower.
For example:
Flatter Slope = Slower Speed
Distance (m)
Distance (m)
Steeper Slope = Faster Speed
Tim e (s)
Tim e (s)
http://www.sycd.co.uk/dtg/
Create a data table and graph the following:
Alex and Ed left home at 1:00 PM and walked
to the movie theatre which is 2.5 miles away.
This took them 60 minutes. The movie lasted
two hours. The boys left the theatre and
walked an additional 2 miles to the store. This
took them 90 minutes because they met up
with some friends and talked for a while.
They stayed at the store 1 hour and then their
dad picked them up to take them home. They
arrived home at 7:30 PM.
Hour
Time
Distance
Activity
Describing Motion: Newton’s Laws
Newton’s First Law of Motion
(also known as Law of Inertia)
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An object at rest tends to stay at rest and an
object in motion tends to stay in motion with
the same speed and in the same direction
unless acted upon by an unbalanced force.
The behavior of all objects can be described
by saying that objects tend to "keep on doing
what they're doing" unless something
interferes.
www.3dkingdom.org
There are two forces that can affect the
movement (speed and direction) of an object.
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Gravity, which is a property of all matter, is a force that
pulls objects toward each other without direct contact
or impact. Objects on Earth are pulled toward the
center of Earth and when raised above the surface of
Earth, they fall “down” toward Earth. As objects fall
toward Earth, their speed increases at a definite rate.
www.3dkingdom.org
Friction is a force that opposes motion. It can slow
down or stop the motion of an object. The slowing
force of friction always acts in the direction opposite to
the force causing the motion.
For example, friction slows or stops the motion of moving
parts of machines. Most tires are designed to increase
friction for better traction on the road.
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www.fierydarts.com
http://www.rockcrawler.com/techreports/bfgmtkm/tire-tread.jpg
Inertia is the tendency of objects to resist any change in
motion. It is the tendency for objects to stay in motion if
they are moving or to stay at rest if they are not moving
unless acted on by an outside force.
http://www.glenbrook.k12.il.us/GBSSCI/PHYS/mmedia/newtlaws/il.html
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Inertia causes a passenger in a car to continue to
move forward even though the car stops.
Inertia is why seat belts are so important for the
safety of passengers in vehicles.
Inertia is why it is impossible for vehicles to stop
instantaneously.
http://www.glenbrook.k12.il.us/GBSSCI/PHYS/mmedia/newtlaws/il.html
Inertia is a property of the object;
it is not a force.
A pl
The force of gravity, in combination with the property of inertia, is
responsible for the orbits of moons and planets.
http://www.unitedstreaming.com/search/assetDetail.cfm?guidAssetId=27DE45E9-9B3D-478E-A546-D893FC4D2B92
http://liftoff.msfc.nasa.gov/a
cademy/rocket_sci/orbmec
h/orbit/orbit.html
Varying the amount of force or mass will
affect the motion of an object.
Force
The greater the force exerted on an object, the faster an
object will move. For example, racecars have very
large engines to produce the force needed to move
the cars so fast.
The smaller the force, the slower the object will move.
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Mass
The greater the mass of an object with the same force
exerted on it, the slower the object will move. Less
massive objects can move faster with less force.
For example, in football, backfield players who must
move faster are often less massive than linemen
who do not have to move fast.
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A tennis ball vs. bowling ball is another example.
The same force on the small mass of a tennis ball
will make it move much faster than the same force
on the larger mass of a bowling ball.
Forces have a magnitude (strength) and a direction.
Think of forces as arrows with the length of the
arrow representing the magnitude (strength) of the
force and the head of the arrow pointing in the
direction of the force. Using such arrows, the
resulting size and direction of the force can be
predicted.
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Forces occur in pairs and can be balanced or
unbalanced. They affect the magnitude (speed)
(illustrated by the length of the arrow) and
direction (illustrated by the direction of the
arrow point) of moving objects.
Balanced
Unbalanced
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Balanced forces
Balanced forces act on an object in opposite
directions and are equal in size as shown in
the arrows below. Balanced forces do not
cause a change in the magnitude or
direction of a moving object. Objects that
are not moving will not start moving if
acted on by balanced forces. Balanced
forces will cause no change in the motion
of an object.
Examples:
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In a tug of war, if there is no movement in
the rope, the two teams are exerting
equal, but opposite forces that are
balanced.
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In arm wrestling, the force exerted by
each person is equal, but they are pushing
in opposite directions.
Draw each of these forces using arrows
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Unbalanced forces
Unbalanced forces are not equal, and they
always cause a change in the magnitude and
direction of a moving object. When two
unbalanced forces are exerted in opposite
directions, their combined force is equal to
the difference between the two forces and is
exerted in the direction of the larger force.
For example, if a soccer ball (small arrow) is
kicked as it moves toward a player (long
arrow), it will move in the opposite direction
because of the force of the kick (smaller arrow
to the right of the =) as shown below:
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Or, if in a tug of war, one team pulls harder
than the other, the rope will move in that
direction as shown below:
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If unbalanced forces are exerted in the same
direction, the resulting force will be the sum
of the forces in the direction the forces are
applied. For example, if two people pull on an
object at the same time, the applied force on
the object will be the result of their combined
forces (resulting force) as shown below:
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When forces act in the same direction, their
forces are added. When forces act in opposite
directions, their forces are subtracted from
each other.
and
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Unbalanced forces cause a nonmoving object
to start moving.
nd
2
Law
Newton’s Second Law of Motion states that if
an unbalanced force acts on a body, that body
will experience acceleration ( or deceleration),
that is, a change of speed.
One can say that a body at rest is considered to
have zero speed, ( a constant speed). So any force
that causes a body to move is an unbalanced force.
Also, any force, such as friction, or gravity, that
causes a body to slow down or speed up, is an
unbalanced force.
nd
2
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Law
When mass is in kilograms and acceleration
is in m/s/s, the unit of force is in newtons (N).
One newton is equal to the force required to
accelerate one kilogram of mass at one
meter/second/second.
nd
2
Law (F = m x a)
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How much force is needed to accelerate a
1400 kilogram car 2 meters per second/per
second?
Write the formula
F=mxa
Fill in given numbers and units
F = 1400 kg x 2 meters per second/second
Solve for the unknown
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2800 kg-meters/second/second or 2800
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N
If mass remains constant, doubling the acceleration, doubles the force. If force
remains constant, doubling the mass, halves the acceleration.
rd
3
Law
 For
every action, there is an
equal and opposite reaction.
rd
3
Law
According to Newton,
whenever objects A
and B interact with
each other, they exert
forces upon each
other. When you sit in
your chair, your body
exerts a downward
force on the chair and
the chair exerts an
upward force on your
body.
rd
3
Law
There are two forces
resulting from this
interaction - a force
on the chair and a
force on your body.
These two forces are
called action and
reaction forces.
Newton’s 3rd Law in Nature
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Consider the propulsion of a
fish through the water. A fish
uses its fins to push water
backwards. In turn, the
water reacts by pushing the
fish forwards, propelling the
fish through the water.
The size of the force on the
water equals the size of the
force on the fish; the
direction of the force on the
water (backwards) is
opposite the direction of the
force on the fish (forwards).
rd
3
Law
Flying gracefully
through the air, birds
depend on Newton’s
third law of motion. As
the birds push down on
the air with their wings,
the air pushes their
wings up and gives
them lift.
Other examples of Newton’s Third
Law
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The baseball forces
the bat to the left
(an action); the bat
forces the ball to
the right (the
reaction).
3rd Law
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Consider the motion
of a car on the way to
school. A car is
equipped with wheels
which spin
backwards. As the
wheels spin
backwards, they grip
the road and push the
road backwards.
3rd Law
The reaction of a rocket is
an application of the third
law of motion. Various
fuels are burned in the
engine, producing hot
gases.
The hot gases push against
the inside tube of the rocket
and escape out the bottom
of the tube. As the gases
move downward, the rocket
moves in the opposite
direction.