Motion and Forces

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Motion and Forces
Displacement in Time and Space
Focus Questions
All animations www.exrx.net
What is motion? How can you tell if an object is
speeding up or slowing down?
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Motion Vocabulary

• Balanced Forces
• Unbalanced Forces
• Inertia
• Gravity
• Friction
• Force
• Mass
• Magnitude
• vd/t

• Total Distance
• Total time
• Motion
• Position
• Reference Point
• Direction
• Speed
• Average speed

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Motion is a change in position of an objectwith
respect to time.
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• 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.
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Cowpens to Downtown Spartanburg 14 miles
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Examples of units of speed aremeters per
second (m/s)kilometers per hour
(km/h)andmiles per hour (mph).
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Direction - the relationship of the position of a
moving object to another position.
www.fierydarts.com www.telegraphics.com
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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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www.fierydarts.com www.telegraphics.com
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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.
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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.
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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
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Calculate the average speed of an object in
motion

• http//sunshine.chpc.utah.edu/javalabs/java12/fnm/
  act1/lab.htm

Snowmobile Distance (in km) Time (in hrs) Avg. Speed (in km/hr)
Mangler 500 15.82  
Otter Pop 15 0.45  
Slider 50 1.41  
Snowflake 240 6.38  
White Fang 30 0.75  
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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
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This data can then be represented on
a time-distance graph
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This graph can then be used to describe the
position, direction and speed of the motion of
the object.
http//teacherline.pbs.org/teacherline/resources/a
ctivities/race/readings/race.htm
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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.
B
B
A
C
A
C
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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.
B
B
A
C
A
C
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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
http//www.sycd.co.uk/dtg/
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Create a data table and graph the following

• Alex and Ed left home at 100 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 730 PM.

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Hour Time Distance Activity
         
         
         
         
         
         
         
         
         
         
         
         
         
         
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Describing Motion Newtons Laws
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Newtons First Law of Motion(also known as Law
of Inertia)

• 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
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There are two forces that can affect the movement
(speed and direction) of an object.

• 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
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• 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.

www.fierydarts.com
http//www.rockcrawler.com/techreports/bfgmtkm/tir
e-tread.jpg
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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
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Inertia is a property of the object it is not a
force.
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A pl
The force of gravity, in combination with the
property of inertia, is responsible for the
orbits of moons and planets.
http//liftoff.msfc.nasa.gov/academy/rocket_sci/or
bmech/orbit/orbit.html
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cfm?guidAssetId27DE45E9-9B3D-478E-A546-D893FC4D2B
92
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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.

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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.

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Examples

• 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.
• 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.

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• 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.
• Unbalanced forces cause a nonmoving object to
  start moving.

and
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2nd Law
F m x a
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Newtons 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.
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2nd 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.

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2nd Law (F m x a)

• How much force is needed to accelerate a 1400
  kilogram car 2 meters per second/per second?
• Write the formula
• F m x a
• Fill in given numbers and units
• F 1400 kg x 2 meters per second/second
• Solve for the unknown
• 2800 kg-meters/second/second or 2800 N

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If mass remains constant, doubling the
acceleration, doubles the force. If force remains
constant, doubling the mass, halves the
acceleration.
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3rd Law

• For every action, there is an equal and opposite
  reaction.

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3rd 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.

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3rd 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.

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Newtons 3rd Law in Nature

• 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).

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3rd Law
Flying gracefully through the air, birds depend
on Newtons 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.
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Other examples of Newtons Third Law

• The baseball forces the bat to the left (an
  action) the bat forces the ball to the right
  (the reaction).

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3rd Law

• 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.

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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.