5/12/15 11.1 Measuring Motion
S  Key Ideas
Chapter 11: Motion
S  What is motion?
Physical Science - Doerfler
S  What is the difference between velocity and speed?
S  How can you use graphs to show the motion of an object?
S
11.1 Key Terms
What is Motion?
S  Displacement
S  Motion must be defined relative to other objects
S  Frame of reference
S  Motion
S  Frame of reference – the objects used to define motion
S  In physics, any object can be a frame of reference
S  Objects in frame of reference are treated as if they are not moving
S  Speed
S  In science, motion is the change in position of an object relative
S  Velocity
Distance and Displacement
S  Distance is how far an object moves
S  Displacement – a measure of how far the starting point is from
the ending point
S  Need to give both length and direction when describing distance
and displacement
S  Can use cardinal directions: north, south, east, west
to a frame of reference
S  Scientists describe how far an object has moved using two terms:
S  Distance
S  Displacement
How are Speed and Velocity
Different
S  Speed – describes how far an object travels in a certain amount of
time
S  Describes how fast an object moves
S  Velocity – describes how fast an object is moving and in what
direction it is moving
S  Can describe direction using up, down, left, right
S  Or relative to reference point: away from the goal
1 5/12/15 Adding Velocities
S  Use positive and negative numbers to determine person’s velocity
when walking on bus
S  Bus is moving eastward at 15 m/s
Calculating Speed
S  To calculate speed, need to measure two quantities:
S  Distance traveled
S  Time it took
S  “positive” motion is when you walk in same direction the bus is
S  SI unit for speed is m/s
S  “negative” motion is when you walk in opposite direction the
S  Some objects move at constant speed
S  Others at various speeds
heading
bus is heading
S  If walking velocity is -1m/s and bus is heading east at +15 m/s
S  (+15 m/s) + (-1 m/s) = +14 m/s east
S  Horse travels 19 m every second, therefore constant speed is 19
m/s
S  If it stops, now at 0 m/s, a horse has variable speed
Average Speed
S  Most objects travel at multiple speeds
S  Average speed is the total distance traveled divided by the total
time it took to travel that distance
S  Speed = distance
time
S 
Instantaneous Speed
S  Instantaneous speed of an object is its speed at a given instant, or
point in time
S  Cannot use equation for speed for instantaneous speed because
we can’t divide by 0
S  Speedometers can measure instantaneous speed
S  If object is moving at a constant speed, its instantaneous speed is
S  V = d/t
S  Example: sledder moves 132 m downhill in 18 s. What is
its constant speed
average speed?
Calculating Velocity
S  RECALL – velocity is speed of an object in a particular direction
S  To calculate velocity, first find speed of object
S  Then, indicate direction
S  Let’s MATH!!
Velocity Example
S  Alaska’s Black Rapids glacier moved at 89 m per day down the
valley
S  What is the velocity of the glacier in meters per second?
S  Step 1: List given and unknown values
S  Given: d=89 m; t=1 day; direction = down the valley
S  Unknown: velocity (v)
S  Perform conversions, and write the equation
S  1 day x 24h/1 day x 60 min/1 hr x 60 s/1 min = 86,400 s
2 5/12/15 Velocity Example cont.
S  Step 3: insert known values and solve for unknown
S  V = d/t
Showing Motion on a Graph
S  Show motion on graph by
S  Recording distance on the vertical axis
S  V = 89 m/86,400 s
S  V = 0.0010 m/s down the valley
S  Time on the horizontal axis
S  Use shape of the line on graph to learn about motion of object
Calculating Speed from Graphs
S  On graph of distance vs. time
Speed from Graph
S  Example
S  Motion of object moving at constant speed is a straight line
S  Slope of the line is equal to speed of the object
S  Calculate slope by:
S  Dividing vertical change of the line by horizontal change
S  Slope = change in distance/change in time
Speed from a Graph
S  Step 1: choose two points to use to calculate slope
S  Point 1: time, t = 1s; distance, d = 6 m
S  Point 2: time, t = 4 s; distance, d = 12 m
S  Step 2: calculate the vertical change and horizontal change
S  Vertical change = 12 m – 6 m = 6 m
S  Horizontal change = 4 s – 1 s = 3 s
S  Step 3: divide the vertical change by the horizontal change
Interpreting Slope
S  The steeper the slope, the faster the object is moving
S  On a graph of distance vs. time:
S  Motion of object moving at variable speed is a curved line
S  Even if object’s speed is changing, you can use graph to to find
average speed
S  find total distance and total time from graph
S  Then, divide distance by time to calculate average speed
S  Slope = vertical change/horizontal change = 6m/3s = 2 m/s
3 5/12/15 Example #1
S  Polar bears are extremely good swimmers and can travel as
Example #2
S  A baseball is pitched at a speed of 35.0 m/s. How long does it
long as 10 hours without resting. If a polar bear is swimming
at an average speed of 2.6 m/s, how far will it have traveled
after 10.0 hours?
take the ball to travel 18.4 m from the pitcher’s mound to home
plate?
11.2 Acceleration
11.2 Key Terms
S  Key Ideas
S  Acceleration
S  What two things may change when an object accelerates?
S  How can you calculate constant acceleration?
S  How can graphs show acceleration?
How is Acceleration related to
Velocity?
S  Acceleration – occurs when an object changes velocity
S  RECALL – velocity has both speed and direction
S  Acceleration has two components:
S  A magnitude
S  A direction
S  When speed or direction of object changes, the object is
accelerating
Acceleration and Speed
S  An object that changes speed is accelerating
S  Accelerating object may speed up or slow down
S  Object that is speeding up has positive acceleration
S  Object slowing down has negative acceleration
S  A cyclist is peddling south and speeds up
S  Every second, velocity of cyclist increases by 1 m/s
S  After 1 s, cyclist is traveling at 1 m/s south; after 2 s, cyclist is
traveling 2m/s south, and so on
4 5/12/15 Acceleration and Speed
S  The cyclists’ acceleration is described as his velocity is increasing
Acceleration and Direction
S  When an object changes direction, it is accelerating
by 1 m/s per second (1 m/s/s or 1 m/s2)
S  The cyclist is speeding up
S  Even if speed is constant
S  Skaters below are going at a constant speed
S  However, they must change direction to stay on the track
S  Therefore, acceleration is +1 m/s2
S  As they go around the curves, they accelerate
Centripetal Acceleration
S  Centripetal acceleration is the acceleration that occurs when an
Calculating Acceleration
S  For object moving in straight line, acceleration only occurs due
object moves in a circular path
to change in speed
S  Calculate using speed at two points of time
S  You might think it is uncommon, but all of us are currently
experiencing it right now
S  Earth is rotating on its axis
S  Earth also experiences this as it orbits the sun
S  The moon as it orbits the Earth
S  Acceleration = final speed – initial speed
S  Triangle symbol is “delta” or “change in”
Calculating Acceleration from
Velocity
S  A cyclist slows along a straight line from 5.5 m/s to 1.0 m/s in
Calculating Acceleration
S  Step 2: write the equation
3.0 s.
S  What is the average acceleration of the cyclist?
S  Step 1: List given and unknown
S  Given: initial speed: vi = 5.5 m/s
S  Final speed: vf = 1.0 m/s
S  Time: t = 3.0 s
S  Unknown: acceleration, a
time
S 
S  A = (vf – vi)/t
S  Step 3: insert known values and solve for unknown
S  a = (1.0 m/s – 5.5 m/s)
S 
3.0 s
S  a = (-4.5 m/s)/3.0 s
S  a = -1.5 m/s2
5 5/12/15 Acceleration from Distance vs.
Time Graphs
Graphing Accelerated Motion
S  Use a graph to determine acceleration when graph is speed vs.
S  Can identify acceleration by examining a graph of distance vs.
time
time
S  Straight line on speed vs. time graph indicates constant
S  Curved line indicates acceleration
acceleration
S  Constant acceleration is acceleration that does not change with time
S  Line with positive slope indicates object speeding up
S  Line with negative slope indicates object slowing down
S  Look at example on handout
Math Day Manners
S  No talking out of turn
Math Practice 1
S  In 1970, Don “Big Daddy” Garlits set what was then the world record
for drag racing. He started at rest and accelerated at 16.5 m/s2 (about
1.68 times the free-fall acceleration) for 6.5 s. What was Garlits’s final
speed?
S  Raise hands
S  Write everything down when we work as a class
S  Step 1:
List the given and unknown values.
S  Actually try and do the problem on your own
S 
Given: acceleration, a = 16.5 m/s2
S  Lastly, NO WHINING!
S 
time, t = 6.5 s
initial speed, initial v = 0 m/s
S 
S 
Unknown:
Practice 1
S  Step 2:
Rearrange the acceleration equation to solve for
final speed, final v = ? m/s
Practice 1
S  Step 3:
Insert the known values into the acceleration
equation, and solve.
final speed.
S 
final v = (16.5 m/s2 × 6.5 s) + 0 m/s
S 
final v = 110 m/s
6 5/12/15 Practice 2
S  A child sleds down a steep, snow-covered hill with an acceleration of
2.82 m/s2. If her initial speed is 0.0 m/s and her final speed is 15.5 m/
s, how long does it take her to travel from the top of the hill to the
bottom?
S  Step 1:
List the given and unknown values.
S 
Given: acceleration, a = 2.82 m/s2
Practice 2
S  Step 2:
Rearrange the acceleration equation to solve for
time.
initial speed, initial v = 0.0 m/s
S 
final speed, final v = 15.5 m/s
S 
S 
Unknown:
time, t = ? s
Practice 2
S  Step 3:
Insert the known values into the equation, and
solve.
#2
S  2.
S 
S 
#3
S  #3
final v = at + initial v
= (–0.33 m/s2)(23 s) + 18.0 m/s
= 10.4 m/s
#4
S  #4
initial v = 49.8 km/h
7 5/12/15 #5
S  #5
#6
S  #6
Practice #3
S  An automobile manufacturer claims that its latest model can “go
from 0 to 90” in 7.5 s. If the “90” refers to 90.0 km/h, calculate
the automobile’s acceleration.
S  Step 1:
List the given and unknown values.
S 
Given: time, t = 7.5 s
S 
S 
S 
Practice #3
S  Step 2:
the value for speed by the number of meters in a kilometer and
divide by the number of seconds in an hour.
initial speed, initial v = 0.0 km/h
final speed, final v = 90.0 km/h
Unknown:
acceleration, a = ? m/s2
Practice #3
S  Step 3:
Perform any necessary conversions.
S  To find the final speed in meters per second, you must multiply
Write out the equation for acceleration.
Practice #3
S  Step 4:
Insert the known values into the equation, and
solve.
8 5/12/15 11.3 Motion and Force
S  Key Ideas
S  What are the four fundamental forces in nature?
S  How can forces affect the motion of an object?
S  Why is friction sometimes necessary?
What are the fundamental
forces?
S  Force – means any action that can affect the motion of an object
S  Force has both magnitude and direction
S  There are four fundamental forces in nature
S  Gravity
S  Electromagnetic force
S  Strong nuclear force
S  Weak nuclear force
Properties of the fundamental
forces
S  Four fundamental forces are similar
S  They act to change in motions of objects
Forces of Different Strengths
S  Strong nuclear force is strongest of all forces
S  Holds together protons and neutrons in nuclei
S  Each has different properties and work in different ways
S  Works over short distance only; the distance of atomic nucleus
S  Strong and weak nuclear forces work over short distance
S  Weak nuclear force is about 10 trillion times weaker than strong
S  Hold atoms together
S  These forces are important but we do not experience them directly in
everyday life
nuclear force
S  Affects some kinds of nuclear decay
S  Gravity and electromagnetic forces are felt everyday
S  Gravity acts over long distances, pulls objects to each other
S  Electromagnetic forces produce friction, magnetism, and static
electricity
Forces of Different Strengths
S  Electromagnetic forces act over long distances
Contact Forces and Field Forces
S  Forces are put into two main groups:
S  About 1% the strength of strong nuclear force
S  Contact forces
S  Hold electron near nucleus of atom and holds molecules together
S  Field forces
S  Gravity is weakest of fundamental forces
S  Contact forces require two objects to physically be in contact
S  About 1040 times weaker than strong nuclear force
with each other
S  Works over longer distances than any other fundamental force
S  When you push or pull an object
S  Shapes structure of our galaxy and universe
S  Friction
9 5/12/15 Field Forces
Forces Affect Motion
S  Field forces act over distances
S  Forces act to change the motion of an object
S  Do not require two objects to be in direct contact
S  Most situations have several forces acting on object at once
S  Examples:
S  Net force – combination of all forces acting on an object
S  Gravity
S  Object will accelerate in the direction if the net force
S  Magnetism
S  Field forces can attract two objects together or push two objects
S  An object will not accelerate if the net force is zero
apart
Balanced Forces
S  Balanced forces produce net force of zero
S  Object experiencing balanced forces will not change its motion
Unbalanced Forces
S  When net force acting on object is greater than zero, object will
accelerate in direction of net force
S  Object at rest will remain at rest with balanced forces
S  Object in motion will remain in motion if the forces are balanced
Force of Friction
S  Friction – force that opposes the relative motion between two
objects in contact
S  Results from electromagnetic forces
S  Occurs as a result of the interactions between atoms on the
surface of two objects
S  Rougher the surface, greater the friction between them
S  Friction can produce heat
S  Rubbing hands together; heat from friction causes match to light
S  Even if surfaces look smooth, they are rough at molecular level
Types of Friction
S  Two main types:
S  Static
S  Kinetic
S  Kinetic friction – friction between tow moving surfaces
S  Two main types of kinetic friction:
S  Sliding
S  Rolling
S  Sliding friction – occurs when two objects slide past each other
S  Rolling friction – occurs when a round object rolls over a flat surface
10 5/12/15 Types of Friction
S  Most cases, rolling friction is less than sliding friction
S  Why its easier to push chair on wheels
S  Static friction – friction between two surfaces that are not sliding
past each other
S  Forces act between molecules on the surface of two objects,
holding them together
S  Static friction is usually greater than kinetic friction
Friction Impacts Daily Life
S  Friction allows you to hold a pencil and write on paper
S  Keeps you from slipping when you walk
S  Between tires and the road
S  Due to friction:
S  A constant force must be applied to keep the car moving.
S  Also keeps car from moving when parked
S  Takes greater force to start object moving than to keep it moving
Increasing and Decreasing
Friction
S  Sometimes, friction between surfaces needs decreased
S  Car engine
S  Many moving parts and cause extreme heat, possible damage
S  Put oil in engine; oil is lubricant (reduces friction)
S  Sometimes friction needs to be increased
S  In cold climates, sand is put on icy roads and sidewalks
11