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									TAKS Objective 5

    Energy and
  Energy Changes
  Physics is the study of energy and
provides answers to our energy needs:




• Mechanical energy to do work such as grind
  wheat or churn milk into cheese saves fossil
  fuels and electricity and can be produced by
  wind.
• Using solar energy to produce heat or hot water
  also saves electricity.
            Energy
Is defined as
  the ability to
  do work
Energy has two
types:    Kinetic (Energy of Motion)
          and
          Potential (Stored Energy)
 Law of Conservation of Energy

• Energy can change forms, but is never
  created nor destroyed
• Loss in one form = gain in an another form
         6 Simple Machines
          make work easier
Wheel & Axle   Wedge
                          Lever




Screw
               Inclined      Pulley
               Plane
    Potential Energy
         2 possibilities
Gravitational PE -
 Object lifted to
 some height
Elastic PE - A
 stretched or
 compressed object
 (spring or rubber
 band)
         Elastic Potential Energy
5 According to the data in the
  table, about how far can a
  spring be expected to
  stretch when a force of
  3.92 N is applied?
A 10.5 cm
B 13.6 cm
C 14.0 cm Although this question is about reading the
D 17.3 cm table, it does show that a stretched spring can
             do work (force x distance). Look at the
             numbers, as Force increases by 0.98, the
             distance doubles, so the answer would be:
Gravitational Potential Energy
          PE = mgh
                  41 What is the
                    potential energy
                    of the rock?
                      m = 95 kg
                  A 59,900 joules
                             m/s2
                     g = 9.8joules
                  B 64,600
                      h = 100 m
                  C 93,100 joules
                95 kg x 9.8 m/s2 x 100
                  D 121,600 joules
                  =93,100 joules C
      Kinetic Energy
KE = m    v 2

         2
Ex: The man is doing
 work, using kinetic
 energy.
Weight= Mass (m) X gravity (g)
•   Weight Unit of mass = kg
•   gravity = 9.8 m/s/s
•   Unit of weight = Newton
•   1 Newton= about ¼
    pound
On Earth gravity = 9.8 m/s/s
  For every second that
  an object falls it
  accelerates 9.8 m/s
  each second.
  A maximum speed is
  reached when the air
  resistance (friction)
  slowing the object
  increases to prevent
  further acceleration.
  This is called terminal
  velocity.
Universal Law of Gravitation
 All objects in the universe
  attract each other by the
  force of gravity
 Gravity varies depending
  on two factors:
   1) the mass of the
      object doing the
      pulling, and
     2) the distance from
      the center of that
46 Which of these would cause the
  gravitational force between Earth
  and the sun to decrease?
F An increase in the length of a day
  on Earth
G An increase in the distance
  between Earth and the sun
H An increase in the number of
  planets orbiting the sun
J An increase in the masses of
  Earth and the sun
What are the 2 factors that effect gravity?
Distance and Mass; decrease distance,
increases gravity. increase mass,
increase gravity. . . .
Energy Conversion
        • Changing from one type of
          energy to another is the
          basis of all machines, and
          life processes on earth.
        • Our main energy use,
          electricity is converted from
          mechanical energy, nuclear
          energy or chemical energy.
        • During conversions, some of
          the energy is converted to
          heat by friction so it is not a
          100% efficient process.
Motion can be described as
• a change in an
  object’s position.
• Average speed is the
  change of position
  (distance) of an
  object over time
• If speed also has a
  direction it is called
  velocity.
    Definition of a Force
• A Force is a push or a pull
Velocity Graphs V = distance
             Pink,     time
 • Velocity (v) is steeper
   the slope (rise slope!                           Velocity
   over run) of a
                       60




                        Distance (m)
   distance (d) vs.
                       40                                        Series1
   time (t) graph                                                Series2
                                       20
Which line, pink or                    0
blue has the greatest                       1 3 5 7 9 11 13 15

velocity?                                        Time (sec)
40 The diagram represents the total travel of a
teacher on a Saturday. Which part of the trip is
made at the greatest average speed?
FQ
       How do we work this one?
GR
HS     Calculate v = d/t for each segment.
J T
    4 The picture shows the position of a ball
   every 0.25 second on a photogram. Using a
     ruler, determine the velocity of the ball.




Use the ruler on the side of the chart to measure from
the front of ball 1 to the front of ball 2 and multiply by 4
(since the pictures are at ¼ seconds). OR front of ball 1
to the front of ball 3, and multiply by 2 since two
pictures would be ½ a second. V = d/t
    Acceleration is a change in an
      object’s velocity over time
• When an object’s
  speed changes over
  time it is accelerating
  (or decelerating)
• A = vfinal – vinitial
           time
• Units for acceleration
  m/s/s or m/s2
     Acceleration Graphs
• Acceleration (a) is
  the slope of a                                            Acceleration
  velocity (v) vs. time



                          Velocity ((m/s)(m)
                                               60
  (t) graph                                    40

• When plotted on a                            20
                                               0
  distance vs. time                                 1   3    5   7    9    11   13   15
                                                                 Time (sec)
  graph, acceleration
  is an exponential
  curve (J curve)
  Read the graph first, what is each
        segment telling you?
In the first second?
 41 According to this
   graph, what 4
During the nextwas the
   bicycle’s acceleration
seconds, from 1 to 5 or
6 seconds? 6 and 10
   between
   seconds?
What about between 6-
A seconds?
10 0.0 m/s2
Between 102and 12.5
B 0.65 m/s
seconds? 2
C 1.6 m/s
What about 12.5 to 15
D 6.5 m/s2
seconds?
         Balanced Forces

• A force that
  produces no
  change in an
  object’s motion is
  balanced by an
  equal, opposite
  force.
  F=ma, the mass is 40kg and the acceleration is
    Remember to read diagrams first! What
   happened in the pictures? Now read 40kg
  3.0m/s – 0.0m/s divided by 1.2s. So it is the x
  2.5m/s/s or 100N
                   question . . .




32 Starting from rest at the center of a skating
  rink, two skaters push off from each other over a
  time period of 1.2 s. What is the force of the
  push by the smaller skater? ANSWER
F 16 N       G 32 N      H 88 N         J 100 N
         Friction


A force that acts to oppose
 motion. It occurs between any
 two surfaces in contact with
 each other, or as air resistance.
           Friction
Friction causes an
 object to slow down
 and stop.
Since the amount of
 energy stays
 constant, the energy
 becomes heat.
To reduce friction oil
 or other lubricants
 are used.
44 A wet bar of soap slides 1 meter across a
wet tile floor without appearing to slow down.
Which of these statements explains why the
bar of soap fails to slow down?
F A constant force on an       This is an example
  object produces a constant   of how a lubricant
  positive acceleration.       works, reducing
G An object in motion tends    friction so the
  to remain in motion in the   inertia keeps the
  absence of an external       soap moving.
  force.
H A moving object having
  constant velocity contains
  kinetic energy.
J An object’s weight is
  proportional to its mass.
  Momentum = Mass x Velocity




• The larger the mass OR the faster an object
  moves, the more momentum it has. That means
  it will hit harder, or do more damage.
• If a penny is thrown it probably won’t hurt very
  much, but if it is shot from a sling shot with a
  higher velocity, it will hurt a lot!
  27 A ball moving at 30 m/s has a
momentum of 15 kg·m/s. The mass of
            the ball is —
 A 45 kg

 B 15 kg

 C 2.0 kg   Formula Page says that
            Momentum = Mass x Velocity

 D 0.5 kg   So 15 kg.m/s = M x 30 m/s
            solving for M it is:
    Newton’s 1st Law of
         Motion
• Objects in motion
  stay in motion
• And objects at rest
  stay at rest
• Until they are
  acted upon by an
  unbalanced force.
        Newton’s 2nd
        Law of Motion
The greater the force applied
 to an object, the greater it
 will accelerate.

Force = Mass X Acceleration
or F=ma
20 How many Newtons of force does a 70.0 kg
 deer exert on the ground because of gravity?
  Record and bubble in your answer on the
              answer document.
• This is an example of
  a weight problem
  since the weight is the
  force is due to gravity.
• F = mg or
   = 70kg x 9.8m/s/s
   = 686 Newtons
    Example 2nd Law Problem
18 What is the net force exerted on a 90.0
  kg race-car driver while the race car is
  accelerating from 0 to 44.7 m/s in 4.50 s?
F 9.8 N          F = ma, where
G 20 N          m= 90.0kg
H 201 N
                a = (44.7 – 0)/ 4.5s
J 894 N
                So, the answer is:
  Newton’s 3rd Law of Motion
• For every
  action force
  there is an
  equal and
  opposite
  reaction
  force.
52 When the air is released from a balloon, the
  air moves in one direction, and the balloon
  moves in another direction. Which statement
  does this situation best illustrate?
F What goes up must come down.
G For every action there is an equal and
  opposite reaction.
H The shape and size of an object affect air
  resistance.
J The acceleration due to Earth’s gravity is 9.8
  m/s2.
     Your turn!!
1. The typical automobile’s kinetic energy is
converted to heat energy when the brakes are
applied. A braking system that converts kinetic
energy to electrical energy instead of heat energy
has been designed. The electrical energy can be
used later to move the car again. How does a
system like this benefit the environment?
F Greater use of solar energy
G Ability to harness wind energy
H Decreased use of fossil fuels
J Use of renewable hydroelectricity
2. Assuming the chart
  contains all energy
  transformations in
  the Earth system,
  how much solar
  radiation goes
  toward evaporating
  water?
F 133,410 terajoules
G121,410 terajoules
H 92,410 terajoules
J 40,000 terajoules
3. Why is the sum of the products’ energy in this reaction
   less than the sum of the reactants’ energy?
A Energy is trapped in the reactants.
B The products absorb available energy.
C Energy is given off as heat.
D The reactants’ energy is less than the melting point of
   glucose
                4. What is the approximate
                  difference in gravitational
                  potential energy of the
                  two shaded boxes?
5.0 m




                A 79 J
                B 59 J
        2.0 m




                C 39 J
                D 19 J
5. A catapult was designed to project a small metal ball at a
   target. The resulting data are shown in the table. Which of
   these might explain the difference between the calculated
   and actual distances?
A The ball landed short of the calculated distance because of
   an increase in momentum.
B Air resistance caused the ball to land short of the calculated
   distance.
C Initial mass of the ball changed with each trial.
D The metal ball was too small for accurate measurements to
   be made.
6. A 1-kilogram ball has a kinetic energy of
  50 joules. The velocity of the ball is —
F 50 m/s
G 25 m/s
H 10 m/s
J 5 m/s
7.  A 0.50 kg ball with a speed of 4.0 m/s
 strikes a stationary 1.0 kg target. If
 momentum is conserved, what is the total
 momentum of the ball and target after the
 collision?
 A 0.0 kgm/s              B 0.5 kgm/s
 C 1.0 kgm/s              D 2.0 kgm/s
8. The table contains data for two wrecking balls
  being used to demolish a building. What is the
  difference in momentum between the two
  wrecking balls?
F 300 kgm/s
G 200 kgm/s
H 150 kgm/s
J 0 kgm/s
Car velocity = 5.5m/s    Car velocity = 0m/s      Car velocity = 0m/s
Driver velocity = 5.5m/s Driver velocity = 5.5m/s Driver velocity = 0m/s
Driver mass = 100kg      Driver mass = 100 kg     Driver mass = 100 kg




 9. The pictures show how an air bag functions in a
   collision. How much momentum in kg m/s does
   the air bag absorb from the crash-test dummy if
   all the crash-test dummy’s momentum is
   absorbed by the air bag? Record and bubble in
   your answer to the nearest whole number on the
   answer document. (Just write it down)
10. The picture above shows the directions in which water
  leaves this scallop’s shell. Which picture below shows
  the direction the scallop will move?
           Check your answers.
1. H: Cars usually run on gasoline, so any heat
   energy saved, reduces use of fossil fuels.
2. J: Subtract each of the lines from the total
   energy, as Energy is Conserved.
3. C: This is the equation for cellular
   respiration and the “lost energy” is often
   used as body heat.
4. B: Calculate PE for each and subtract.
5. B: Air resistance is a form of friction which
   is where the rest of the energy dissipates as
   heat.
6. H: KE=½(mv2) so 50= ½(1 x v2) or
  100 = v2.
7. D: use the formula page, mass x velocity
8. J: again, mass x velocity, they are both
   300 kg m/s.
9. Momentum = mass x velocity or 5.5 x
   100 or 550 kg m/s.
10. D: For each action an equal opposite
   reaction, since the force is out and down,
   the shell will move up.

								
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