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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
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
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?
A seconds?
10 0.0 m/s2
Between 102and 12.5
B 0.65 m/s
seconds? 2
C 1.6 m/s
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?
• 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
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.
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
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
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
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
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?
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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