Download Work and Energy

Science 10
Acceleration, Work and Energy
Acceleration Due to Gravity
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Sir Isaac
, determined that the acceleration due to the pull of the
Earth’s gravity was
This is called the
What this means is that gravity
The formula remains the same
Example:
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Ms. Godley throws a coin from a building and finds that the coin has an initial velocity of
5.0m/s and takes 5.0seconds to reach the ground. Determine the coins final velocity.
Example:
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A plant falls from a balcony and takes 3.0seconds to reach the ground. At what velocity does the
plant hit the ground?
Example
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If a toy falls off a balcony and its final speed before impact with the ground is 17.5m/s ,
then the time it took to hit the ground is…
Example:
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Find the velocity in m/s of a skydiver after 6.0seconds of free fall. What would this
velocity be in km/hr?
Your Task
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Example problem B1.9 p 149
Practice Problem # 16 p 149
Example problem B1.10 p 153
Practice Problem # 17 p 153
Check and Reflect p 154 # 1, 4, 6, 8, 9, 11
Check and Reflect #9 – 11,14 – 16 p 162
Practice Problem Worksheet
 IN ORDER TO RECEIVE FULL MARKS YOU MUST SHOW ALL OF YOUR
WORK!!!!!!!
Work and Energy
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NRG (
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Force: a
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Balanced Forces: equal in
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Unbalanced Force: are
and do not cancel out
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Kinematics - the study of
things move
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Dynamics – the study of
): capacity to do work
or
on an object. The measurement of force is (
and cancel each other out
things move
)
Isaac Newton
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Developed the branch of mathematics
that we call calculus when he was 23 years old.
worked in the area of physics called dynamics and developed three laws
that bear his name.
Newton’s 1st Law
st
1 LAW OF MOTION:
 “
in a straight
line and a body at rest tends to stay at rest – unless they are acted upon by an
unbalanced force
 This
will result in an
of the object
 This tendency to remain at rest or to remain in motion in a straight line is called
"
“
 Inertia - the tendency of an object to
a change in its motion. (an
object's mass is a measure of its inertia)
Newton’s 2nd Law
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The acceleration of an object will vary
force and inversely with the object’s mass.
o What this really means
MATHEMATICALLY, WE USE THE FORMULA:
a α F  Δ force yields Δ acceleration
[α means "proportional to“]
a α 1/m  Δ mass yields inverse Δ acceleration
2
F = ma (units are kg m/s or Newtons, N)
with the applied unbalanced
Mass and Weight are Distinct
 Mass - the
change
in an object - it does not
- the unit of mass is the
 Weight - weight is the
object
exerts on a given
- weight is a force thus the units are

on earth all objects accelerate downward at
the weight of an object is given by:
(g) so
Example:
What mass must an object have if 75 N of force are required to give it an acceleration of 3.0
m/s2?
Example:
A 500g object is accelerating at a rate of 0.45m/s2. What force is needed to accomplish this?
Example:
A force of 6.5 x 10 3N is exerted on an object resulting in an acceleration of 10 m/s 2. What is
the mass of the object?
Newton’s 3rd Law
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FOR EVERY ACTION THERE IS AN EQUAL ACTION IN THE OPPOSITE DIRECTION (
)
1) As you push on a wall, the wall pushes back on you with the same force
2) As a book lies on a table: the book exerts a
force due to gravity
on the table. But there is also a force that the table exerts on the book
.
This second force is just enough to
the gravitational force on the
book so the book lies still.
(Forces always occur in pairs.)
Work:
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Work is a measure of the
of
produced by a
motion when a force is applied
Work is a measurement of the energy used during the movement  Therefore an object
has to
for work to be done
Work is measured in
(units are
, or
, or
For work to be done there must be:
1) Movement
2) A force is applied
3) The force and displacement must be in the same direction
There is no work done if:
1) A force is applied and there is no movement
2) There is movement but no force (i.e. free floating in space)
3) The force and the direction of movement are not in the same direction
)
Example:
A) Holding a textbook?
B) Walking with a heavy backpack?
C) Lifting a pail out of a well?
D) Pushing against a heavy box that won’t move?
Example:
Find the work done to push a large container 20m across a room given that 20N of Force is
required
Example:
Calculate the amount of work done by Superman when he needs to haul Lois Lane around which
requires a force of 16N to lift her 240m off the ground.
Example:
A 15kg bale of hay is tossed at 2.5m/s2 across a 2.0m space to a waiting cow. What’s the
amount of work done by the worker?
Example:
A 75kg object is pushed up a frictionless ramp at a constant velocity. The height of the ramp is 5.0m
and its incline length is 10m. How much work is done?
Quantifying Energy
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Work - the
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Energy - a measure of an object's (or system)
of energy from one object (or system) to another
to do work
GRAPHICAL REPRESENTATION OF FORCE VS. DISTANCE
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The area under a Force vs., Distance graph represents work input
Input and Output
 If work is done on an object it will
energy. [Note: Units of both work
and energy are
]
 If the system is
the work
will be
to the work
.
 In “real world” systems 100% efficiency is impossible.
Example:
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What is the efficiency of a system that uses 15,000J to raise a 200N object 10m?
Your Task
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Example Problems B 1.11 & 12
Practice Problems p 160 18-20
Check & Reflect p 161 # 1,4, 6-10, 12
Section Review p 162-163 #1-8,10, 13, 15, 16-20,22-24
Science 10
Forms of Energy
2 Groupings of Energy
1) Exhibits Motion (
 Example: Anything the
)
2) Doesn’t Exhibit Motion (
 Lots of types of this kind of energy
 “
”
)
Mechanical Energy
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Mechanical energy is the
energy
of
and
Potential Energy
 It is energy due to position or condition
– It is never associated with motion
 The energy possessed is gained by doing work on the object
Example:
A student exerts an average force of 85 N on a slingshot, pulling it back a distance of 0.42 m.
Did the student do work? If so, how much work did he do?
Types of Potential Energy
1) Elastic Potential Energy - the energy
in a stretched elastic band
2) Chemical Potential Energy – the energy stored in
3) Electric Potential Energy – the energy in a
4) Gravitational Potential Energy- the energy stored as an object moves upwards
Gravitational Energy
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Imagine that you lift the Grey Cup over your head and then hold it there
You have done work on the object because you have exerted a force (
over a distance
When an object is lifted from rest to a point farther from the earth, work is done
against gravity causing the object to gain potential energy
The cup has
and stored potential energy due to its
Formula:
Example:
A 20kg ball is held at a height of 7.0m
a) Determine the ball’s potential energy
b) The ball is dropped. What is the ball’s potential energy when it hits the
ground?
)
Example:
What is the potential energy on a 1.4kg IPod sitting on a shelf that is 2.3m above the floor?
Example:
Determine the mass of an object that sits 100m above the ground and has a potential energy of
5,000J?
Elastic Potential Energy:
o Energy stored in springs or elastic bands
 Example: Diving Boards
Chemical Potential Energy
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Energy stored in chemical bonds.
- Measured using a bomb calorimeter.
- Heat of Combustion (ΔH ) - the amount of energy released by a substance in a
c
combustion reaction (equal to the chemical E stored in its bonds).
p
-
Chemical energy can be calculated using:
Q = mcΔt
(not required in this course)
Electrical Potential Energy
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The energy which results from the force between two charged objects, which gives the
objects (usually electrons) the potential to travel between two points.
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[more in Physics 30]
Nuclear Potential Energy
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Energy released by
atoms) reactions.
(splitting atoms) or
(combining
- Conversion of mass to energy is described by Einstein's formula:
E = energy (J)
m = mass converted (kg)
8
c = speed of light (3.00 x 10 m/s)
Potential Energy
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All forms of potential energy must be converted before they can produce a useful
form of energy.
Example: battery, stick of dynamite, gasoline, water behind a hydroelectric dam, etc.
Your Task
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Example Problems p 174
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Practice Problems p 174 # 1-3
Do Practice Problems # 1-3 on p. 174 – 175
Do Check and Reflect pg. 178 # 4 -9
Physics Slides: 86-140