Transcript h i =h v i =0 v f >0 K i + U gi W = K f + U gf + DE K i + U gi W = K f + U

How to Make an Energy Bar Chart
1. Determine what is in the system
2. Choose a height for each bar. Draw a bar for each kind of energy in
the system.
• there is no kinetic and elastic potential energy in the initial state
• there is no gravitational and elastic potential energy in the final
state
• there is no work done by external forces
• the sum of the heights on the right (5 units) equals the sum of the
heights on the left (5 units)
3. If energy leaves or enters the system, make a bar for work done
by the system (example: heat lost because of friction)
Or work done on the system (skier’s poles moving a skier faster)
Stephen is pulling David up at a
constant speed on a frictionless
water slide.
Construct a statement involving
energy and work that best describes
Which of the following statements
the situation.
best describes the situation?
System
40o
Earth
1 The energy in the system changes. This change is equal to the force
that Stephen exerts on David multiplied by the distance David moves.
2 The energy in the system changes. This change is equal to the force
that Stephen exerts on David multiplied by the distance David moves
multiplied by the cos 40o.
3 David is moving at a constant speed, thus the net force exerted on
David is zero. Zero net force means zero work. Thus, the energy of the
system does not change.
situationisisdescribed
describedwith
withaabar
barchart
chart.and
Construct
AAsituation
in words.a verbal
description
matchesmatches
this bar chart.
Which
verbalthat
description
this bar chart?
Kinitial + Ug initial + Us initial
+
W = Kfinal + Ug final + Us final +DEint
1 An object fell off a cliff and energy in the system was transferred
from gravitational potential energy to kinetic energy.
2 A spring on a table shot an object. You measured the kinetic
energy of the object when it landed at the same elevation.
3 A spring shot an object vertically from a height above the ground.
You then measure the kinetic energy at ground level just before the
ball hits the ground.
AAsituation
situation is
is represented
represented pictorially
pictorially below.
below. Construct
Which bara
bar
chart
best matches
the picture.
chart
bestthat
matches
the picture?
system
y
t, y = h
ti, yi = 0
v = constant
Earth
Ko +
1
2
3
Ugo
W
K + Ug + Us
Tim is carrying a basket of laundry to be washed. This motion
is represented by a free body diagram and a motion diagram.
y

Fhand on basket

v = constant
x

Fearth on basket
Earth
Which
verbal
description
best that
Construct
a verbal
description
matches
the diagrams?
(The(The
best matches
the diagrams.
earth
and
basket
are
the
system)
earth and basket are the system)
1
Since the basket is moving, work is done on the basket and that is all
we are able to tell.
2
Tim does work, but the systems energy does not change.
3
The velocity is constant, the height is constant and Tim is doing work.
The internal energy of the system must be increasing.
4
No work is being done on the system which implies the system’s
energy does not change.
Alan is on a frictionless kiddie coaster.
Which energy
bar chart
Construct
an energy
bar
best matches
chart
that bestthis
matches
situation
as
the
car
this situation as the car
goes to
to the
the bottom.
bottom?
goes
1
2
Ki + Ugi W = Kf + Ugf
Ki + Ugi W = Kf + Ugf
h>hf >0
vf >0
3
hi=h
vi=0
Ki + Ugi W = Kf + Ugf
the same
same coaster
coaster was
was
IfIf the
not frictionless,
frictionless, which
not
bar chart would
construct
a bar chart
describe
that process?
would
describe
that
process.
1
Ki + Ugi W = Kf + Ugf + DE
2 Ki + Ugi W = Kf + Ugf + DE 3
h>hf >0
vf >0
hi=h
vi=0
Ki + Ugi W = Kf + Ugf
A roller coaster car’s trip is described by the
energy bar chart shown below.
Ki + Ugi W = Kf + Ugf
Which final point in the
Construct a picture
picture below matches
showing the final and
the bar chart?
initial points below that
matches the bar chart.
Initial
1 Final
3 Final
y
2 Final
5 Final
0
4 Final
The energy bar chart below represents a car’s
motion.
Ki + Ugi W = Kf + DE
Which
motion
diagram
matches
Construct
a motion
diagram
thatthis
matches this
situation?
situation.
1

vfinal
3
vfinal

vinitial

vinitial
2

vfinal

vinitial

4
vinitial vfinal  0
A car is moving horizontally as described by
the motion diagram below.

vfinal

vinitial
Which
energy
bar chart
matches
this?
Construct
an energy
barbest
chart
that best
Ki + Uthe
= Kf + Umotion
2
K
gi W
gf
1
i + Ugi W = Kf + Ugf
matches
above
diagram.
3
Ki + Ugi W = Kf + Ugf
4
Ki + Ugi W = Kf + Ugf
Tim is moving a basket of laundry and his hands
exert a force on the basket equal in magnitude to
the force of the Earth on the basket.
Which energy bar chart(s) could match this situation?
Construct an energy bar chart that could match this
(The earth and basket are the system)
situation. (The earth and basket are the system)
1
3
5
Ki + Ugi W = Kf + Ugf
Ki + Ugi W = Kf + Ugf
3&4
2
4
6
Ki + Ugi W = Kf + Ugf
Ki + Ugi W = Kf + Ugf
2, 3 & 4