Transcript 8PE and KE review - Mr-Hubeny

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Objective:
MCAS reviewKinetic and Potential
Energy
Homework: O.R. question
All need to know about KE and
PE to do well on MCAS
all
 Potential Energy. –
 Conservation of Energy
(Energy is conserved!)
Energy of Position
(how high)
P.E. = mgh
 Energy start= Energy end
(may lose some to friction
(heat) and air resistance,
etc.
 Kinetic Energy-
Energy of motion
(how fast)
K.E. = 1/2mv²
 P.E
K.E.
Mechanical Energy- either P.E. or K.E.
P.E. = mgh
Since boxes being lifted, we need P.E.
Give us weights,
so already have
mg, just need
multiply by h
Use P.E. equation to solve for each box
to see which hs the most change.
Three different boxes are lifted to
different heights.
Box X weighs 115 N and is lifted to 15m.
Box Y weighs 210 N and is lifted to 10m.
Box Z weighs 305 N and is lifted to 5m.
Which of the following statements
best describes the boxes’ change in
mechanical energy?

A.Box X had the
greatest change in
mechanical energy
 B.Box Z had the
smallest change in
mechanical energy.
 C.Boxes X and Y had
the same change
in mechanical
energy.
 D.Boxes Y and Z had
the same change in
mechanical energy.
P.E. box X = 115x15 =1725J; P.E. box Y = 210x10 = 2100J;
Figure out P.E. had at first (at h = 50m), then
figure out P.E. had after dive (at h = 20m). Then
subtract two answers to get how P.E. lost
2. A 9 kg model airplane flies
horizontally at a constant
speed. If the plane suddenly
dives from its altitude of 50 m
and levels off at 20 m, how
much potential energy does it
lose in the dive?
A.450 J
B.1800 J
C.2700 J
D.9000 J
 Need 2 students to
come up to board.
One to calculate
P.E. at before dive,
the other to
calculate P.E. at
end of dive.
 Remember:
P.E. = mgh
 A cart at the top of a hill is released




and rolls down the hill. Which of the
following describes the energy of the
cart just as it reaches the bottom of
the hill?
Still moving so has energy
A.The cart has no energy.
B.The cart has maximum kinetic energy.
Yes! Goes
C.The cart has maximum gravitational
fastest at
potential energy.
At lowest point bottomso no P.E.
all
D.The cart has equal gravitational
P.E. into K.E.
potential and kinetic energy.If no P.E., then P.E. and K.E.
cannot be equal.
4. Which of the following increases when a
metal spring is stretched horizontally?
A.potential
Horizontally = across
B.kinetic energy
= change in position
C.gravitational energy
= spring has potential
D.electrical energy
to do something
= change in P.E.
As get lower, less
P.E., but also moves
faster, so…
5. The figure below represents a
pendulum’s motion with the lowest point
of its swing labeled P.
P.E.
What happens to most of the pendulum’s
gravitational potential energy as it
reaches the lowest point P?
A.It is transformed into inertia.
B.It is transformed into kinetic energy.
C.It is transformed into thermal energy.
D.It is transformed into chemical energy.
K.E.
Use what learned from #5 to
answer #6.
6. The diagram below shows the path of a
student on a sled starting from rest at
point W.
The student slides down a frictionless,
snow-covered hill past point Z, which is
at ground level.
Which of the following statements best
describes the energy of the student and
sled from point W to point Z?
A.The total energy at point W is less than
at point Z.
B.The total energy at point W is greater
than at point Z.
C.The potential energy at point W
becomes all kinetic energy at point Z.
D.The kinetic energy at point W becomes
all potential energy at point Z.
Energy is conserved!
All P.E. becomes K.E.
Conservation of Energy is
E start = E end…
The diagrams below
a man
a be more
Inshow
order
forswinging
K.E. to
golf club.
The kinetic energy started
of the golf with…
club at point Y is
more than the potential energy of the club at
point X. Which of
the following
statements
Energy
must have
been added!
best explains why this fact does not violate
the law of conservation of energy?
A.Gravity is constant from point X to point Y.
B.Air resistance is greater at point Y than at
point X.
C.Acceleration due to gravity is greater at point
Y than at point X.
D.Energy is added by the man to the golf club
from point X to point Y.
than the P.E.
Write in notes
Write the law of conservation of energy in your
own words.