Download Lesson 2 Handouts and Instructional Notes

Lesson 2 - 0
Lesson 2
Handouts and Instructional Notes
Lesson 2 - 1
Discovering the Principles of Energy
ACTIVITY 1: The Drop of Doom
Materials: 1 basketball, 1 “fake” bowling ball (shhhhhh)
Procedure (Part A):
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Step 1 – Have one person lie on their back on the floor. Have the second person drop a
basketball from 1 foot above their head. The person lying on the floor can catch the ball.
Step 2 – Have the second person stand on a chair close to where their head is. Have the person
on the chair drop a basketball from as high up as possible towards the head of the person lying
on the floor. The person lying on the floor can catch the ball.
Which step was the person on the floor more afraid of? Why?
Procedure (Part B):
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Step 1 – Have one person lie on their back on the floor. And have the second person stand on a
chair close to where their head is.
Step 2 – Have the person on the chair first drop a basketball from as high up as possible towards
the head of the person lying on the floor. The person lying on the floor can catch the ball.
Step 3 – Have the person on the chair now drop a “fake” bowling ball (person on floor does not
know it is fake) from as high up as possible towards the person lying on the floor.
Which ball was the person on the floor more scared of? Why?
Observations - _____________________________________________________________________________
__________________________________________________________________________________________
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Gravitational Potential Energy -______________________________________________________________
__________________________________________________________________________________________
__________________________________________________________________________________________
Gravitational Potential Energy Formula - ______________________________________________________
Relate findings of activity to Gravitational Potential Energy Formula - _____________________________
__________________________________________________________________________________________
__________________________________________________________________________________________
__________________________________________________________________________________________
ACTIVITY 2: Speed
Materials: 1 racquetball
Procedure (Part A):
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Step 1 – Have the class close their eyes and imagine they just finished skiing down a black
diamond at Sunshine.
Step 2 – Have student imagine when they are at the bottom of the hill and look up for their
friend they are instantly surprised. Instead of their friend they see an apple come barreling
towards them. How do you feel?
Lesson 2 - 2
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Step 3 – Have student imagine when they are at the bottom of the hill and look up for their
friend they are instantly surprised. Instead of their friend they see a very large person barrelling
towards them. How do you feel?
Procedure (Part B):
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Step 1 – Have two people sit approx. 6 feet away from each other, facing each other (one person
holding the racquetball). A different person stands approx. 6 feet away from the pair.
Step 2 – The two people first roll the ball back and forth to each other as fast as they can. The
person standing attempts to stop the ball as it is rolling (he / she has to start from 6 feet away).
Step 3 – The two people then roll the ball back and forth to each other slowly. The person
standing again attempts to stop the ball as it is rolling (he / she has to start from 6 feet away).
Which situation was easier? Why?
Observations - _____________________________________________________________________________
__________________________________________________________________________________________
__________________________________________________________________________________________
Kinetic Energy -____________________________________________________________________________
__________________________________________________________________________________________
__________________________________________________________________________________________
Kinetic Energy Formula - ___________________________________________________________________
Relate findings of activity to Kinetic Energy Formula - ___________________________________________
__________________________________________________________________________________________
__________________________________________________________________________________________
__________________________________________________________________________________________
Energy Concepts
TOTAL MECHANICAL ENERGY = ____________________+______________________+____________________
The law of ____________________________ states that energy cannot be ________________________ or
_________________________, but it can be transferred or transformed from one form to another. The total
amount of energy in a closed system never changes. Therefore the total mechanical energy in a closed system
remains _______________________.
Work = _________________________ x __________________________
In order to accomplish _______________________ on an object there must be a ______________________
exerted on the object and it must move (___________________________) in the direction of the force.
Lesson 2 - 3
1.
A ball falls from a height of 2 meters in the absence of air resistance. Which form of energy
did the ball posses first? Which form of energy did the ball possess as it was falling?
a. The ball is losing height (falling) and gaining speed. Thus, the internal or conservative
force (gravity) transforms the energy from PE (height) to KE (speed).
2. A baseball is traveling upward towards a man in the bleachers. Which form of energy did the
ball posses first? Which form of energy did the ball possess second?
a. The ball is gaining height (rising) and losing speed (slowing down). Thus, the internal or
conservative force (gravity) transforms the energy from KE (speed) to PE (height).
3. Rusty Nales pounds a nail into a block of wood. The hammer head is moving horizontally when
it applies force to the nail. Did this situation change the potential energy, kinetic energy or
both?
a. The applied force of the hammer causes the nail to gain speed. Thus, the external or
non-conservative force alters the kinetic energy of the nail.
4. The frictional force between highway and tires pushes backwards on the tires of a skidding car.
Did this situation change the potential energy, kinetic energy or both?
a. The friction force on the car causes the car to lose speed. Thus, the external or nonconservative force alters the kinetic energy of the car.
5. A shopping cart full of groceries is sitting at the top of a 2.0-m hill. The cart begins to roll until it hits a
stump at the bottom of the hill. Upon impact, a 0.25-kg can of peaches flies horizontally out of the
shopping cart and hits a parked car with an average force of 500 N. How deep a dent is made in the car
(i.e., over what distance does the 500 N force act upon the can of peaches before bringing it to a stop)?
a. Initially:
PE = 0.25 kg * 9.8 m/s/s * 2 m = 4.9 J
KE = 0 J (the peach can is at rest)
Finally:
PE = 0 J (the can's height is zero)
KE = 0 J (the peach can is at rest)
The work done is 500 N*d*cos 180 = -500*d
Using the equation,
TMEi + Wext = TMEf
4.9 J + (-500*d) = 0 J
d = 0.0098 m (9.8 mm)
Lesson 2 - 4
6. A 510-kg roller coaster car starts at a height of 32.0 m. Assuming negligible energy losses to friction
and air resistance, determine the PE, KE, and speed of the car at the various locations (A, B, C, D, and E)
along the track.
Location
Height (m)
PE* (J)
KE* (J)
velocity (m/s)
Start
32.0
160 000 J
0J
0
A
28.0
140 000 J
20 000 J
8.9 m/s
B
11.0
55 000 J
105 000 J
20.5 m/s
C
20.0
100 000 J
60 000 J
15.5 m/s
D
5.0
25 000 J
135 000 J
23.2 m/s
E
15.0
75 000 J
85 000 J
18.4 m/s
F
0
0J
160 000 J
25.3 m/s
*rounded to the second significant digit
Answers: See above table (answers in red)
The potential energy for every row can be found using the equation PE = m*g*h where m=510 kg and g = 9.8
m/s/s. In the first row, the total mechanical energy (KE + PE) equals 160 000 J (rounded). Since no work is done
by non-conservative forces, the total mechanical energy must be 160 000 J in all the other rows. So the KE can
be computed by subtracting the PE from 160 000 J. The velocity can be found using the equation:
KE = 0.5*m*v2 where m=510 kg.
7. Use the work-energy theorem to determine the force required to stop a 988-kg car moving at a speed of
21.2 m/s if there is a distance of 45.7 m in which to stop it.
a. Answer: F = 4.86*103 N
The work energy theorem can be written as
KEi + PEi + Wnc = KEf + PEf
The PEi and PEf can be dropped from the equation since they are both 0 (the height of the car
is 0
m). The KEf can also be dropped for the same reason (the car is finally stopped). The equation simplifies to
KEi + Wnc = 0
The expressions for KE (0.5*m*v2) and Wnc (F*d*cos[Theta]) can be substituted into the equation:
0.5*m*vi2 + F*d*cos[Theta] = 0
where m=988 kg, vi=21.2 m/s, d=45.7 m, and Theta = 180 degrees (work and displacement acting in opposite
directions). Substituting and solving for F yields 4.86*103 N