### Harry Potter and the Prisoner of Askaban: The Knight Bus

```Harry Potter and the Prisoner of
Scene Description
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Harry Potter decides to take a ride on the magical Knight Bus. This bus defies
gravity many times during the scene.
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The braking force of this speed is too great
–
The stopping distance of the bus going this velocity would require a much
longer stopping distance
–
The time is still wrong in order to stop within this distance
–
The force to stop in the movie does not match up with the actual force it would
take the bus to stop
–
Harry should fly through the windshield after stopping that instantly
The Scene
The Knight Bus
Physics Concepts
Stopping distance of bus
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●
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Determine velocity of bus using a distance interval and
the travel time of that interval
Determine the bus' momentum with momentum formula
Impulse momentum theorem to determine the braking
force needed to stop the bus
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Braking force to stop bus in the time it stops in movie
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Find actual stopping distance for time in movie
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More Physics Concepts
Force to stop a bus going this speed
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Determine force of friction
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Determine net force (ma)
Prove windshield would break
●
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Impulse momentum to determine force at which Harry would be
traveling
Compare to force it takes to break a windshield
Estimations
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1 city block~ 100,000 sq.ft.
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1.93s to travel 1 block
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20,360kg estimate for triple decker bus
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Average male mass: 70kg
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About 10 passengers on bus (70kg each)
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100kg for fuel tank
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5m estimated distance bus takes to stop
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1.5s estimate stop time of buss in scene
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.055s estimate time face hits glass for
Calculations
Timing the bus as it travels
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1 city block~ 100,000sq ft
100,000 = 223.62ft
223.62
3
= 74.54m
Velocity=
V=

74.54
1.93
V= 38.6218

Determining the mass of the bus
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1 double decker bus~ 14,560kg +
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1000kg extra floor +
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70kg per person x 10 people +
●
100kg fuel tank and fuel = 20,360kg estimate mass of triple decker
bus
Braking force of the bus in the scene
Impulse Momentum Theorem:
Stopping distance the bus needs in the scene to
avoid collision with the woman
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Kinematic equation:
1
2
1
2
●
=
+
●
=
●
= 28.96635
38.6218 + 0 1.5
Time the bus should take to stop
if it does only have about 5m like
the scene
1
2
5 = 38.6218 + 0
5 = 19.3109
= .2589
Determining the force needed to stop
the bus in the scene
Coefficient of friction:
Fnet=Fapp-Fk= µFn
−
• Fnet=ma (acceleration=
)
0−38.6218

Fnet=20,360(
)
.5
Fnet=1,572,679.696N
•
Rubber on wet concrete: µs=0, µk=.5
Fk=µkFn (Fn=mg)
Fk=.5(20,360×9.81)
Fk=99,865.8N
•
Fnet=Fapp-Fk
1,572,679.696=Fapp-99,865.8
Fapp=1,672,545.496N
~1,6722.5455kN to stop the bus
Harry would crash through the windshield
during brake of the bus
Momentum: F∆t=mvf-mvi
=
(70×0)−(70×38.6218)
.055 (  )
−2,703.526
.055
=
= −49,155.0181
-Convert into lbs /10 ×2.2= 10,814.104lbs
120-200N/mm2 =Force needed to break a windshield
●
●
The braking force of this speed is too great for the bus
The stopping distance of the bus going this velocity would require a much longer
stopping distance
–
●
The time is still wrong in order to stop within this distance
●
●
Only a quarter of a second of time can pass to have a 5m stopping distance
The force to stop in the movie does not match up with the actual force it would take
the bus to stop
–
●
~5m in movie, but it would need 28.97m
~524kN to stop in the movie, it should take ~1,000kN to stop in real life
Harry should fly through the windshield after stopping that instantly
–
49,155.0181N of force greatly exceeds the 120-200N Force needed to break a
windshield
Works Cited
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●
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"AEC Regent III RT." Wikipedia. Wikimedia Foundation,
25 Mar. 2012. Web. 04 Apr. 2012.
<http://en.wikipedia.org/wiki/AEC_Regent_III_RT>
<http://www.land4ever.com/block.htm>.
"Knight Bus." Knight Bus- Hary Potter Wiki. Wikja
Entertainment. Web. 4 Apr. 2012.
<http://harrypotter.wikia.com/wiki/Knight_Bus>.
Works Cited
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"London Bus Museum." 1952 AEC Regent III Bus â€
“
RT3491 -. The London Bus Preservation Trust Ltd. Web. 04
Apr. 2012. <http://www.londonbusmuseum.com/museumexhibits/double-deck-buses/aec-regent-iii-rt-rt-3491/>.
Yirka, Tonya. "Tempered Glass Strength Specifications."
EHow. Demand Media, 29 Nov. 2010. Web. 21 Apr. 2012.
<http://www.ehow.com/list_7575774_tempered-glassstrength-specifications.html>.
Works Cited
Pictures:
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http://news.bbc.co.uk/cbbcnews/hi/pictures/galleries/newsid_2821000/2821417.stm
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157
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http://hypertextbook.com/facts/2006/DanielTouger.shtml
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