GRAVITY VEHICLE
SCIENCE OLYMPIAD 2012
Coaches Workshop
October 22, 2011
EGG-LESS SCRAMBLER
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Similar to former event “The Scrambler”
Come closest to reaching target
 Scoring based on time and distance
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Lowest score wins
Distance announced after impound
Braking mechanism can be employed
No electrical components
Major Differences:
Propelled only by its own mass
(gravitational potential energy)
 Teams must build ramp with release mechanism
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CONSTRUCTION: THE RAMP
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Material: smooth to reduce friction (waste energy)
Plastic, metal, wood, plexiglas, etc
 Too smooth=bad tracking
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Ideally adjustable
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Perhaps hinged to adjust slope (i.e. lounge chair)
Shape: Where is mass concentrated?
Flat- simple design, easily adjustable
 Curved- maximize PEg
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CONSTRUCTION:
VEHICLE— CHASSIS
Rigid is good– propulsion will cause stress
 Material: plastic, wood, metal, plexiglas, etc
 Structure: flat board, box style…
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Make sure axles are ‘square’ to direction of travel and
parallel to each other
Design idea:
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Build car in 2 halves with bolt connecting axles
CONSTRUCTION:
VEHICLE — AXLES
Rigid and straight
 Crooked axle = crooked path
 Materials: wooden/plastic dowels, metal rods
 Axle-Chassis attachment:
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1.) Drill holes in chassis, feed axle through
2.) Attaches tubes to chassis– straws
CONSTRUCTION: VEHICLE — WHEELS
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Friction is the enemy
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Straight and light-weight
Ideas:
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CDs- spin well, but need traction
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Strips of latex balloon
Hobby airplane wheels
 Home-made– only if perfectly circular
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How many? 3 or 4?
 Thick or thin?
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Depends on chassis design
BRAKING — GENERAL PRINCIPLES
Braking mechanism can’t be used to propel vehicle
 Don’t lose contact/normal force
 Higher normal force=faster braking
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Keep center of mass closer to front axle
Minimize Skidding
Kinetic friction dependent wheel-surface friction
instead of braking mechanism
 Decreases stability
 Fix it by relocating/adding mass- modeling clay
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Brake pads for smoother stops
 Too much brake=Newton’s 3rd Law
 Take tips from Scrambler
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CONSTRUCTION:
BRAKING SYSTEMS — STRING METHOD
String from one axle unwinds and wraps around the
second axle. Fed through, axles lock, car stops. Distance
traveled controlled by the amount of string.
Easy construction
 Easy to implement
 Easy length adjustment
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Poor accuracy
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Backlash
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wrapping variation
When string goes taut
Skid
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Relies on wheel traction
CONSTRUCTION:
BRAKING SYSTEMS — THREADED ROD METHOD
Uses a threaded rod for the axle, wing nut on the axle.
Rotation of wheels = horizontal motion wing nut. Wing
nut reaches a barrier, stops the axle. Distance set by
wing nut placement.
High accuracy
 Consistency
 Easy length adjustment
 Little backlash
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Harder to build
 Axle friction
 Skid
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GENERAL TIPS
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Test on various surfaces—gym floors, hallway tile
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Chart data for each; better idea on race day
Clean floor— dirt/dust affects braking
 Calculations– divide race length by wheel
circumference to get # of rotations
 TEST and TEST AGAIN!
 Don’t DQ! Read rules carefully
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Dimensions include starting point
 Release mechanism- actuated by pencil
 No loose pieces
 Vehicle mass < 2.500 kg
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Have fun!
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Event Coordinator: Betsy White– bwhite6@utk.edu