Transcript A f - Athlete-or

Athlete or Machine?
www.raeng.org.uk/athleteormachine
Presented by Dominic Nolan. Education Programme Manager. The Royal Academy of Engineering
CHALLENGE
•Make a model of a bob skeleton sled
•See how far you can launch a Barbie!
•Present an answer to the question:
Athlete or Machine?
Which is more important in the sport of bob
skeleton?
Bob Skeleton
•1500m track
•150 m vertical drop
•143 km/h (40 m/s, 89 mph)
•Athletes times differ by tenths of seconds
•Rules for sled’s dimensions, mass and
materials
•33 – 43 kg sled
•Amy Williams - Olympic gold 2010
•www.youtube.com
Make a 1:5 bob skeleton sled
•Make the runners by bending the
metal rod
•Attach runners to pod with cable ties
•Make sled’s launch tube using
acetate sheet, tape and a plastic nose
cone (check that it fits onto the pump’s
launch tube)
•Fix the launch tube to the pod with
double-sided sticky pads
Factors
Weight
The athlete’s shape
The athlete’s position
Aerodynamic lift
Steering
Clothing and equipment
Starting
Corners
Ergonomics (how the body fits a product)
Track incline (the slope down the length of the track)
Friction on the ice
Aerodynamic drag (air resistance)
Tuning the characteristics of the skeleton
Material choice
Sled runners
Energy transfer
Potential Energy (PE) = m x g x h
144 639 Joules (J)
Gravity (g) =
9.81 m/s2
Mass (m) of athlete
and sled = 97kg
Change in PE for our athlete and sled =
1450m
Vertical drop of
track (h) =
152m
Kinetic Energy (KE) = ½ x m x v2
(diagram not to scale)
0.5 x 97 kg x (40.23 x 40.23) = 78495 J
The bob skeleton: kinetic energy gained during a run
Max speed if all PE
transferred into KE
200000
Why isn’t the all of the athlete’s and
180000
sled’s potential energy transferred into
kinetic energy?
Kinetic energy (Joules)
160000
140000
120000
100000
80000
60000
Amy
Williams
max
speed
40000
20000
0
5
10
15
20
25
30
35
40
Speed in metres per second (m/s)
45
50
55
60
Which two forces resist
the forward movement of
the athlete and sled down
the track?
friction
aerodynamic drag
(air resistance)
Friction force
Friction is a force that resists the movement of two surfaces against each other.
Which combinations provide a lot or a little friction?
rubber / concrete
A little friction
steel / ice (0.03)
felt / wood
A lot of friction
rubber / rubber
steel / ice
steel / wood
rubber / rubber (1.16)
rubber / concrete (1.02)
steel / wood (0.2 - 0.6)
felt / wood (0.22)
Calculating friction force
Ff =  x m x g
What is the friction force acting on the runners of a bob skeleton sled and
athlete with the combined mass of 97 kg (athlete = 68 kg, sled = 29 kg)?
Ff = …………………………
=
Mu, the coefficient of friction (steel on ice = 0.03).
m=
Mass (kg).
g =
The acceleration due to the gravity, which is 9.81 m/s2.
Aerodynamic drag force
The resistance provided by the air passing over a shape is a force called
aerodynamic drag.
Which shapes have a higher or lower coefficient of drag?
Lower CD
Higher CD
CD = 0.42
CD = 1.05
CD = 0.47
CD = 0.5
Calculating drag force
Calculate the drag force acting on the athlete and
sled as they travel down the track at 40 m/s?
FDRAG = ½ x  x CD x Af x V2
FDRAG = ………………………….
 =
1.2 kg/m3
(density of air)
CD =
0.45
(drag coefficient of athlete and sled)
Af =
0.139 m2
(frontal area of athlete and sled)
V =
40 m/s
(velocity)
What is the total force resisting the
forward movement of the athlete and
her sled down the track?
88.56N
FTOTAL = ……………………………………
80
Force in Newtons (N)
70
Between which velocities is friction
60
force dominant?
50
………………………………………………..
40
Between which velocities is drag force
30
dominant?
20
………………………………………………..
10
0
You can compare the two forces on the
5
10
15
20
25
30
35
Speed in metres/second (m/s)
40
45
graph here.
Prove that it is better to be heavy and narrow when competing in
The sport of bob skeleton.
ATHLETE 1
ATHLETE 2
Total mass: 97 kg
Total mass: 100 kg
Af :
0.139 m2
Af :
0.129 m2
Athlete or Machine?
Which is more important in the sport of bob
skeleton?
•Discuss this question with your partner/team
•Present your answer to the rest of the group