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

FactorsWeight

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

Potential Energy (PE) = m x g x h

Change in PE for our athlete and sled =

144 639 Joules (J)

Kinetic Energy (KE) = ½ x m x v2

0.5 x 97 kg x (40.23 x 40.23) = 78495 JThe bob skeleton: kinetic energy gained during a run

0

20000

40000

60000

80000

100000

120000

140000

160000

180000

200000

5 10 15 20 25 30 35 40 45 50 55 60

Speed in metres per second (m/s)

Kin

etic

en

erg

y (J

ou

les)

Amy Williams max speed

Max speed if all PE transferred into KE

Mass (m) of athlete and sled = 97kg

Vertical drop of track (h) =

152m

1450m

(diagram not to scale)

Gravity (g) = 9.81 m/s2

Energy transfer

Why isn’t the all of the athlete’s and

sled’s potential energy transferred into

kinetic energy?

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?

A lot of frictionA little friction

rubber / rubber (1.16)

rubber / concrete (1.02)

steel / wood (0.2 - 0.6)

felt / wood (0.22)

steel / ice (0.03)

rubber / concrete

felt / wood

rubber / rubber

steel / ice

steel / wood

Calculating friction force

Ff = x m x g

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.

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)?

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?

Higher CDLower CD

CD = 1.05

CD = 0.5CD = 0.47

CD = 0.42

Calculating drag force

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)

Calculate the drag force acting on the athlete and

sled as they travel down the track at 40 m/s?

What is the total force resisting the

forward movement of the athlete and

her sled down the track?

FTOTAL = ……………………………………

Between which velocities is friction

force dominant?

………………………………………………..

Between which velocities is drag force

dominant?

………………………………………………..

You can compare the two forces on the

graph here.

10

0

20

30

40

50

60

70

80

5 10 15 20 25 30 35 40 45

Speed in metres/second (m/s)

Fo

rce

in

Ne

wto

ns

(N

)

88.56N

Prove that it is better to be heavy and narrow when competing inThe sport of bob skeleton.

ATHLETE 1

Total mass: 97 kg

Af: 0.139 m2

ATHLETE 2

Total mass: 100 kg

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