Athlete or Machine? 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

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

Potential Energy (PE) = m x g x h Change in PE for our athlete and sled = Joules (J) Kinetic Energy (KE) = ½ x m x v x 97 kg x (40.23 x 40.23) = J 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/s 2 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 ( ) felt / wood (0.22) steel / ice (0.03) rubber / concrete felt / wood rubber / rubber steel / ice steel / wood

Calculating friction force F f =  x m x g F f = …………………………  =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/s 2. 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 C D Lower C D C D = 1.05 C D = 0.5 C D = 0.47C D = 0.42

Calculating drag force F DRAG = ½ x  x C D x A f x V 2 F DRAG = ………………………….  =1.2 kg/m 3 (density of air) C D =0.45(drag coefficient of athlete and sled) A f =0.139 m 2 (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? F TOTAL = …………………………………… Between which velocities is friction force dominant? ……………………………………………….. Between which velocities is drag force dominant? ……………………………………………….. You can compare the two forces on the graph here Speed in metres/second (m/s) Force in Newtons (N) 88.56N

Prove that it is better to be heavy and narrow when competing in The sport of bob skeleton. ATHLETE 1 Total mass: 97 kg A f : m 2 ATHLETE 2 Total mass: 100 kg A f : m 2

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