NSF-funded scientists Paul Doherty, Deborah King, and George Tuthill, along with bobsled designer Bob Cuneo, use an Olympic bobsled run, from starting push to the finish line, to illustrate acceleration, velocity, gravity, and drag.
Banking On Speed: Bobsled
LESTER HOLT, Anchor:
The Winter Games in Vancouver provide a chance for the United States four-man bobsled team to win its first gold medal in more than sixty years. Team members and an Olympic bobsled designer with help from a materials science researcher and a sports scientist funded by the National Science Foundation explain how they’ll accomplish this feat.
The Bobsled got its name when it appeared in Switzerland in the 1890s – its riders “bobbed” back and forth to try to increase their speed.
Today’s Olympic bobsledders use precision moves…and an intuitive understanding of physics…to maximize velocity, the speed of the bobsled as it plummets downhill.
STEVE HOLCOMB, U.S. Bobsled Team: There's more than just four fat guys getting into a sled and sitting there and hoping to win.
HOLT: By hurtling around the icy bends of the course at 5-Gs – five times the force of gravity…akin to what fighter pilots experience. Physicist – and Air Force veteran -- Paul Doherty did the Olympic bobsled run in Lillehammer, Norway --site of the 1994 Winter Games.
PAUL DOHERTY, The Exploratorium: We were being slammed first to the right at five G's and then to the left at five G's. And when you bend your head forward under five G's, which I did through one of the turns, you can't lift it – your neck muscles are not strong to lift it, and bring it back up.
HOLT: What gives the bobsled its velocity is acceleration – initially created during the powerful 50-meter push at the race start…by the team’s feet pushing forcefully against the ice.
DR. DEBORAH KING, Ithaca College: the start is where you are going to try to pick up speed. So you are getting your acceleration.
DR. GEORGE TUTHILL, Plymouth State University: The team is trying to get that bobsled moving as rapidly as possible, by applying maximum force to the track.
HOLT: Then, in a tightly choreographed succession, the bobsledders jump in – while trying to keep the sled steady, and on a straight trajectory.
STEVE MESLER, U.S. Bobsled Team: There's a lot of disruption, a lot of forces that can cause the sled to go back and forth and slow yourself down.
HOLT: In the push phase, everything the bobsledders do is important: this is the only chance they have to generate force.
BOB CUNEO, Bo-Dyn Bobsled Project: other than the athletes’ starting the sled at the top, there’s no way to gain more force going down so you have to conserve the force that you began with.
TUTHILL: You might think that if they save a tenth of a second at that start, how could that translate into much as they go down the track. But you have to remember that they're accelerating all the time. And so if they have a small advantage at the start, they continue to maintain that small advantage most of the way down the track.
HOLT: The force of gravity accelerates all bobsleds down the course in exactly the same way – Galileo is said to have demonstrated this by dropping two objects of different mass from the Tower of Pisa. So gravity is constant for all the sledders.
HOLT: The only way sled teams can affect their run-time once on the course is by minimizing the loss -- of even a fraction of a second -- to forces that can slow the sled…especially air resistance: drag.
HOLT: Everything about the bobsled is designed to reduce drag – from the sleek sled-body…to the team’s tight hooded suits…to the way the crew “tucks” into close formation, like peas in a fiberglass pod.
KING: It needs to be nice and streamlined and smooth, with nothing sticking out, so the air will flow over the bobsled really smoothly, won’t cause a lot of resistance, and so they are able to go really fast down the mountain.
HOLT: Each steep, banked turn on the track is also a potential speed “trap”…
HOLCOMB: Every time you hit the walls, it slows you down. You want to minimize any sort of --of you know, of-rubbing.
HOLT: Scraping against the ice wall increases friction…The sled loses momentum…momentum established during that first push – and remember, momentum it can’t get back.
CURT TOMASEVICZ, U.S. Bobsled Team: We just try to be as loose as possible, try to not fight the pressures as we go into the curves, try to anticipate the curves a little bit but not actually lean with all the curves.
HOLT: …trying to find the perfect “racing line” around the frozen curves…
HOLCOMB: Unfortunately, there really isn’t a perfect place to be--if you take the shortest distance, that doesn't necessarily mean um, you're going to be the fastest. You may be, creating too much friction on the ice that will slow you down or you may be creating not enough friction when you're actually moving too far.
HOLT: So if you can push the bobsled without stumbling… Get into a tight space in a careful formation…Careen down a treacherous ice trail without crashing or hitting a wall…Consider yourself lucky.
But -- if you can do it in less time than everyone else…Consider yourself a winner.
Science Activity (Grades 6-9) from Lessonopoly
BANKING ON SPEED: BOBSLED
Objective: Build and test a bobsled model to understand the acceleration of gravity on objects of differing mass.
Introduction notes for teacher:
This activity is designed to reinforce the ideas in the “BANKING ON SPEED & THE BOBSLED” video by NBC Learn. An additional goal is to strengthen ideas about the acceleration of gravity on objects of differing mass. Student instructions are given below are given below.
Part One: A bobsled simulation
(1) Build a bobsled “chute” with a ten foot length of 1½ inch diameter PVC. Build about five bobsleds out of identical, small test tubes with stoppers. The test tubes should be thin enough and short enough to easily slide down the inside of the PVC pipe. The test tubes are to be filled (with water, metal fillings, etc.) so that they each have different masses. Obtain a stopwatch or some other timing device.
(2) Label the bobsleds so that they can be individually identified. Make a prediction on which of the bobsled tubes will make the shortest trip down the chute
(3) Drop each sled down the chute and measure the time for its trip to the bottom. (a) Be sure you have a cushion of some kind at the bottom so the glass tubes won’t break. (b) Make sure that the condition for dropping are the same for each sled, so none will have an advantage or disadvantage. (c) Have paper/pencil ready to record the data neatly and completely.
(4) Each sled should have at least five timed runs, preferably alternated between the other sleds.
(5) When determining the “winner”, the average of three timings will be calculated. The top and lowest times for each sled will be discarded. Calculate average to the hundredths place.
(6) Subtract the shortest average from the longest average. This difference is called a range. Use this range and the middle value of the five sled averages to calculate a percent. (e.g. range divided by middle average multiplied by 100)
Part Two: Analyzing the data (questions to answer)
(1) Did your hypothesis prove correct? (e.g. your prediction from step two above)
(2) How is the “start off” of this simulation different from the real run shown in the video?
(3) Would you say that one of the bobsleds was faster than the others or was there a tie?
(4) How would you use the percentage calculated in step six to determine if there was a tie?
(5) In most cases, a tie would be declared. Friction would be the cause of time differences. If any sled would lose, it might be the heaviest sled. Why?
(6) Since there is a tie, what can you say about the speeds and accelerations of different mass objects when they move downwards under the influence of gravity?
(7) The video pointed out that the bobsled that was pushed the fastest at the top of the run had the advantage of winning. Under what condition would the “fastest at the top” sled not win the race to the bottom of the chute?
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