# The Naked Scientists Forum

### Author Topic: Bobsleigh  (Read 4329 times)

#### turnipsock

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##### Bobsleigh
« on: 06/01/2008 22:18:57 »
Bobsleigh rules say that the maximum weight for crew and bob is 625kg (I think), yet all the teams try to get as near to the maximum weight as possible.

If all objects fall at the same rate, due to gravity, then if you take the sine of the slope, then all bobsleighs should accelerate the same amount down the track.

They have to push at the start so a light bob would make this easier, yet they all add lead to bring the weight up...why?

#### Soul Surfer

• Neilep Level Member
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• keep banging the rocks together
##### Bobsleigh
« Reply #1 on: 06/01/2008 22:26:42 »
To understand why you need to look at the things that make the bob go slower.  Friction on the ground is a usually approximately proportional to the weight and has no major effect. The critical thing is the air resistance.  This is essentially the same whatever the weight of the bobsleigh and crew so the heavier the crew and sled the less effect it has.

#### kalayzor

• Jr. Member
• Posts: 24
##### Bobsleigh
« Reply #2 on: 07/01/2008 02:25:08 »
I'd also think of this in terms of energy.  Great initial mass = greater potential energy to start with, so greater kinetic energy at the end.

#### lyner

• Guest
##### Bobsleigh
« Reply #3 on: 09/01/2008 21:54:56 »
I'd also think of this in terms of energy.  Great initial mass = greater potential energy to start with, so greater kinetic energy at the end.
That is too simplistic; the top speed of a light or heavy object will be the same, in the absence of friction, at the bottom of the slope. KE per kg at the bottom will be the same as the PE per kg at the top. The only difference, in practice will be, as Soul Surfer said, the effect of air resistance will be less, proportionally for the larger mass.

#### turnipsock

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##### Bobsleigh
« Reply #4 on: 09/01/2008 23:01:13 »
This is starting to get quite complicated.

If the driver makes a mistake and brushes against the side wall, the lighter bob will be more affected.

During the pushing part of a bob run, the lighter bob will accelerate more quickly and will cover more of the track in a given time.

If the driver has to steer, a greater amount of energy will be lost from the heavier bob.

#### lyner

• Guest
##### Bobsleigh
« Reply #5 on: 10/01/2008 11:09:18 »
Yes - there are a lot of factors in the final result. Nothing is ever straightforward when you dig deep.
It's all down to Newton's Second Law.
Acceleration = Force / Mass
The 'main' forces, at the start, are due to 1. gravity; this is proportional to the total mass of the rig plus crew and 2. the crew, pushing; fit athletes will have similar power/weight ratios so large crew will contribute more to the total mass and 'dilute' the effect of the bob. By the time the crew have stopped pushing and jumped on board, the speed is not great enough to bring aerodynamics into play.
All the other forces are due, much more, to aerodynamics and friction; They limit the terminal velocity and are not dependent on mass. I suggest that steering would have a similar effect for all masses because of Newton 2, again; a similar change in velocity would involve a similar loss of Kinetic Energy per kg.
« Last Edit: 10/01/2008 11:11:18 by sophiecentaur »

#### The Naked Scientists Forum

##### Bobsleigh
« Reply #5 on: 10/01/2008 11:09:18 »