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Homework Help: Why do sprinters start in a crouched position?

  1. Sep 26, 2008 #1
    I know that when the sprinter takes off, the friction acts on you that causes his acceleration, same but opposite force that you put on the ground. But why does he start in a crouched position?

    Is it because to increase Fn? How would crouching increase the Fn? If one's crouched, the Fn would be cos(theta)Fn because we want the force normal to the surface? Wouldn't his Fn be less than if he was just standing straight up because of cos(theta)?

    And why do they use starter blocks?

    Can somebody elaborate on this so i can understand this concept?
  2. jcsd
  3. Sep 27, 2008 #2
    Because it's faster than standing straight?
  4. Sep 27, 2008 #3


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    Starting in the crouched position with the starter blocks makes your acceleration at the start of the race be applied by a sort of "side-ways jump" if you will, instead of the usual standing start push.

    Also there is less chance that you can slip at the start due to pushing off the starting pads rather than pushing off the ground which is slanted to your motion.
  5. Sep 27, 2008 #4
    yes but please refer to my question.. my question is why?

    why do they have a great acceleration with crouched position and by using the block? Can you explain in terms of Fn?

    Ff = uFn since friction is the force acting on you in the direction of motion.
  6. Sep 27, 2008 #5


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    You might want to draw a force diagram and think of the sprint start more as a launch problem, accelerating to maximum speed being the most efficient time-wise.

    Viewed in that fashion what is the dependence of starter forward velocity acceleration component compared with the normal weight component and the angle that a sprinter can affect the force through?
  7. Sep 27, 2008 #6


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    Hi pikapika1! :smile:

    I don't think the crouch affects the linear acceleration … I think it's to minimise the angular acceleration …

    you'd look pretty stupid if your feet shot forward so fast that you fell over backward! :biggrin:
    Maybe the friction on the ground wouldn't be enough to prevent slipping? Or maybe to avoid straining the ankles?
  8. Sep 27, 2008 #7

    This is really a biomechanics problem. Try to think of your legs a spring and the spring equation (F=kx) where k is the spring stiffness (assume constant) and x is the displacement of the sping. Since F=ma also, we can arrange to get a= (k/m)*x. SO, since k and m are constant, the only thing you can really change is x to increase your acceleration. If you were to start running standing up, your legs aren't as bent so you would have a little x. However, if you crouch you are compressing your body and legs like a spring, thus resulting in a bigger x and acceleration.

    Think if you had a sping.... the more you push down on it, the more force you get.

    So the starting block helps you crouch and angle this force in the x-dircetion (otherwise if you took all your force in the y-dir you would be high jumping instead!)

    This is a VERY basic analysis.... the runners physique and biology also plays a huge role in (remember k and m would be different for everyone)

    I'm not sure how much friction plays a role , Im sure it does somehow but I would think that would be a function of what kind of shoes your wearing and the dynamics of the runner.

    Hope this helps
    Last edited: Sep 27, 2008
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