Impulse of changing direction

#1

I’ve got a question about parkour.

The common language around parkour is that you can save your knees and reduce impact after a big jump by transferring your momentum forwards.

How does this work? Wouldn’t the impulse vector required to change a vertical momentum to an equal horizontal momentum be sqrt(2) times bigger than the impulse required to just stop the downward momentum?

Am I wrong in thinking I can do simple vector addition with the equation pf = p0 + j ?

#2

I think you are right that adding horizontal momentum doesn’t address the fact that you need to stop your vertical momentum somehow. But just to note, the horizontal component of momentum isn’t really a problem, as it is slowed and stopped by friction. It won’t cause the same kind of damage to your body.

I think rolling is designed to address your vertical momentum. Any change to horizontal momentum is coincidental.

Think of the problem of reducing the force of your impact in general. For simplicity, let’s say your center of mass is moving down with a constant velocity. You require a fixed impulse to stop that speed, so you want to spread the force of your impulse over the longest period of time possible, i.e. decelerate over a long period of time. Ideally, you want to decelerate constantly over the maximum time interval possible. You also want to spread the impulse of the landing over as much of your body as possible, reducing the damage to your legs which will inevitably receive the most.

The earliest you can start to decelerate is when your feet touch the ground. Your deceleration must ends when your center of mass has reached its lowest point. That physical distance and your velocity determine the maximum time window for deceleration. Then the closer to the ground you can get your center of mass, the better.

So there are a few reasons to roll:

  1. Rolling allows you to decelerate as constantly as possible
  2. Rolling spreads the impulse of the landing over many parts of your body
  3. Rolling allows your center of mass to get as close to the ground as possible before coming to a complete stop
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#3

According to the page below, it appears to be a question of spreading your impact out over time.

http://ffden-2.phys.uaf.edu/webproj/212_spring_2017/Cameron_Showalter/Cameron_Showalter/rolling.html

#4

I think this is it. That makes sense to me. I don’t suppose there would be a good way to crumple that would get you as low as you can with rolling.

There is also, as always with these kinds of questions, the problem of physiology. The fact that there are certain ways our bodies are good at absorbing impact and these kinds of athletes are particularly careful about their knees.

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