Originally Posted by jack97
Re read technote 10, Lind prefaces the section in terms of physical principle associated with work and energy, the term "pressure" did not come into play. He also loosely couples "angular velocity" with "velocity" and is still valid since each is a measure of distance travel over time, for the angular case the distance is measures in radians and can be shifted to a linear coordinate system by means of trig functions. Another way to look at angular velocity is the distance in an arc over time, as you make the arc smaller in the limit it becomes a straight line, hence angular velocity (in this limit as arc ==>0) is velocity.
Again, in technote 10, first paragraph, Lind mentions pumping up and down in the mogul field and shifting the COM to change the kinetic energy and so on. His example in this section is only for the case of pumping to increase velocity but one can use the same principles to show a decrease in velocity can occur.
He is very clear about stating the change is increase:
the name of the chapter is "Pumping to increase velocity"
He says the following things at various points:
"there is a reservoir of potential energy in the human body that the skier may convert to kinetic energy"
Note he does not say anywhere that is possible to convert that kinetic energy back into the human reservoir of potential energy.
"a skier may increase his kinetic energy and acceleration during a turn or when he runs through the troughs in a mogul field by pumping up and down"
No word on slowing down, only about propulsion...like the pump track.
"The rider can pump himself and the cart to a higher velocity if anywhere in the transit of a curve he pushes his center of mass towards the center of that curve"
"When the rider moves his mass towards the center of rotation on each turn, work is done against the centrifugal force in an amount exactly equal to the increase in kinetic energy"
When going through bumps and pumping them to speed up, an example scenario is stated with equations to back it up:
"reaction force on the skier's legs when he performs this maneuver is 3.5 times his weight"
Last stuff in the section:
"An expert skier will use this same effect to his advantage and pump his body up and down at appropriate places on a race course. This body motion increases the skier's speed going through rolls on the race course or during the carving of a turn, particularly at the end of a turn where the radius of the curvature is shortest and the potential for increasing kinetic energy, and thus velocity, by doing work against the centrifugal force is greatest.
Nowhere in this chapter is slowing down mentioned, only increasing speed. Its very clear that kinetic energy is added to create accelerations. No mention of sucking kinetic energy back into your body somehow.
We can say that you can, however, create accelerations in the opposite direction to slow yourself down but that will require some kind of work output from your muscles to convert their stored potential energy into kinetic energy that accelerates in the opposite direction. Pumping allows you to work against centrifugal forces to create that kinetic energy in the right direction for increasing speed.
Think this through some more and think about how your body can create more work to slow down by pushing against something. The author did not discuss slowing down at all here.