Kinesiology 384

Kinesiology 384: Biomechanics at Penn State University

[Kinesiology 384 @KINES384]

Okay, that's it for me for this evening. I will have office hours tomorrow 2-4, but otherwise I will see you at the exam at 6:50 in 101 Ag Sci.

Good night!

[Kinesiology 384 @KINES384]

I'm using "increasing" to mean "getting more positive" and I'm clarifying this each time I use it. If vel is neg and getting more neg (decreasing), then then the speed is incr (speeding up). If vel is pos and getting more neg (decreasing), then the speed is decr (slowing down).

[Ashley Struble @StrubeDube22]

@KINES384 if velocity is becoming more negative, does that technically mean that it’s increasing in the opposite direction?

[Kinesiology 384 @KINES384]

You're welcome!

[allyssa @allyssatome]

@KINES384 thank you!

[Kinesiology 384 @KINES384]

We say similar things about position and velocity - whether position is increasing, decreasing, or neither determines whether velocity is positive, negative, or zero.

[Kinesiology 384 @KINES384]

In this class we never talk about acc increasing or decreasing. If vel is incr (getting more +), then acc is positive. If vel is decr, then acc is neg. If vel is neither inc nor dec then acc is 0. Saying acc is increasing is different from saying acc is positive.

[allyssa @allyssatome]

@KINES384 how do you know if acceleration is decreasing compared to acceleration staying at constant but nonzero value?

[Kinesiology 384 @KINES384]

Thanks - it's been my pleasure! It's been four years since I last taught 384 and it's good to be back. It's been nice to be able to get into more depth and discuss more applications than I can in Kines 101.

[Kinesiology 384 @KINES384]

One more thing: The practice test is there to give you an idea of the format, scope, depth, and difficulty you can expect. It's not intended to be a comprehensive inventory of everything you could be tested on.

[Kinesiology 384 @KINES384]

For example, look at 9-13 and 18-21 on the prac test. We talked about force plates since MT 3, but much of this material (esp the basic mechanics) is from earlier in the semester. Study only the recent lectures and you're not giving yourself the best chance on these questions.

[Kinesiology 384 @KINES384]

I just looked at the practice test and the test tomorrow and I would say that the practice test is a good representation of what you will see. However you characterize the practice test, I think it would be a mistake to pay little attention to material from before MT3. (cont'd)

[Luke Rarig @lukerarig02]

@KINES384 So looking at the practice final, it isn’t a good representation since the majority of it is lectures 19-32?

[Kinesiology 384 @KINES384]

Just emailed about this, but you can now find the first 3 questions from the final on Canvas in the Review Materials folder.

[Kinesiology 384 @KINES384]

I'll be here until 10 pm answering whatever questions are submitted. You can tweet questions, mentioning @KINES384, or if you don't have a Twitter you can email them to [email protected].

[Kinesiology 384 @KINES384]

My advice is to study material from throughout the semester. There is definitely an emphasis on the more recent material on the final, but at least half of it is from before the last MT. Some material could be classified both ways, so it's difficult to say precisely.

[Kinesiology 384 @KINES384]

Q: MT 1, #6, why is instantaneous acc negative and not zero?

A: That's a graph of vel vs time, and at C the vel is 0. The question asks about acc, and acceleration is the change in acceleration with time - the slope of the v vs. t graph - and this is negative at point C.

[Kinesiology 384 @KINES384]

PF #45. FL and FR are 120 N and 600 N, so the dn frc P = 120+600=720 N. From #44, moment of FR about the L end is 360 Nm (600 N x 0.6 m). Frc FL has no moment about L end (d=0) so the moment of P about the L end (Px) is 360 Nm. Px = 360, or x = 360/720 = 0.5 m = 50 cm.

[Kinesiology 384 @KINES384]

(2/2) You can have V = 0 but acc not be zero (see #31). Constant V means acc = 0, and this means no net force by the 2nd law. The vertical acceleration depends on not only the force on the feet N but also the body weight W, according to N - W = ma.

[Kinesiology 384 @KINES384]

(1/2) PF #30 - #35. As you slow from +5 m/s to +2 m/s, you move in the + dir, so position increases. At pk height it is vert vel that is zero, not acc. During landing (while feet are on the ground), the vel goes from downward to 0, an upward change in V so it's an upward acc.

[Kinesiology 384 @KINES384]

(3/3) Magnitude of acceleration is found by F = ma, so 180 = 90a, and a = 2 m/s/s.

[Kinesiology 384 @KINES384]

(2/3) This h force on the foot is the only h force on the body at B, C, and D, so the force in the graph dictates the h acc. There is force at D, so the vel is changing (there is acc), there is backward force at B, so the person is slowing down then, and at C the h acc is zero.

[Kinesiology 384 @KINES384]

(1/3) PF #9 - #13. Keep in mind that this is the h force on the foot while the foot is on the ground during walking. The v force on the foot will have a different pattern. When the h force goes to zero at C, that doesn't mean the v force goes to zero - foot stays on the ground.

[Kinesiology 384 @KINES384]

Practice Final explanation for #5 - #8. See lecture notes. Screws are designed so supination drives screws, reason is that we are stronger in supination than pronation. The hum head is stabilized more by musc than lig - these muscles and their tendons form the rotator cuff.