Enquiries/help with some assignment questions

[Wei]

Hi everyone, my name is Tze Wei Lim and i am a exercise science undergraduate from Deakin University. I am currently undergoing my Master in Exercise Science (Strength and Conditioning) at ECU, and am having some problems with questions from my biomechanics assignment. I was wondering if anyone here could help me out with some of the questions that i have difficulties with ?

Projectile Motion:
1. Neglecting the height of release, a baseball is thrown vertically upwards at 20 m/s, find:
a) vertical velocity after 2 s
b) maximum height
c) total flight time
d) time (seconds) that the projectile is at height 10.5 m.

What would happen to the values obtained in question 1 if the baseball was thrown at an angle of 30 degrees to the ground? (circle one option)
a) Increase, decrease, no change
b) Increase, decrease, no change
c) Increase, decrease, no change

Basic equations for a strength and conditioning coach:
2. At the instant an athlete is cutting, he is applying 200 N of medio-lateral force and 400 N of anterior-posterior propulsive force. Neglect vertical force in this problem.
a. Based on your knowledge of 3-D axis. Draw the how these two forces would be represented on a diagram, using both direction and magnitude.
b. Calculate the resultant horizontal force of this athlete. Resultant horizontal force = √((400N)2 + (200N)2) = √(160000+40000) = 447.21N (Solved!)
c. Calculate and describe the angle the force is being applied (you may show this on the diagram)

4. Show your work for the following and calculate what happens if an athlete is pulling a sled angle attached to his hip, resulting in an angle of force of pull at 15 degrees to the ground. If the coefficient of sliding friction between the sled and the grass is 0.8 and the sled has a mass of 25 kg, what is the acceleration of the sled if he is constantly pulling with a resultant force of 300 N? (this is a multi-step problem).

So far, i have managed to calculate these steps:
25kg * 9.81 = 245.25N
300sin15° = 77.65N (Vertical component)
245.25N – 77.65N = 167.6N (Reaction force)
300cos15° = 289.78N (Horizontal component)
F = μR, F = 0.8 * 167.6N, F = 134.08N (Friction force)

However, i am really very unsure as to whether i am just supposed to just utilise the horizontal force component of 289.78N to get the acceleration value, as Force = Mass * Acceleration. This is because, there is a reason that the coefficient of sliding friction was provided and therefore, i do not think that it is so simple as to just divide 289.78 over 25kg.

Really tearing my hair out over these questions, as i have been working on these questions for quite a fair bit (3-5 weeks or more ?, for the whole assignment). The worse thing is that my assignment is due on the 25th of May and i am a little worried. Hoping that some one here can simplify and explain these questions to me so that i can understand these biomechanics concepts much better, as i really wanna be well versed in these concepts and not just get these questions over and done with. Thank you.

Regards,
Tze Wei Lim

[Ted Andresen]

Re: Enquiries/help with some assignment questions

Hi Lim,

First of all, do not pull your hair out. Hair is important, especially, as we mature and get wiser.

I do not want to solve the problem for you, but let me recommend a method for approaching physics problems that helped my students.

Write down all the algebraic equations that you can remember from the problem area you are working in.
Read the problem carefully.
Every time it gives a variable (y(0)=0 m, v(0)=20 m/s, t = 2 sec) look through all the equations where that variable appears.
Put a check mark above that variable.
Put a check mark above all the constants, such as, g, pi, or c.
Look for any equation that has all the variables checked except for one.
Solve that equation for the unchecked variable.
Place a check mark on that variable and in any other equation where it appears.
Put a question mark above the variable that you are trying to find, v(t).
Search for any equation where your target unknown, v(t), is the only non-checked variable.
Do not put numbers in the equations, yet.
Instead, solve the equations algebraically, substituting the expressions for the checked variables in the equations with the question marks.
Here, it is useful to draw serpentine arrows from one equation to the next, before you do the algebraic substitution.
One equation should have all the variables marked.
Solve that equation for the unknown variable.
Check the final equation for dimensional consistency; make certain that the units on the one side equal the units on the other.
(When working with forces, you may find it helpful to enter gravity as 9.8 Nwt/kg, instead of 9.8 m/s^2)
Now put in the given values with their units. Again, cancel the units and crank out the numeric value.

Also, when substituting the units in the final equation, do not write units in horizontal format, such as, (ml of O2)/(kg-min). Instead write them in vertical, format, as in,

ml of O2
----------
Kg*m

Then you will be able to see clearly which units are in the numerator and which are in the denominator. It will make cancellation much easier.

If you maintain some organization while solving physics problems, it can be a satisfying and fulfilling experience.

Ted Andresen
St. Petersburg, FL

[Jessica Newman]

Re: Enquiries/help with some assignment questions

I was wondering how you went with these questions as I have the exact same questions just different values and am struggling with them altogether!
Are you please able to provide me with some assistance?