Download Ph101_Lab-Jumping-Impulse

Ph101: Fundamentals of Physics Laboratory
Instructor: Tony Zable
1
Laboratory #6: Push-Ups, Human Jumping & Hang-time - A Tale of Exercise,
Impulse & Momentum
Purpose:
1. To observe the force profile for various types of “push-ups”
2. To measure the hang time for a human jump
3. To determine calculate the launch velocity and impulse for a jump
4. To calculate the area under the force-time graph during a jump and compare it to
direct calculation of impulse
Introduction:
Forces as we study them in textbooks appear rather uncomplicated, simple, and generally
easy to work with. With limited exception, the forces we encounter in every day life are
far from uncomplicated, simple, and easy to work with (especially from a mathematical
perspective). Even the simplest motions such as walking or jumping involve erratic and
complex forces as our muscles generate basic locomotion. Application of Newton’s 2nd law
to simple everyday forces (which can be complex force problems) is often impractical and
in many cases impossible. In some instances, the concepts of impulse and momentum
can be applied and prove to be powerful tools for studying problems involving real-life
forces, especially collisions. In this laboratory exercise, the forces that occur during
“simple” activities such as push-ups and jumping will be studied. You will find that
“simple” movements aren’t as simple as they appear and the forces our bodies generate
are amazingly complex. Hopefully, you will develop a new appreciation for the amazing
mechanics of our human machinery… and in the process, develop a better understanding
of the concepts of impulse and momentum.
Preliminary Questions:
1) Consider person jumping directly upward into the air from ground level and landing
back on the ground (on her feet).
a) How does the momentum of the person just after leaving the ground compare to
her momentum just before landing?
b) How does the impulse exerted on the person during the launch compare to the
impulse received during landing?
c) How does her launch force and landing force compare for the jump?
2) How is impulse related to net force and elapsed time?
3) How is impulse related to momentum?
File Name: \ph101\Ph101_Lab6-Jumping-Impulse.doc
Ph101: Fundamentals of Physics Laboratory
Instructor: Tony Zable
2
Apparatus:
 Force plate & LabPro
 Computer w/LoggerPro Software
Part 1: Physics of Push-Ups:
1)
Connect 2 force plates to a LabPro interface (Be sure the force plate is set to 850N)
2)
Open LoggerPro Software and open the “Phy101_Lab6-Pushups” experiment file
3)
Zero force plates (with nothing on them!)
4)
Click on “collect” then place one hand flat on each plate and perform a normal
push-up (back straight, balance on your toes, go down slowly until your elbows are
bent at ~90o angle then back up). You should be able to perform several push-ups
during a single data collection run.
5)
While the subject is performing push-ups, the other lab partners should pay special
attention to the both the push-up motion & the force graphs. Partners: Notice
anything interesting during the push-ups?
6)
Observe the force-time graphs and record your observations: (Be sure to note how
the force of your arms compare, how the total combined force varies throughout
the motion, etc…). You can number your observations.
7)
Print the graph (be sure to put your name on the print-out).
8)
Label the “up” phase of the motion and “down” phase. Ask your partners for
assistance if necessary.
9)
Now click on “collect” and perform a “cheater” push-up (back straight, balance on
your knees, go down slowly until your elbows are bent at ~90o angle then back up)
10)
Observe the graphs and record your observations: (make similar comparisons as
above).
11)
Print the graph (be sure to put your name on the print-out).
12)
Try other types of push-ups and observe the force-time graphs. You don’t need to
print them out.
13)
{Note: All members of the group should perform this activity. Don’t grumble, it’s a
great mini-work-out…}
File Name: \ph101\Ph101_Lab6-Jumping-Impulse.doc
Ph101: Fundamentals of Physics Laboratory
Instructor: Tony Zable
3
Part 2: Jumping and Impulse:
In this series of experiments, you will perform a series of jumps on the force plate and
compare the forces involved for different types of jumps. Calculated and measured values
of impulse for each jump will be compared and % error calculated. {Warning: All steps
should be completed by a single individual before deleting the graph and starting over.}
1)
Since only one force plate is needed per group, break-up into smaller workgroups
(3 or 4 per group).
2)
Remove one of the force plates from the LabPro interface.
3)
Set the force plate to 3500 N
4)
Zero force plate (with nothing on it!)
5)
Stand on the force plate and record your weight (in newtons)
6)
Zero the force plate with you on it
7)
Crouch into a pre-jump position (while still on the force plate)
8)
Click on “Collect” while still crouching on the force plate
9)
After “Collect” is selected, perform the following maneuvers:
a. Jump directly upward and land back down onto the force plate. {The force
plate reading should disappear until you land on the plate.}
b. Pause briefly, then jump quickly and land back down onto the force plate.
c. Pause briefly, then jump slowly and land back down onto the force plate.
10)
You should be able to perform all of the actions during a single data collection run.
11)
Print the graph.
12)
Observe the force-time graph for the various jumps. Record your observations:
13)
For each of the jumps on the graph, label the motions curves correspond to.
Calculating Impulse from the Hang Time and Launch velocity:
14)
Use the mouse to click-and-drag will measure the hang time (i.e. the time interval
when student is off the force plate). The “dx” value at the bottom left corner of the
graph is the value of time high-lighted (your hang time).
15)
Record the hang time in table 1.
16)
Calculate your launch velocity for this jump using the hang time and record this
value in table 1. {Do you remember last week’s Nerf gun lab? Use vlaunch = ½ g.t}
17)
Determine your mass in kilograms from the weight measurement. Record this
value in table 1.
18)
Use the launch velocity to calculate your momentum just after your feet leave the
force plate and record this value in table 1.
File Name: \ph101\Ph101_Lab6-Jumping-Impulse.doc
Ph101: Fundamentals of Physics Laboratory
Instructor: Tony Zable
19)
4
Determine the launch impulse from your change in momentum during the launch
phase of your jump, using the momentum value in step 13.
Impulse (calculated) = (momentum)
Measuring Impulse from the Average Force and Time:
20)
Use the mouse to highlight the region of the force-time graph where you pushed off
from the force plate. Be sure to only select the region of data where you are still in
contact with the plate. (This will correspond to roughly the region of the force graph
that is greater than zero and just to the left of the jump)
21)
Click on the “Statistics” button from the menu bar on the top of the screen. This
button will calculate the average (mean) force over the region of the graph selected
22)
The “dx” value at the bottom left corner of the graph is the value of time highlighted (time the launch force was applied).
23)
Record the average force value of the area in table 2. This is the average force for
the push-off phase of your jump.
24)
To determine the measured impulse, multiply the average force value (from step
18) by the time (step 17)
Impulse (measured) = (Average Force) x Time
25)
Repeat steps 4-19 and try to get even more impressive hang times. Record all of
your values in the table below.
26)
{Note: All members of the group should perform this exercise and record his/her
own data.}
File Name: \ph101\Ph101_Lab6-Jumping-Impulse.doc
Ph101: Fundamentals of Physics Laboratory
Instructor: Tony Zable
5
Table 1. Calculating Impulse from Launch Velocity
Weight (N) =
Trial #
(change in momentum)
Mass (kg) =
Hang-time
(sec)
Launch
Velocity
(m/s)
Launch
Momentum
(kg.m/s)
Calculated
Impulse
(
)
Pre-crouch
Quick jump
Slow jump
Table 2. Measuring Impulse from Average Launch Force & Time
Trial #
Average
Force (N)
Time
(s)
Measured
Impulse
(F.t)
Pre-crouch
Quick jump
Slow jump
File Name: \ph101\Ph101_Lab6-Jumping-Impulse.doc
Calculated
Impulse
(
)
%
Error
Ph101: Fundamentals of Physics Laboratory
Instructor: Tony Zable
6
Summary Questions:
1) How much easier are “cheater” push-ups than normal ones? What is the basis for
your answer?
2) How does your push-up technique affect the applied force you exert when
performing push-ups? Explain in terms of applied force and motion.
3) What have you learned from your answer in (2) about how an individual should
perform push-ups?
4) In this experiment, do your calculated and measured values for impulse agree with
each other? Why or why not?
5) What are the units of impulse for your calculated and measured values? Are they
consistent with each other? Why or why not?
6) What might you do in order to maximize your hang-time during a jump? Try some
different approaches. Describe them.
File Name: \ph101\Ph101_Lab6-Jumping-Impulse.doc