Planking Physics Style
Instructor Notes
Introduction
In this lab we will be utilizing two force plates and a 2x6x8 plank. Make sure both
force plates are set on the -850 N to 3500N setting (switch on side) before you open
logger pro. Once in logger pro open the file called “planking.cmbl”.
1. Place the force plates 1.5m apart. Zero the force plates. Stand on one of the
force plates and get your weight (do this for each team member so you can use
the later). Record the values below.
Sample data: 950 N
Center the plank on the force plates. Make sure there are equal amounts of the plank
extending beyond the force plates. Once centered on the plates record the readings
on each of the force plates (FP) using on the diagram below.
Sample data: FP1 – 20 N;
FP2 – 20N
Plank
FP 1
FP2
Draw the force diagram for the plank.
F GP
F N2
Sample data: F N1 = 20N; F GP = 40N;
F N2 = 20N
F N1
What is the net force on the plank?
2. If you were to stand on one of the force plates, predict what the force plate
readings would be?
Sample Data: FP1 = 925 N;
FP2 = 60 N
We suggest they stand on FP1.
Now stand on one of the plates and record the value you are getting for both
force plates. Stand with your feet pointing along the board.
See above.
Draw the force diagram for the plank now.
F N1
F Gs
F GP
What is the net force on the plank?
F N2
Sample data: - 5 N (should be zero)
Force
3. Predict what would happen if you walk along the board from one force plate to
the other. Remember to predict the force for both force probes.
FP2
Do it! Record your actual data on the above graph.
time
FP1
Sample data: see above, where the student is starting on FP1.
Does the actual data match your predicted data? Why or why not.
Pretty much. As the student walks away from FP1, we expect the force to decrease
and roughly linearly.
4. Measure the distance between the “midpoints” of the two force plates. Using
masking tape, mark the positions that are ¼, ½, and ¾ the way from one force
plate midpoint to the other force plate midpoint.
Sample data: we used 1.6 m between the “midpoints”.
Using what you saw in 3, predict what the force plate readings should be at ¼, ½,
and ¾ the way from the force plate you stand on to the other force plate.
Sample predictions: ¼ or .4 m from FP1 – FP1 = 750 N and
½ or .8 m , from FP1 – FP1 = 500 N and
FP2 = 250 N
FP2 = 500 N
3/4 or 1.2 m, from FP1 – PF1 = 250 N and FP2 = 750 N
Now actually walk from one force plate to another stopping at ¼, ½, and ¾ the
distance between the plates. Record your readings at each of these positions.
Sample data:
¼ or .4 m from FP1 - FP1 = 756 N and
FP2 = 256 N
½ or .8 , from FP1 -
FP1 = 500 N and
FP2 = 510 N
¾ or 1.2 m from FP1 – PF1 = 225 N and
FP2 = 775 N
5. Now draw a force diagram for each of these positions.
a. at ¼ the way from the starting force plate to the ending force plate.
F N1
What is the net force?
F Gs
F GP
F N2
Sample data: the net force was 22 N (should be zero).
b. at ½ the way from the starting force plate to the ending force plate.
F N1
What is the net force?
F Gs F GP
F N2
Sample data: the net force was 20 N (should be zero)
c. at ¾ the way from the starting force plate to the ending force plate.
F N1
F GP
What is the net force?
F Gs F N2
Sample data: 10 N (should be zero)
6. What general rule is implied about the net force in these cases?
That the net force should be zero for a static case no matter where the student is
standing.
7. What about torque? On the force diagram from 2, predict what would be net
torque of the system?
Redraw the force diagram of you standing on the plank at the midpoint of one of
the force plate as in 2.
pivot
F N1
F Gs
F GP
F N2
Consider placing the pivot at FP1.
Show your calculations for finding the net torque for this case.
Using sample data and placing the pivot at FP1, the net torque would be:
F N2 Δx – F GS Δx’ - F GP Δx” = 0 ⇒ (256 N)(1.6 m) – (756 N) (.4 m) - (256 N)(.8 m) = 0
409.6 Nm – 302.4 Nm – 204.8 Nm = -97.6 Nm
8. What would be the net torque for the plank with you standing on the plank at ½
the distance to the other force plate?
Redraw the force diagram.
Show your calculations for finding the net torque for this case.
9. What would be the net torque for the plank with you standing on the plank at
3/4 the distance to the other force plate?
Redraw the force diagram.
Show your calculations for finding the net torque for this case.
10. What general rule is implied about the net torque in these cases?