Force and motion
Force and Motion
Marker
Force pushes the lever arm down. More force pushes the lever arm down further. Force stretches the rubber band. The more the right side of the lever arm is pushed down, the more the left side of the lever arm comes up, and the more the rubber band is stretched.
Experiment D
The goal of each trial in Experiment D of Lesson 9 is to put the same force on the rubber band by clips at 8” from the fixed pivot as was done by clips at 4” from the fixed pivot. Then the number of clips and the distance at each position is compared.
This procedure is repeated for five trials. For each trial the number of clips at 4” from the fixed pivot is increased. This means that the lever arm is pushed down more and the rubber band is stretched more. For each trial, the marker must be reset for the clips at 4”, but kept in that position to find out how many clips are needed at 8” from the pivot to lower the lever arm to that point.
The result of each trial is a separate equation: We can say that 2 clips at 8” from the pivot put the same force on the rubber band as 4 clips at 4” from the pivot only if the rubber band is stretched exactly the same amount. For the rubber band to be stretched exactly the same amount, the lever must be depressed to exactly the same point. Thus the marker must be carefully placed when the clips are at 4” and it must not be moved when clips are removed from 4” and placed at 8”.
If the marker is not set carefully at the beginning of each trial, and kept in the same position for the rest of that trial, then the number of paper clips recorded will be wrong.
Experiment E
Whereas each trial in Experiment D changed the force at 4" from the fixed pivot (and changed the placement of the marker), trials 2, 3, 4, and 5 of Experiment E all compare the number of clips at varying distances from the fixed pivot that are needed to depress the lever to exactly the same point as 12 clips at 2" from the fixed pivot, which was trial 1.
Thus, in Experiment E it is crucial that the placement of the marker in trial 1 be made very carefully. This is done by making a horizontal line on the end of the lever arm, making a line on the marker, and placing the marker so the two lines are even. Then, the marker must not be moved throughout the remainder of the trials.