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Homework Statement
An small doll is attached to an elastic cord which is suspended from a support as shown. An accelerometer is attached to the doll. The doll is allowed to fall and produces the graph shown. When the recording system is started the doll is stationary at the upper support and released approximately 5 sec later.[/B]
https://bb9.waukesha.k12.wi.us/bbcswebdav/pid-692076-dt-content-rid-2436426_1/xid-2436426_1
-The graph can also be found here: https://docs.google.com/document/d/1k0hjDdK4kqCp05ZFZt-aNn_T9xFhHaF-teH7M08jH7Q/edit?usp=sharing
A) At what time on the graph does the elastic cord begin to pull upward on the doll?
B) At what time on the graph is the doll at its lowest point?
D) Calculate the approximate impulse given to the doll during the first falling portion of the jump.
E) How would each of the following be different if a larger mass doll were attached to the same elastic cord?
- maximum speed of the doll before the cord acted
- total impulse given to the doll during the falling portion of the fall
- width of the pulse on the graph for the first oscillation of the doll.
Homework Equations
I=mvf-mvi[/B]
The Attempt at a Solution
A. The cord pulls upwards on the doll beginning at 8 seconds, because the acceleration shifts from negative to positive
B. The doll is at its lowest point at 10 seconds because this is when the positive acceleration it experiences from the bungee cord reaches its peak.
D. I am not entirely sure how to find the impulse without knowing the mass of the doll, but I would assume that I could find it by finding its change in velocity, and multiply each of these by the constant m. However, I don't know how to find the initial velocity of the doll before the jump, since it appears that its negative acceleration was not a constant 9.8m/s^2 between 5-8 sec. Does this meant that the cord was working on the doll before 8 seconds and my answer to A is wrong as well? I am assuming that the final velocity can be found by estimating the area under the curve from about 8 to 11 seconds, which would be
vf=area under max height (26m/s^2)
0.5*26*3s=39m/s
But how do I find the initial velocity? Is my approach and my thought process entirely wrong?
E.
-The maximum speed of a larger mass for the doll would not be any different from a smaller mass because acceleration due to gravity is constant regardless of the mass of an object
-The total impulse would be higher because impulse is equal to mvf-mvi
-I'm not entirely sure what this is asking, but I would guess that it is referring to the time between each positive acceleration due to the bungee jump and the negative acceleration due to gravity during freefall, which would be unaffected by the mass of the object in question.
Ultimately, I am only fairly confident in my answer for B. I am especially thrown off by the fact that the acceleration of the doll does not shift immediately from negative to positive and vice versa. Does this mean that the cord begins pulling on the doll when the acceleration begins to increase even when it is still negative, or when the acceleration shifts from negative to positive?
[/B]
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