course Phy 121 G^Szh\assignment #008
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22:36:33 QUESTION FROM STUDENT--Please define the differnece between Fnet and Force. See if you can answer this question.
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RESPONSE --> Fnet is the net force on mass or the total of all forces acting on the object. Force is simply the push or pull that causes an object to accelerate. For example, the gravitational force on an object is equal to the weight.
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22:37:41 ** Net force is the sum of all forces acting on an object. If you're pushing your car you are exerting a force, friction is opposing you, and the let force is the sum of the two (noting that one is positive, the other negative so you end up with net force less than the force you are exerting). Your heart rate responds to the force you are exerting and the speed with which the car is moving; the accel of the car depends on the net force. **
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RESPONSE --> Ok
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22:44:55 In terms of the equations of motion why do we expect that a * `ds is proportional to the change in v^2, and why do we then expect that the change in v^2 is proportional to Fnet `ds?
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RESPONSE --> The equations of motion illustrate that acceleration from rest over a known distance increases v^2 by 2(a)(`ds), and from another standpoint, to increase 0.5v^2 by (a)(`ds). Since net force equals (m)(a), we know that (Fnet)(`ds) = (m)(a)(`ds). So, `ds is proportional to v^2, Fnet is proportional to `ds, therefore Fnet is proportional to change in v^2 and directly proportional to acceleration..
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22:45:43 ** It's very important in physics to be able to think in terms of proportionality. To say that y is proportional to x is to say that for some k, y = k x. That is, y is a constant multiple of x. To say that a * `ds is proportional to the change in v^2 is to say that for some k, a * `ds = k * ( change in v^2)--i.e., that a * `ds is a constant multiple of the change in v^2. In terms of the equations of motion, we know that vf^2 = v0^2 + 2 a `ds so a `ds = 1/2 (vf^2 - v0^2), which is 1/2 the change in v^2. So a `ds is a constant multiple (1/2) of the change in v^2. Formally we have a `ds = k ( change in v^2) for k = 1/2. Now since Fnet = m a we see that Fnet is proportional to a for a given mass m, and it follows that Fnet `ds = k * change in v^2, for the appropriate k (specifically for k = mass / 2. **
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RESPONSE --> I had a little trouble explaining this concept, but I understand it.
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22:47:08 How do our experimental results confirm or refute this hypothesis?
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RESPONSE --> If the results follow and support the hypothesis which was formed before the experiment, then the hypothesis was confirmed. If, however, the results do not support the hypothesis but actually are not as expected, the hypothesis is refuted. A graph of our results should be a sufficient illustration to disprove or support the hypothesis.
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22:48:56 ** We didn't actually do this part of the experiment, but on a ramp with fixed slope a `ds is simply proportional to `ds. When we measured `dt for different distances from rest down the same ramp, we were then able to determine the average and final velocities. The change in v^2 for each timing would be from 0 to vf^2. The change would therefore be just vf^2. If a `ds is proportional to the change in vf^2 then a graph of vf^2 vs. a `ds should be linear. Since a is constant we don't even need to determine it--a graph of vf^2 vs. `ds would be linear. This would confirm the hypothesis. **
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RESPONSE --> I did not know we were to describe the experiment that was explained in the notes. I simply described how a hypothesis is supported or disproved by an experiment. NOTE: If a `ds is proportional to the change in vf^2 then a graph of vf^2 vs. a `ds should be linear. Since a is constant we don't even need to determine it--a graph of vf^2 vs. `ds would be linear. This would confirm the hypothesis.
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