course Phy 201 x????????^?????assignment #008r{?j??????????Physics I
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13:48:31 QUESTION FROM STUDENT--Please define the differnece between Fnet and Force. See if you can answer this question.
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RESPONSE --> My guess would be that Force is undescript and can mean any force acting on the observed object whereas Fnet is the total of all forces that are acting on the observed object.
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13:49:40 ** 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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14:14:24 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 --> We expect a * `ds to be proportional to change in v^2 because the definition of acceleration is change in velocity divided by time duration, so any change in velocity would directly affect acceleration, and since displacement is the product of average velocity and time duration - again average velocity being caluculated by using the change in velocities - directly relating change in velocity to displacement, therefore, a change in velocity would directly affect displacement. Since both acceleration and displacement are directly affected by a change in velocity, changing the velocity would also directly affect the product of acceleration and displacement. The change in velocity also then is proportional to the product of Fnet and displacement since we already know that displacment is directly affected by a change in velocity due to its definition.
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14:20:07 ** 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 --> ok, so using proportionality, we should think that, for example, A = k B with A and B being the proportional equations and k being the constant.
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14:24:52 How do our experimental results confirm or refute this hypothesis?
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RESPONSE --> The experimental results confirm this hypothesis because a graph of the results of 1/2 v^2 vs. a * `ds and a graph of v^2 vs. Fnet * `ds both give straight lines.
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14:25:52 07-14-2006 14:25:52 ** 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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NOTES -------> ok
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