course Phy 201
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09:43:58 Physics video clip 01: A ball rolls down a straight inclined ramp. It is the velocity the ball constant? Is the velocity increasing? Is the velocity decreasing?
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RESPONSE --> The velocity is increasing
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09:44:09 ** It appears obvious, from common experience and from direct observation, that the velocity of the ball increases. A graph of position vs. clock time would be increasing, indicating that the ball is moving forward. Since the velocity increases the position increases at an increasing rate, so the graph increases at an increasing rate. **
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RESPONSE --> Ok
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09:44:26 If the ball had a speedometer we could tell. What could we measure to determine whether the velocity of the ball is increase or decreasing?
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RESPONSE --> The position of the ball in regards to time.
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09:44:37 ** STUDENT RESPONSE: By measuring distance and time we could calculate velocity. INSTRUCTOR COMMENTS: The ball could be speeding up or slowing down--all you could get from the calculation you suggest is the average velocity. You could measure the time to travel the first half and the time to travel the second half of the ramp; if the latter is less then we would tend to confirm increasing velocity (though those are still average velocities and we wouldn't get certain proof that the velocity was always increasing). You would need at least two velocities to tell whether velocity is increasing or decreasing. So you would need two sets of distance and time measurements. **
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RESPONSE --> Ok
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09:45:24 What is the shape of the velocity vs. clock time graph for the motion of the ball?
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RESPONSE --> The shape of the velocity is a curve
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09:45:32 ** If the ramp has an increasing slope, the velocity would increase at an increasing rate and the graph would curve upward, increasing at an increasing rate. If the ramp has a decreasing slope, like a hill that gradually levels off, the graph would be increasing but at a decreasing rate. On a straight incline it turns out that the graph would be linear, increasing at a constant rate, though you aren't expected to know this at this point. All of these answers assume an absence of significant frictional forces such as air resistance. **
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RESPONSE --> Ok
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09:46:23 A ball rolls down ramp which curves upward at the starting end and otherwise rests on a level table. What is the shape of the velocity vs. clock time graph for the motion of the ball?
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RESPONSE --> This curve would be increasing at a decreasing rate once the ball reaches the flat plane
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09:46:39 ** While on the curved end the ball will be speeding up, and the graph will therefore rise. By the time the ball gets to the level part the velocity will no longer be increasing and the graph will level off; because of friction the graph will actually decrease a bit, along a straight line. As long as the ball is on the ramp the graph will continue on this line until it reaches zero, indicating that the ball eventually stops. In the ideal frictionless situation on an infinite ramp the line just remains level forever. **
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RESPONSE --> Ok
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09:51:14 For the ball on the straight incline, we would certainly agree that the ball's velocity is increasing. Is the velocity increasing at a constant, an increasing, or a decreasing rate? What does the graph of velocity vs. clock time look like?
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RESPONSE --> The velocity would be increasing at a decreating rate.
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09:51:24 ** It turns out that on a straight incline the velocity increases at a constant rate, so the graph is a straight line which increases from left to right. Note for future reference that a ball on a constant incline will tend to have a straight-line v vs. t graph; if the ball was on a curved ramp its velocity vs. clock time graph would not be straight, but would deviate from straightness depending on the nature of the curvature (e.g., slope decreasing at increasing rate implies v vs. t graph increasing at increasing rate).**
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RESPONSE --> ok
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x}{s}쯩Ҥ assignment #001 呌騶~Q Physics I 06-06-2006
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09:55:50 Briefly state what you think velocity is and how you think it is an example of a rate.
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RESPONSE --> Velocity describes speed and motion of a body. It describes how fast and in what direction a body is moving. This is an example of rate because velocity is the rate of change of position.
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09:56:01 ** A rate is a change in something divided by a change in something else. This question concerns velocity, which is the rate of change of position: change in position divided by change in clock time. **
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RESPONSE --> Ok
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10:00:24 Given average speed and time interval how do you find distance moved?
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RESPONSE --> Given average speed and time interval, you calculate distance moved by multiplying the average speed and the time interval.
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10:00:33 ** You multiply average speed * time interval to find distance moved. For example, 50 miles / hour * 3 hours = 150 miles. **
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RESPONSE --> Correct
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10:01:26 Given average speed and distance moved how do you find the corresponding time interval?
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RESPONSE --> Given average speed and distance moved you calculate the time interval by dividing the average speed into the distance moved.
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10:01:41 ** time interval = distance / average speed. For example if we travel 100 miles at 50 mph it takes 2 hours--we divide the distance by the speed. In symbols, if `ds = vAve * `dt then `dt = `ds/vAve. Also note that (cm/s ) / s = cm/s^2, not sec, whereas cm / (cm/s) = cm * s / cm = s, as appropriate in a calculation of `dt. **
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RESPONSE --> Correct
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10:02:38 Given time interval and distance moved how do you get average speed?
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RESPONSE --> Given time interval and distance moved, by dividing the distance moved in that interval by the time taken you can caluculate the average speed.
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10:02:43 ** Average speed = distance / change in clock time. This is the definition of average speed. For example if we travel 300 miles in 5 hours we have been traveling at an average speed of 300 miles / 5 hours = 60 miles / hour. **
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RESPONSE --> Ok
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10:03:36 You should have written up Video Experiment 2 as directed and submitted it. If you have done so within a couple of days of submitting this query then you need not answer any question here that was answered on your writeup. This is a general principle for experiments and can be applied on all queries. Give your slopes and your rates of velocity change as rate vs. slope ordered pairs according to the y vs. x convention (slope first, rate second), and specify the slope of the straight line you got at the end.
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RESPONSE --> Ok
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10:03:47 ** It's very important to express all quantities in terms of the correct units. Slope, being rise / run in units of cm / cm, ends up unitless (the cm 'cancel'). Velocities are expressed in cm / sec. Rates of velocity change are expressed in (cm / sec) / sec or cm / sec^2. Slopes are expressed in units of rise / units of run, in this case giving just cm / sec^2. **
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RESPONSE --> Ok
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10:07:44 Explain briefly how you calculated your slopes.
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RESPONSE --> I calculated my slopes as rise/run, defined as the change in the y-coordinates divided by the change in the x-coordinates. I named my first point (x1, y1) and our second point (x2, y2), with formula m = (y2-y1)/(x2-x1). If I foudn the slope of the lines is 2, on graph that the line moves up two spaces for every space that it moves to the right:
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10:07:54 ** slopes are caclulated by using the basic rise over run formula. For instance one student reports that when the paper stack was 1.26 cm tall and the desk was 44 cm long the slope was 1.26/44 which is equivalent to 0.0286 **
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RESPONSE --> Correct
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10:11:17 Explain briefly how you determined your rates of velocity change.
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RESPONSE --> The rate of change of velocity with respect to time, or the change in velocity over a given period of time is called acceleration. determining the acceleration of an object require that the mass and the net force are known. If mass (m) and net force (Fnet) are known, then the acceleration is determined by use of the equation a= Fnet/m.
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10:11:26 ** rates of velocity change are calculated by dividing change in velocity by change in clock time. From your timing you got average velocity. Since accel is uniform and initial velocity is zero, final velocity must be double average velocity. Change in velocity is obtained by subtracting init vel from final vel. Change in clock time is the time required to accelerate down the incline. Note units: when you divide change in velocity by change in clock time you are dividing cm/sec by sec, giving cm / sec * 1/sec = cm/sec^2. **
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RESPONSE --> Correct
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10:13:08 What were the units of the slope of your straight line on the graph of rate of velocity change vs. slope? Hint: The slope of a ramp has no units; what were the units of the rise between two points? (you won't be timed on this one, so don't worry about taking a few minutes if you need it).
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RESPONSE --> M was the units used in this particular graph for the exercise.
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10:14:18 ** Rise is change in the rate of velocity change, in cm/sec^2. Ramp slope is unitless. So graph slope is in units of cm/sec^2. The ideal value would be very close to the acceleration of gravity, which is 980 cm/sec/sec. Typical range when timing with the pendulum is from 600 to 1500 cm/sec/sec. Using very accurate timing (electronic equipment in the lab) with these clips we still end up with an error of about 8% on the high side, which shouldn't happen. The experiment done with accurate timing should be quite accurate. **
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RESPONSE --> I misunderstood the question, now I see that the units are cm/sec^2. I also now know that the ramp slope does not have any units.
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