I will continue to add captions. All should be added by 10:00PM, 9/23/18.
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- PhyFl18SS1.6.1 The resultant vector needs to be in a different color. A proper description of a vector requires magnitude, units,angle, quadrant.
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- PhyFl18SS1.6.2 All angles are measured from the horizontal (with some exceptions) If it is a map view, this is how you describe the quadrant that you are in.
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- PhyFl18SS1.6.3 A displacement vector (or any vector for that matter always starts at the begining and is a straight arrow to the end. The path that the object takes does not matter. ∆s in blue is a displacement vector. It only cares about the beginning position and the final position. The displacement vector of my life would start in Ponca City Hospital and probably end where I am teaching at NHS. It doesn’t care where I have visited and lived in my life. It only cares about where I was born and where I died. It would be a 124 milelong straight displacement vector.
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- PhyFl18SS1.6.4 A vector at an angle is the hypotenuse of a right triangle. So if you are going at 4 yds/sec at 45° N of E, you represent that with an arrow that has a length of 4 yds/sec (at whatever scale you assigned that velocity vector), but you are also going 2.8yds/sec east and 2.8 yards per second north. You are actually going all three velocities at once. Weird, huh? Either we do it this way with 2D motion or you learn how to plot everything in 3D (∆x,∆y, and ∆t).
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- PhyFl18SS1.6.5 We went out to the courtyard and did a couple of pinwheels. From this exercise, you should start to get a feel of circular world vs. linear world. We all went the same omega (angular velocity), but we went different 2D linear velocities.
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- PhyFl18SS1.6.6 Still on the courtyard pinwheels. If we say it took us 10 seconds, then we had an angular velocity of 2π/4 radians per 10 seconds (=π/20 radians per second)
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- PhyFl18SS1.6.7 So far we have talked about displacement and velocity being vectors (magnitude and direction) and time is a scalar (no direction). What about baby omega? Is it a vector or a scalar?
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- PhyFl18SS1.6.8 It turns out that omega (angular velocity) IS a vector. But what about it’s direction. Since the object is rotating (or at least going in circles) how do you represent this circular motion with a straight arrow? You gotta go third dimension bro. So you represent the omega with an arrow coming out of the page for counter clockwise rotation (CCW). You represent an arrow coming out of the page with a dot and a circle around it (sometimes just a dot)
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- PhyFl18SS1.6.9 You represent clockwise rotation (CW) with an arrow going INTO the page. An arrow into the page looks like the arrows tail feathers left an impression on the page (like an “x”). The arrow is along the axis of rotation.
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- PhyFl18SS1.6.10 Circular world vs. linear world. For wevery relationship (think equation) in linear world, there is a corresponding equation in circular world.
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- PhyFl18SS1.6.11 Definition of a radian. The question came up . . . “Why don’t radians have any units?” Because they are a ratio of the arc length of a circle (∆s) and the radius of that d
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- PhyFl18SS1.6.12 From the three base equations (on the left) we derived a very useful, very important equation in Physics which bridges linear world to circular world. You will have to know this derivation for TEST 1B. .
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- PhyFl18SS1.6.13 Here’s the good old what is the omega of the second, minute, and hour hand which you will find in every Physics textbook.
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- PhyFl18SS1.6.14 converting radians per second to rpm (revolutions per minute) Like what your tachometer measures on the dashboard of your car.
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- PhyFl18SS1.6.15 From that bridge equation we derived, here is another useful minibridge equation relating linear velocity to angular velocity.
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- PhyFl18SS1.6.16 I spun the wheelchair tire in front of the classroom and you timed it. We got 5.7 radians/sec. when multiplied by the radius of the tire we see that the outside of the tire is rotating at 1.6m/s
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- PhyFl18SS1.6.17 The period (Tp) is the time it takes an object to complete one revolution. Like the period of the earth is 24 hours or the period of the earth around the sun is 365 days. Period can also mean the time it takes a pendulum to come back to its original position. Period is the inverse of frequency.
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- PhyFl18SS1.6.18 Here we were looking at the period of the spinning wheel in front of the class.
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- PhyFl18SS1.6.19 Period talk.
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- PhyFl18SS1.6.20 The wheel spinning up front. Using the brige equation to determine its velocity.
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- PhyFl18SS1.6.21 Here we are trying to figure out how many miles per hour the spinning wheel would be going it it were attached to a bicycle.
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- PhyFl18SS1.6.22 The point I was trying to make with this discussion was that the moon is moving at 2300mph, but appears to us to be moving hardly at all. The reason for this is the very long radius. Since omega = v/r. Since r is soooo big, it wipes out the huge v. the omega is what we perceive as we stand below an object rotating above our head. A jet may be going at 600mph, but because its radius (from us) is, say, 50,000 ft, it doesn’t have a very big omega so it doesn’t seem like it is going that fast to us. There are probably those out there who would call its speed of 600mph “fake news” because they themselves do not understand circular motion kinematics.
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- PhyFl18SS1.6.23 so when the radius is large compared to the velocity, the object appears to be going slow to us down below. This sounds like a good essay question for Test 1B.
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- PhyFl18SS1.6.24 THT1A.18
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- PhyFl18SS1.6.25 THT1A.18
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- PhyFl18SS1.6.26 If you see “SH” on your test from ym grading it means you should have used a ruler. It stands for “shaky hands”
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- PhyFl18SS1.6.27 If you see this on your test from my grading it means that I followed your mistake so you missed less than you would have if I was a computer and was grading your test.
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- PhyFl18SS1.6.28 It is much better the search the Facebook group for what your are looking for than the scroll scroll scroll.
Askey Physics 