WEBVTT physics/ap-physics-1-2/fullerton
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Hi everyone and welcome back to Educator.com.
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In this mini-lesson, we are going to go through the first page of the kinematics defining motion worksheet from the APlusPhysics site, which you can find the link for down below.
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Let us dive right in.
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A student on her way to school walks four blocks east, three blocks north, and then another four blocks east.
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Compared to the distance she walks, the magnitude of her displacement from home to school is...?
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The distance she walks is how far she has traveled, so 4 + 3 + 4 were 11 blocks.
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The displacement, however, is the straight line distance from where you start to where you finish, so that would be the displacement.
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A displacement is obviously less than the distance that she walked, so compared to the distance she walks, the magnitude of her displacement is less.
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Number 2 -- A motorboat which has a speed of 5 m/s in still water, is headed east as it crosses a river flowing south at 3.3 m/s.
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What is the magnitude of the boat's resultant velocity with respect to the starting point?
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Well, if it is going 5 m/s east and it is also crossing a river that is flowing at 3.3 m/s south, the boat's resultant velocity with respect to the starting point -- Well to do that, I am just going to add these two vectors.
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They are already lined up tip to tail, so I can draw a line from the starting point of the first to the ending point of the last.
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Now to figure out that magnitude, I am going to use the Pythagorean Theorem, so to figure out that magnitude, that is going to be the square root of 5 m/s² + 3.3 m/s²...
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...which is going to be 5² + 3.3² and the square root of that gives me about 6 m/s -- Answer 3.
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Number 3 -- A speedometer in a car does not measure a car's velocity because velocity is a...?
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Velocity is a vector quantity, so we can get rid of 3 and 4 because velocity is not a scalar and because it is a vector, it has a direction associated with it.
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Velocity is a vector and has a direction associated with it, a speedometer in a car does not measure that because the speedometer does not tell you the direction, so best answer here is Number 1.
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Number 4 -- A person observes a fireworks display from a safe distance of 0.75 km, so the distance is going to be 0.75 km, which is 750 m.
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Assuming that sound travels at 340 m/s in air -- so the velocity or speed of 340 m/s -- what is the time between the person seeing and hearing a fireworks explosion?
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Well, our relationship there is our velocity is going to be the distance traveled divided by the amount of time, therefore, time is going to be our distance traveled divided by the velocity or 750 m/340 m/s.
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If I plug that into my calculator, I come up with right around 2.21 m/m or 1/1/seconds, which will leave us with units of seconds -- Answer 2.
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Number 5 -- On the surface of Earth, a space craft has a mass of 2 × 10^4 kg.
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What is the mass of the spacecraft at a distance of 1 earth radius above Earth's surface?
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Now, that is a trick question there. Remember mass is how much stuff something is made up of.
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In this case, that is not going to change even though its position is changing. It still has the same mass.
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So our answer is Number 2, 2 × 10^4 kg.
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Number 6 -- An airplane flies with a velocity of 750 km/h, 30 degrees south of East.
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What is the magnitude of the eastward component of the plane's velocity?
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Let us draw ourselves a little axis here and it is going to fly 750 km/h at an angle of 30 degrees south of East.
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If I go south of East, it is probably going to be something right around that and its magnitude is 750 km/h and our angle here is 30 degrees.
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What is the magnitude of the eastward component of the plane's velocity?
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If we want the eastward component, well that means we are going to want this component, the component of our 750 km/h vector.
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The way we get that -- the magnitude of the eastward component of the plane's velocity, which I am going to call the x-component of the velocity is just going to be our total velocity times the cos(30 degrees) or 750 km/h × cos(30 degrees)...
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...which implies then that our velocity in the east direction is 750 × cos(30) or about 650 km/h -- Answer 2.
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Number 7 -- One car travels 40 m due East in 5 s and a second car travels 64 m due West in 8 s.
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During their periods of travel, the cars definitely had the same...?
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Well, let us take a look -- car 1 travels 40 m due East in 5 s, so that means its velocity is going to be its distance traveled -- let us call that Δx -- divided by the time it took or 40 m/5 s = 8 m/s and the velocity will be East.
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Our second car has a velocity of Δx/t, but it is going to travel 64 m due west, so let us call that -64 m/8s, which is going to be -8 m/s East or 8 m/s west.
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They have the same speed, but different velocities because they are going in different directions, so our correct answer -- they must have the same average speed.
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Let us check out one more -- State the two general characteristics that are used to define a vector quantity.
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Vectors are made up of two things -- they must have a magnitude or a size and a vector also has a direction, so magnitude and direction gives you a vector.
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That covers the first page of our kinematics defining motion worksheet.
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We will talk to you next time. Make it a great day!