WEBVTT physics/ap-physics-1-2/fullerton
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Hi everyone and welcome back to Educator.com.
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Today's mini-lesson is going to be on projectile motion as we work through page 1 of the APlusPhysics worksheet on projectile motion and you will find the link to that worksheet down below.
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Let us dive in with Number 1 -- A volleyball hit into the air has an initial speed of 10 m/s.
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Which vector best represents the angle above the horizontal that the ball should be hit to remain in the air for the greatest amount of time?
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Well, obviously, if you want it to remain in the air for the longest amount of time, you want it to have as much vertical velocity so it goes as high as possible, so the answer there must be 1 -- you want it being sent completely vertically.
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All of the energy being sent into its vertical motion, so it is in the air for the longest amount of time.
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Now following on from here, on the diagram, use a protractor and a ruler to draw a vector to represent the initial velocity of the projectile, given that it is launched at 250 m/s at an angle of 60 degrees.
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Begin the vector at (P) and use a scale of 1 cm = 50 m/s.
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If one centimeter is 50 m/s, our vector is 250 m/s long, our vector should be 5 cm long, so that should look something like this...
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...where our velocity here is 250 m/s and our angle there -- check with the protractor -- should be 60 degrees.
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We have a projectile fired from the ground again, same projectile, same velocity, and same angle.
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Determine the horizontal component of the initial velocity.
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Well, the horizontal component is going to be the x-component, which will be V × cosine of our angle (60 degrees) or 250 m/s × cos(60 degrees) and cos(60 degrees) is 1/2, so that is going to be 125 m/s.
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Number 4 -- Explain why the projectile has no acceleration in the horizontal direction.
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There is nothing to cause a force in the horizontal direction.
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If we are neglecting air resistance, there is nothing to change its velocity.
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Once it is going horizontally, gravity is only pulling down, it does not affect the horizontal velocity.
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So why is there no acceleration?
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I would write something like gravity only pulls down.
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Number 5 -- We have two stones, (A) and (B) and they are thrown horizontally from the top of a cliff.
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Stone (A) has an initial speed of 15 m/s and (B) has an initial speed of 30 m/s.
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Compared to the time it takes (A) to reach the ground, the time it takes (B) to reach the ground is...?
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For launching these from a cliff, there goes (A) and (B) has a greater initial velocity, but that has nothing to do with their vertical motion.
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Regardless of whether you drop an object, throw an object, shoot an object, as long as you have no initial vertical velocity and you have flat ground for them to land on, they will hit the ground at the same time.
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Similar to the famous thought experiment, where you drop a bullet from a gun and fire the gun horizontally at the same time, assuming that the ground is completely flat, the bullets hit the ground at the same time.
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Comparing the amount of time it takes -- the same.
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One more -- This diagram represents the path of a stunt car driven off a cliff, neglecting friction.
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Compared to the horizontal component of the car's velocity at (A), has some velocity at (A) horizontally, the horizontal component of the car's velocity at (B) is -- well we have hit that a couple of times already just on this page of the worksheet.
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The horizontal velocity is not going to change. Gravity only pulls down, so in this case, the horizontal components of its velocity is still going to be the same.
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That completes page 1 of the worksheet on projectile motion.
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Thanks so much for your time. Make it a great day!