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# Horizontally Launched Projectiles

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Notes from Ms. Parker's class on October 7, 2011.

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• A few years ago some researchers in the U.S. went to elementary schools, junior and senior high schools, and universities and asked them to look at the following:&quot;Ignoring air resistance, which of the following correctly shows what an object would do if it rolled off a cliff?&quot; Figure 1The breakdown of answers they got was almost exactly the same at all ages.About 60% said number 1 was correct. The object will stop in midair, and then start to fall straight down. Because this is what the Coyote always did in cartoons (and because some people actually referred to it in their explanation of why they chose this answer), the researchers called it the Wile E. Coyote Effect. About 25% said number 2 was correct. The object will move forward at first, but will eventually just fall straight down.Only about 15% answered number 3. The object will continue to move forwards the entire time it is falling.
• ### Horizontally Launched Projectiles

1. 1. Good Morning!<br />Today we will:<br /><ul><li> show your completed homework
2. 2. talk about projectile motion
3. 3. begin working on PTG 2.4 (finish for homework)</li></ul>Do BEFORE THE BELL:<br /><ul><li> get out your homework
4. 4. get out your spiral and something to write with</li></li></ul><li>Quick Question<br />Does horizontal velocity influence vertical velocity?<br />
5. 5. Projectile Motion<br />It’s as easy as rolling off a cliff…<br />
6. 6. Make a Prediction<br />If this red ball had a net force to the right applied to it and rolled off the cliff at a constant speed, what would its path look like?<br />
7. 7. Predict<br />Which path will the ball follow?<br />A<br />B<br />C<br />
8. 8. Prediction<br />Determine which path you believe the ball will follow AND WHY. Write your prediction on the back of your homework – we will be adding to this at the end of this lesson, so keep it neat! <br />
9. 9. A<br />B<br />C<br />
10. 10. A few years ago, researchers went to elementary, middle, and high schools as well as universities and showed students this image and asked them, <br />B<br />“Ignoring air resistance, which of the following correctly shows what an object would do if it rolled off a cliff?<br />
11. 11. The Results<br />The breakdown of answers they got was almost exactly the same at all ages.<br />About 60% said letter A was correct. The object will stop in midair, and then start to fall straight down. because some people referred to the coyote in cartoons, the researchers called it the Wile E. Coyote Effect. <br />About 25% said letter B was correct. The object will move forward at first, but will eventually just fall straight down.<br />About 15% answered letter C. The object will continue to move forwards the entire time it is falling. <br />
12. 12. So What’s the Correct Answer?<br />I’m not going to tell you. We’ll revisit the question at the end of these notes. <br />
13. 13. Observations from the lab<br /><ul><li>Both coins fall the same amount vertically in the same amount of time.
14. 14. The projected coin also moved to the right; but it fell downward at exactly the same rate as the dropped coin.</li></ul>In your notes, sketch what the motion of these two coins must have looked like<br />
15. 15. Observations from the Lab<br />As we saw with the coin launcher, both coins – the one that fell straight down andthe one that was flung out horizontally - hit the ground at the same time.<br />So what effect did horizontal velocity have on the time it took (the downward motion) the coin to hit the ground?<br />The best conclusion we can make from this is that the horizontal motion of the coin does not affect downward motion of the coin.<br />
16. 16. Observations from the lab<br />Intuition will tell you that the horizontally launched object will “hang” in the air.<br />But…<br />YOUR INTUITION (at least in this case) IS WRONG!<br />Here’s a video (if you are looking at this on tbirdscience, you can click this link to go to the youtube video we watched)<br />
17. 17. Horizontal and Vertical Motion Don’t Know Each Other Exist!<br />The title of this slide says it all: horizontal and vertical motion are independent of each other – even when they are happening at the same time!<br />So…let’s get back to the question:<br />
18. 18. How does initial velocity affect the time it takes a horizontally- launched projectile to reach the ground?”<br />It Doesn’t!<br />What is the only factor (besides air resistance, which we are ignoring) that DOES affect the time it takes a horizontally-launched projectile to reach the ground?<br />How high off the ground the object is.<br />
19. 19. Revisit Our Thinking<br />Which path will the ball follow?<br />A<br />B<br />C<br />
20. 20. Motion of Every Object<br /><ul><li>The vertical position of a projectile constantly changes, though.
21. 21. Why?
22. 22. The constant force of gravity.
23. 23. As the projectile rises, it slows down. As it falls, it speeds up.
24. 24. The change in velocity is ALWAYS 9.8 m/s every second. (9.8 m/s2)</li></li></ul><li>Motion of Every Object<br />Any projectile travels horizontally and vertically.<br />Ignoring air resistance, the horizontal velocity remains the same<br />REMEMBER: force doesn’t “run out” and the law of inertia says objects in motion remain in motion at the same speed and direction unless a net force is applied.<br />
25. 25. Motion of Every Object<br />To keep the math super simple, let’s round the change in velocity to 10 m/s2<br />If a projectile is thrown straight up at 40 m/s, its velocity will decrease by 10 m/s every second.<br />At the top of its path, its velocity is 0 m/s…<br />but immediately, the object begins to fall back to earth where its velocity will increase by 10 m/s every second.<br />
26. 26. Revisit the Question<br />Does horizontal velocity influence vertical velocity?<br />
27. 27. Reflection<br />1. If a bullet is fired horizontally from a gun at the same instant a bullet is dropped from gun barrel height, which bullet hits the ground first? Explain your answer in terms of horizontal and vertical velocity, and gravity<br />2. Sketch the drawing on the right. Explain<br />the significance of the coins being at the<br />same height at each instance AND the<br />significance of the increasing distances<br />between the “strobe photo.” ADV: if the table<br />is 5m high, how long are the coins in the air?<br />