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Motion in One Dimension

Motion in One Dimension. x. t. v. t. a. Honors Physics Lecture Notes. t. ConcepTest 2.12a Throwing Rocks I. You drop a rock off a bridge. When the rock has fallen 4 m, you drop a second rock. As the two rocks continue to fall, what happens to their separation?.

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Motion in One Dimension

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  1. Motion in One Dimension x t v t a Honors Physics Lecture Notes t

  2. ConcepTest 2.12a Throwing Rocks I You drop a rock off a bridge. When the rock has fallen 4 m, you drop a second rock. As the two rocks continue to fall, what happens to their separation? 1) the separation increases as they fall 2) the separation stays constant at 4 m 3) the separation decreases as they fall 4) it is impossible to answer without more information

  3. ConcepTest 2.12a Throwing Rocks I You drop a rock off a bridge. When the rock has fallen 4 m, you drop a second rock. As the two rocks continue to fall, what happens to their separation? 1) the separation increases as they fall 2) the separation stays constant at 4 m 3) the separation decreases as they fall 4) it is impossible to answer without more information At any given time, the first rock always has a greater velocity than the second rock, therefore it will always be increasing its lead as it falls. Thus, the separation will increase.

  4. Tuesday September 6th Introduction of Free Fall

  5. ConcepTest 2.9b Free Fall II Alice Bill v0 vA vB Alice and Bill are at the top of a building. Alice throws her ball downward. Bill simply drops his ball. Which ball has the greater acceleration just after release? 1) Alice’s ball 2) it depends on how hard the ball was thrown 3) neither -- they both have the same acceleration 4) Bill’s ball

  6. ConcepTest 2.9b Free Fall II Alice Bill v0 vA vB Alice and Bill are at the top of a building. Alice throws her ball downward. Bill simply drops his ball. Which ball has the greater acceleration just after release? 1) Alice’s ball 2) it depends on how hard the ball was thrown 3) neither -- they both have the same acceleration 4) Bill’s ball Both balls are in free fall once they are released, therefore they both feel the acceleration due to gravity (g). This acceleration is independent of the initial velocity of the ball. Follow-up: Which one has the greater velocity when they hit the ground?

  7. Free Fall • Free fall is a term we use to indicate that an object is falling under the influence of gravity, with the force due to gravity being the only force on the object. Air resistance is considered negligible. • Gravity accelerates the object toward the earth the entire time it rises, and the entire time it falls. • Free fall is like any other type of accelerated motion; it’s just that the acceleration is well defined. The acceleration due to gravity near the surface of the earth has a magnitude of -9.81 m/s2. The direction of this acceleration is DOWN.

  8. Symmetry in Free Fall • When something is thrown straight upward under the influence of gravity, and then returns to the thrower at the same position, this is very symmetric. • The object spends half its time traveling up; half traveling down. • Velocity when it returns to the same position is the opposite of the velocity it was thrown up with. • Acceleration is -9.81 m/s2and directed DOWN the entire time the object is in the air!

  9. -g H = -50 m What is the maximum height the ball reaches? vi= 30 m/s -g = -9.8 m/s2 h vi vf = 0 a = -g xf= h H 22

  10. -g H = -50 m How much time to reach the maximum height? vi= 30 m/s -g = -9.8 m/s2 h vi vf = 0 a = -g t= tT H 23

  11. -g H = -50 m What is the speed of the ball at xi =0? vi= 30 m/s -g = -9.8 m/s2 h vi a = -g xi= 0 H Sketch an approximate x-vs-t, v-vs-t, and a-vs-t graph of the above.

  12. -g H = -50 m How much time to reach yi =0? vi= 30 m/s -g = -9.8 m/s2 h vi vf = -30 m/s a = -g t= tyi H 25

  13. -g H = -50 m What is the finalspeed of the ball? vi= 30 m/s -g = -9.8 m/s2 h vi a = -g xf= H H 26

  14. -g H = -50 m How much totaltime in the air? vi= 30 m/s -g = -9.8 m/s2 h vi vf = -43.37 m/s a = -g t= ttotal H 27

  15. Reflex Testing • Who’s got the quickest reflexes in Honors Physics? We’re dying to know! • Using only a meter stick, test the reflexes of each person in your group by dropping the meter stick between his or her thumb and forefinger. One trial only per person, please! • The quickest people from each lab table will participate in a “drop-off” at the end of class. • We will then calculate the reaction time of the meter stick champion!

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