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Constant Velocity and Uniform Acceleration Teacher Excellence Workshop June 18, 2008.

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Presentation on theme: "Constant Velocity and Uniform Acceleration Teacher Excellence Workshop June 18, 2008."— Presentation transcript:

1 Constant Velocity and Uniform Acceleration Teacher Excellence Workshop June 18, 2008

2 Distance  The total length of the path traveled by an object is called distance.  “How far have you walked?” is a typical distance question.  The SI unit of distance is the meter (m).  Distance is always positive. It has no direction. It is an example of a scalar.

3 Displacement (  x)  The change in the position of a particle is called displacement.  “How far are you from home?” is a typical displacement question.  The SI unit for displacement is the meter.  Displacement has a direction. In 1D motion, it can be positive or negative. It is an example of a vector.  Calculation of displacement:

4 A B Distance vs Displacement  Have your students draw the distance and displacement of a particle that moves from A to B around an oval track.

5 A B Distance vs Displacement  Have your students draw the distance and displacement of a particle that moves from A to B around an oval track.

6 Let’s Do a Lab!  Purpose: Figure out a way to make a cart move with an average velocity of as close to 0.200 m/s as possible. Use only the equipment provided. Photogate must be in PULSE mode.  Report: Write your BRIEF lab report. The section I want you to focus on in this lab report is the procedure section. Check my web site for examples of good procedures, and for the lab report guidelines.

7 Practice Problem: You drive in a straight line at 10 m/s for 1.0 km, and then you drive in a straight line at 20 m/s for another 1.0 km. What is your average velocity?

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9 Qualitative Demonstrations 1)Demonstrate the motion of a particle that has an average speed and an average velocity that are both zero. 2)Demonstrate the motion of a particle that has an average speed and an average velocity that are both nonzero. 3)Demonstrate the motion of a particle that has an average speed that is nonzero and an average velocity that is zero. 4)Demonstrate the motion of a particle that has an average velocity that is nonzero and an average speed that is zero.

10 Quantitative Demonstration  You are a particle located at the origin. Demonstrate how you can move from x = 0 to x = 5.0 and back with an average speed of 0.5 m/s.  What the particle’s average velocity for the above demonstration?

11 Graphical Problem Demonstrate the motion of this particle. t x

12 Graphical Problem Demonstrate the motion of this particle. t x

13 Graphical Problem What physical feature of the graph gives the constant velocity from A to B? t x xx tt A B v ave =  x/  t

14 Graphical Review Problem Demonstrate the motion of these two particles. t x

15 Graphical Problem Demonstrate the motion of these two particle. t v

16 Graphical Problem t x What kind of motion does this graph represent?

17 Graphical Problem Can you determine average velocity from the time at point A to the time at point B from this graph? t x A B xx tt v ave =  x/  t

18 Graphical Problem: Determine the average velocity of this particle between 1 and 4 seconds. x (m)

19 Graphical Problem: Determine the average velocity from the graph. x (m)

20 Graphical Problem: Determine the average velocity between 1 and 4 seconds.

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22 Instantaneous Velocity  The velocity at a single instant in time.  If the velocity is uniform, or constant, the instantaneous velocity is the same as the average velocity.  If the velocity is not constant, than the instantaneous velocity is not the same as the average velocity, and we must carefully distinguish between the two.

23 Instantaneous Velocity Draw a tangent line to the curve at B. The slope of this line gives the instantaneous velocity at that specific time. t x B xx tt v ins =  x/  t

24 Practice Problem: Determine the instantaneous velocity at 1.0 second.

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26 Acceleration (a)  Any change in velocity over a period of time is called acceleration.  The sign (+ or -) of acceleration indicates its direction.  Acceleration can be…  speeding up  slowing down  turning

27 Questions  If acceleration is zero, what does this mean about the motion of an object?  Is it possible for a racecar circling a track to have zero acceleration?

28 Questions  If acceleration is zero, what does this mean about the motion of an object?  Ans: It is not changing.  Is it possible for a racecar circling a track to have zero acceleration?  Ans: Nope; the direction of the velocity is changing.

29 Uniform (Constant) Acceleration  In most high school physics courses, we will generally assume that acceleration is constant or uniform.  With this assumption we are free to use this equation:  The SI unit of acceleration is the m/s 2.

30 Acceleration in 1-D Motion has a sign!  The sign indicates direction. Acceleration is therefore a vector.  If the sign of the velocity and the sign of the acceleration is the same, the object speeds up.  If the sign of the velocity and the sign of the acceleration are different, the object slows down.

31 Qualitative Demonstrations 1)Demonstrate the motion of a particle that has zero initial velocity and positive acceleration. 2)Demonstrate the motion of a particle that has zero initial velocity and negative acceleration. 3)Demonstrate the motion of a particle that has positive initial velocity and negative acceleration. 4)Demonstrate the motion of a particle that has negative initial velocity and positive acceleration.

32 Practice Problem: A 747 airliner reaches its takeoff speed of 180 mph in 30 seconds. What is its average acceleration?

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34 Practice Problem: A horse is running with an initial velocity of 11 m/s, and begins to accelerate at –1.81 m/s 2. How long does it take the horse to stop?

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36 Graphical Problem Demonstrate the motion of this particle. Is it accelerating? t (s) v (m/s) 0.50

37 Graphical Problem Demonstrate the motion of this particle. Is it accelerating? t v

38 Graphical Problem What physical feature of the graph gives the acceleration? t v vv tt A B a =  v/  t

39 Practice Problem: Determine the acceleration from the graph.

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41 Kinematic Equations for uniformly accelerating objects


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