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During an interval of time, a tennis ball is moved so that the angle between the velocity and the acceleration of the ball is kept at a constant 120º. Which statement is true about the tennis ball during this interval of time? Choose one answer.

a. Its speed decreases and it is changing its direction of travel.
b. Its speed remains constant, but it is changing its direction of travel.
c. Its speed decreases and it is not changing its direction of travel.
d. Its speed increases and it is changing its direction of travel.
e. Its speed remains constant and it is not changing its direction of travel

In my mind the ball is travelling with a negative velocity, south, all in the y component. At the time it is being accelerated at 120 degrees, thus it is slowing down in the negative Y direction. "Its speed decreases and it is not changing direction of travel." That is, it will eventually change its direction of travel, but just because it is accelerating in the opposite direction of the current vector does not mean that it has changed direction; yet. The answer is given as "a." But couldn't "a." or "c." be true?

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The point is everyone has to start somewhere- and being that I am new to Physics I have to spend time developing my understanding. I am not unaware of the fact that I am green, which is why I am asking for guidance. –  Kurt Apr 15 '12 at 20:04
    
I am not sure you are starting at the right place. You should understand these simple problems by solely read your physics textbooks. And asking only when you have an advanced problem. –  Pygmalion Apr 15 '12 at 20:12
    
That's not very appropriate, Pygmalion. Yes, this is primarily an expert level site, but as long as we're not getting flooded by elementary questions (which we're not), we do allow questions at any level as long as the poster is willing to put in some thought and narrow the question down to a concept. I think this question (and the other one) is acceptable. –  David Z Apr 15 '12 at 20:44
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2 Answers 2

up vote 1 down vote accepted

That is, it will eventually change its direction of travel, but just because it is accelerating in the opposite direction of the current vector does not mean that it has changed direction; yet.

I think this is the core of your confusion. The object is not accelerating in the opposite direction of the current velocity. Try drawing a diagram of the two vectors and you'll see what I'm talking about.

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After drawing the diagram, I see clearly that the object is going to be decreasing in speed and changing direction. I chose some arbitrary values and it is much clearer now. Thanks. –  Kurt Apr 15 '12 at 21:29
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You should never try to solve the problem "in your mind". Even when it is multiple choice.

Follow this procedure

  1. Make a sketch of the situation
  2. Draw relevant quantities in the sketch
  3. Write down the describing equations
  4. Write down initial conditions
  5. Solve the equations

From the solution to the problem, you can easily check for each of the statements whether or not they are true.

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Thanks, I followed those steps and the question in much clearer to me now. My first mistake was trying to solve the problem in my mind. @Pygmalion - I appreciate your feedback and I will spend more time on the basics. –  Kurt Apr 15 '12 at 20:43
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