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Thread: 2D projectile motion help

  1. #1 2D projectile motion help 
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    A boy throws a rock with a velocity of 20 m/s with an angle of (Theta), the rock enters a house's window at a velocity of 12 m/s

    Find:

    1- Theta
    2- The height of the window
    3- The distance between the boy and the house

    Can this problem be actually solved ? If so, how?

    Thanks for the help

    Theta is an unknown angle, can be any angle.
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  2. #2  
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    1. I don't know what theta means

    2. If there is height in 2D, then you can observe height and width, but not the depth. This may be not true, but you would only see 2 of the 3.

    3. If someone can find the distance based on this info, I would be interested to hear the method. The only info I can see is the "rock" slowing from 20 to 12 m/s - what is the cause of slowing in 2D?

    I also am not sure rocks exist in 2D.
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  3. #3  
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    I came across this problem but i couldn't find an answer, It seems to not have enough information.
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  4. #4  
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    Hi, Hani, welcome to the forum!

    Mayflow, where is the 2D thing coming from?
    I would supose that theta is the angle from the horizontal.

    Hani, how far have you got on this problem, we cannot solve homework problems, but we can help if you get stuck.
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  5. #5  
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    Quote Originally Posted by Jilan View Post
    Hi, Hani, welcome to the forum!

    Mayflow, where is the 2D thing coming from?
    I would supose that theta is the angle from the horizontal.

    Hani, how far have you got on this problem, we cannot solve homework problems, but we can help if you get stuck.
    2D is in the topic title. At Hani, I would certainly solve the problem for you if I could, but I don't know how to at this time, and I don't know if there is enough info to solve it.
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  6. #6  
    x0x
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    Quote Originally Posted by Hani View Post
    A boy throws a rock with a velocity of 20 m/s with an angle of (Theta), the rock enters a house's window at a velocity of 12 m/s

    Find:

    1- Theta
    2- The height of the window
    3- The distance between the boy and the house

    Can this problem be actually solved ? If so, how?

    Thanks for the help

    Theta is an unknown angle, can be any angle.
    1. Theta is the angle made by the velocity of the rock with the horizontal.
    2. You have enough data to solve the problem.
    3. Neither Jilan nor mayflow are capable of helping you solve the problem, at least mayflow is honest and admits it.
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  7. #7  
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    X0X, i'll gift this one to you, even though I love these type of questions. (Reminds me my applied maths students in Hong Kong -bless you all whatever you are doing now!)
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  8. #8  
    x0x
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    Quote Originally Posted by Jilan View Post
    X0X, i'll gift this one to you, even though I love these type of questions. (Reminds me my applied maths students in Hong Kong -bless you all whatever you are doing now!)
    You mean, you were an instructor? No way.
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  9. #9  
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    x0x is going to help you on this one. He's great on differential equations.
    .
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  10. #10  
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    Quote Originally Posted by Jilan View Post
    x0x is going to help you on this one. He's great on differential equations.
    .
    This problem does not need differential equations. You seem confused.
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  11. #11  
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    Quote Originally Posted by Hani View Post
    A boy throws a rock with a velocity of 20 m/s with an angle of (Theta), the rock enters a house's window at a velocity of 12 m/s

    Find:

    1- Theta
    2- The height of the window
    3- The distance between the boy and the house

    Can this problem be actually solved ? If so, how?

    Thanks for the help

    Theta is an unknown angle, can be any angle.
    You know that:




    where

    What other equations can you write?
    Last edited by x0x; 10-11-2014 at 02:19 AM.
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  12. #12  
    btr
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    Quote Originally Posted by Hani View Post
    A boy throws a rock with a velocity of 20 m/s with an angle of (Theta), the rock enters a house's window at a velocity of 12 m/s

    Find:

    1- Theta
    2- The height of the window
    3- The distance between the boy and the house

    Can this problem be actually solved ? If so, how?

    Thanks for the help

    Theta is an unknown angle, can be any angle.
    Assume we can neglect air resistance, and that the rock moves in a uniform downwards gravitational field of strength 9.8 m/s2. Proceed in the standard way, as x0x seems about to try, and resolve the motion into its horizontal and vertical components. If the distance along the ground to the house is , the height of the window (above the initial position) is , the time of flight is and the initial angle of the velocity above the horizontal is , you end up with three equations: one from the condition that the final horizontal coordinate is , one from the condition that the final vertical coordinate is , and one from the condition that the final speed is 12 m/s.

    From these three conditions you are looking to obtain the four unknown quantities , , and . You could, for example, write everything in terms of , but you can't solve the problem completely without some additional piece of information (e.g. if we knew the angle at which the rock entered the window). Give it a try and you'll see.

    There is enough to constrain , of course: solutions do not exist if the angle between the initial velocity and the horizontal is below a certain limit (the speed never gets as low as 12 m/s in these cases). You may find it instructive to calculate that limit as an exercise (hint: the speed is minimum when the potential energy is greatest).
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  13. #13  
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    Thanks for the warm welcome Jilan. This isn't a homework question, its for test prep. I tried solving it but i couldn't, any help?
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  14. #14  
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    Thanks for the answer btr, so will this question require more than simple kinematics and trigonometry, or is it simply unsolvable without the angle?
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  15. #15  
    x0x
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    Quote Originally Posted by Hani View Post
    the rock enters a house's window at a velocity of 12 m/s
    What does this tell you?
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