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Homework Help: HELP! Some simple questions from a confused Student!

  1. Sep 18, 2006 #1
    Hey guys...new to the forum and I am in desparate need of some help with some homework problems assigned to me. We are working on 2D Kinematics and I am at a total loss. Here are some examples of the problems we are doing....any help at all would be wonderful!


    1. A rocket is fired at a speed of 97.0 m/s from ground level, at an angle of 53.0 ° above the horizontal. The rocket is fired toward an 27.0-m high wall, which is located 24.0 m away. The rocket attains its launch speed in a negligibly short period of time, after which its engines shut down and the rocket coasts. By how much does the rocket clear the top of the wall?

    2. In the javelin throw at a track-and-field event, the javelin is launched at a speed of 23.0 m/s at an angle of 34.5 ° above the horizontal. As the javelin travels upward, its velocity points above the horizontal at an angle that decreases as time passes. How much time is required for the angle to be reduced from 34.5 ° at launch to 13.8 °?

    3. An airplane is flying with a velocity of 240 m/s at an angle of 30.0° with the horizontal, as the drawing shows. When the altitude of the plane is 2.4 km, a flare is released from the plane. The flare hits the target on the ground. What is the angle ?

    4. The escalator that leads down into a subway station has a length of 19.0 m and a speed of 1.51 m/s relative to the ground. A student is coming out of the station by running in the wrong direction on this escalator. The local record time for this trick is 6.57 s. Relative to the escalator, what speed must the student exceed in order to beat the record?

    5. On a pleasure cruise a boat is traveling relative to the water at a speed of 4.22 m/s due south. Relative to the boat, a passenger walks toward the back of the boat at a speed of 1.69 m/s. (a) What is the magnitude of the passenger's velocity relative to the water? (b) How long does it take for the passenger to walk a distance of 21.0 m on the boat? (c) How long does it take for the passenger to cover a distance of 21.0 m on the water?

    6. Relative to the ground, a car has a velocity of 18.2 m/s, directed due north. Relative to this car, a truck has a velocity of 22.9 m/s directed 47.2 ° south of east. Find the (a) magnitude and (b) direction of the truck's velocity relative to the ground. Give the directional angle relative to due east.

    7. A bullet is fired from a rifle that is held 1.26 m above the ground in a horizontal position. The initial speed of the bullet is 1130 m/s. Find (a) the time it takes for the bullet to strike the ground and (b) the horizontal distance traveled by the bullet.

    8. The highest barrier that a projectile can clear is 12.7 m, when the projectile is launched at an angle of 15.0 ° above the horizontal. What is the projectile's launch speed?
  2. jcsd
  3. Sep 18, 2006 #2


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    Welcome to the Forums,

    One is expected to show ones efforts before requesting help :smile:
  4. Sep 18, 2006 #3


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    In order to get some help, it would be great to show some effort first. If you study the kinematic equations considering projectile motion, relative motion etc., I'm sure you'll know how to start.
  5. Sep 18, 2006 #4
    I would love to show my efforts....but I don't know how to get all the chicken scratch that is taking over my notebooks onto the forum :confused:

    I am headed out but will be back in an hour....when I get back I will try to provide all attempts at answering the questions....sorry! didn't know the rules of the forum, I am a n00b! :biggrin:
  6. Sep 18, 2006 #5


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    You may wish to try reading our thread on [itex]\LaTeX[/itex] which can be found at: https://www.physicsforums.com/showthread.php?t=8997
    Don't worry about it, we're not bringing out the birch just yet!
  7. Sep 18, 2006 #6
    Ok so lets start with the first one:

    The way I attempted it was:

    I found the maximum height the Rocket will reach:

    H = Final Velocity^2 - Initial^2 / 2(a) = 0 - (97.0 m/s)^2 / 2(-9.80 m/s^2)

    = 480 m less the height of the wall (27 m) = 453 m
  8. Sep 18, 2006 #7
    Anyone....help :cry:
  9. Sep 18, 2006 #8
    Okay for question one, here is how I would approach the problem:

    First, draw yourself a diagram. It really explains stuff. Then you need to draw your vectors. Why? Because you need Horizontal stuff and Vertical Stuff (stuff being t,d,v,u,a).

    So you draw your fist velocity vector...magnitude of 97 m/s and angle of 53. That is the hypotenuse (not sure if that's the right spelling :confused: ) of a right angle triangle. The horizontal line of that tiangle is your horizontal velocity and the vertical one is your vertical velocity vector.

    Calculate the magnitude of them using sin, cos, tan where approperiate.

    That should give you your initial velocities for Horz and Vert. Then simply go doing your algebra (sub equations, use formulas) to solve the problem....I'm kindda lazy at the moment so I haven't given a shot at calculations.

    By the way I am also a grade 12 physics student that wanted to solve physics questions and help others. So I might be wrong....not sure but that's the way my teacher's tought me :biggrin:

    EDIT: You have your Horz d which is 24m. Your Vert time and Horz Time are equal (unknown though). Horz and Vert Vs can be found by the method described above, and your vert Acceleration is 10 m/s^2 down. Just sub in the numbers to find Vert Displacement...that's what you really need. Then subtract the 27 from that and you'll get your answer.
    Last edited: Sep 18, 2006
  10. Sep 19, 2006 #9

    Anyone else...I managed to get close to the answer but not quite right
  11. Sep 20, 2006 #10
    What do you mean 'get close'?

    Tell me what you got for the answer...

    EDIT: I calculated the whole thing and ended up with 17.4 m for vertical displacement of the rocket....which is lower than the wall.

    This means that the rocket won't pass over the wall and hit the wall instead unless I'm doing something wrong or I'm missing an element in the question (I tripled checked it so I guess it's the second). So um...the rocket won't pass over the wall.
    Last edited: Sep 20, 2006
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