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Homework Help: 4 course work questions need help

  1. Jul 25, 2014 #1
    dear all,
    I started to study the physics during this summer holiday, I bought some books and I could not slove below questions which found in the books, I do not know to use which equation is correct..........
    does anyone experts can help me ?

    -----(v=s/t; s=1/2(u+v)t; v=u+at; s=ut+1/2at^2; v^2=u^2+2as)
    1. A car starts from rest with a uniform acceleratin of 0.6ms^-2. A second car B starts from the some point, 4 seconds later and follows the same path with an acceleration of 0.9ms^-2. How far will the cars have traveled when B passes A ?
    * I think .....u=0m/s a=0.6ms^-2; a=0.9ms^-2 t=4s g=-9.81m/s^2

    -----(v=u-gt; s=ut-1/2gt^2; v^2=u^2-2gs)
    2. A stone is falling freely, has an instantaneous velocity of 25m/s^-1, having started from rest. Calculate (a) how long it has been falling? (b) how far it has fallen? (c)its velocity after falling for a further 3.5 secnds? (d) the distance it has convered during these 3.5 seconds?
    * I think......u=0m/s v=25m/s^-1 t=3.5s g=-9.81m/s^2

    3. A tower is 100m high. One body is dropped from the top of the tower, and at the same instant another body is projected vertically upwards from the bottom, and they meet half-way up. Find (a) the initial velocity of the projected body? (b) its velocity when it neets the descending body?
    * I think......d or h=100m; d1 or h1=100m/2 g=-9.81m/s^2

    4. A rocket leaves its pad vertically with an acceleration of 7.5m/s^-2 which remains constant until the fuel is exhausted after 8 seconds. It then continues to travel freely in the vertical direction. Find (a) the total time taken for the rocket to reach its highest? and (b) the altitude achived?
    * I think......u=7.5m/s^-2 t=8 seconds g=-9.81m/s^-2
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Jul 25, 2014 #2


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    Try writing an equation for each of the cars' distance (from the starting point) as a function of time.

    Remember, the second car is not moving until t=4seconds

    Have you tried any part of this problem? Are you stuck on part (a)?

    Try writing an equation for each of the bodies' height as a function of time (use "v" in place of the unknown velocity)

    Remember, one of the bodies starts out at a height of 100m

    Have you tried this problem? It will be easier to help you if you show your attempt.
  4. Jul 26, 2014 #3
    I have no idea about slove this question....
    a) Find the total time taken for the rocket to reach its highest point
    b) The altitude achieved.
  5. Jul 26, 2014 #4


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    The rocket accelerates for 8 seconds at a rate of 7.5 m/s per second. What will be the final velocity after that acceleration?

    Then after the first 8 seconds, it stops accelerating upwards and now is accelerating downwards at 9.8 m/s^2 (from gravity) so how long will it take for it to reach it's highest point?

    P.S. If you can, it will really help me to help you if you can be more specific about what parts you don't understand
  6. Jul 26, 2014 #5
    Thank you very much your quick response !
    Last edited: Jul 26, 2014
  7. Jul 26, 2014 #6


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    v does not equal 7.5m/s

    The acceleration is what is 7.5 m/[itex]s^2[/itex]

    You haven't learned that acceleration * time = (change in) velocity?
  8. Jul 26, 2014 #7


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    Sorry, I didn't notice at first that you replied to my other responses.

    That is the distance that car A travelled when car B first starts moving. The car will be moving at 0.6*4=2.4 m/s right? So the equation for the car A's distance will be:

    Now all you need to do is write an equation for [itex]d_B[/itex] as a function of time, and then solve for "t" in the equation [itex]d_A=d_B[/itex]
  9. Jul 26, 2014 #8
    dear sir...... i am shame to tell that i have not study physics in before......i just started to study by myself....... very appreciated your knidly understanding my special situation.......
  10. Jul 26, 2014 #9


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    The equation you want to use is v=u+at where u is the initial velocity (which is zero in this case) and a is the acceleration (in this case it is 9.8)

    This one is a little tricky. What you're going to want to do is find the time that it takes for the top body to fall halfway down the building. Then you're going to need to use that time to find the initial speed of the first body (the reason is that they need to both be halfway at the same time)

    So if we call the time it takes to fall halfway down the building "T" then we have:
  11. Jul 26, 2014 #10


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    Then, these problems will certainly be difficult to solve. If you can't solve them from the advice I have given so far, then I would suggest writing these problems down somewhere and saving them for later while you learn more physics.

    And while you're learning physics, try to understand exactly what the formulas mean
    (it will be very hard to solve problems like these if you are using formulas that you don't fully understand)

    I could try to walk you through these problems but I don't think you're going to completely understand why I did it the way that I did, unless you have a stronger physics foundation.

    But do not throw these problems away! Save them for later! They are excellent practice to give you a strong understanding!

    Don't be ashamed! It's absolutely excellent that you're studying physics on your own!
  12. Jul 26, 2014 #11
    dear sir, i am very appreciated your kindly understanding and explaination as much as you could to me....you are right......to slove the problems i have is too easy for all of your experts. i will study your explaination carefully and try my best to understanding them........by the way, could you teach me anymore web site for the physics beginners just like me......as you mentioned if i could not 100% understanding the physics foundation, i will not understanding physics in the future....
  13. Jul 26, 2014 #12


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    I don't really know of any websites. There is only one I know of (I am not a fan of them, but it's a good start) and that is khanacademy.

    The first few sections should be good a good start.
  14. Jul 26, 2014 #13
    thank you very much with appreciated ! i will visit there first.
    again, many many thanks your great help and value information, wish i can understanding Physics like you and other experts.....
  15. Jul 28, 2014 #14


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    Thread is closed. @gakusei_98 -- check your PMs. You *must* show your work before we can offer tutorial help. Please also only post one question per thread. Post a question using the Homework Help template showing your initial work. Then after you are able to work through it to a solution, you may start another thread with your 2nd question, and so on.
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