1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: A few beginner physics questions

  1. Feb 5, 2007 #1
    A few beginner physics questions~~

    I just started taking Physics and Im already getting stumped on what seem to be the easiest questions...

    heres a few:

    1. A bullet goes 750 km/hr and hits its target 2.0 seconds later. How far away is the target? (Look at the units)

    What I did was change the kilo meters to meters by multiplying 750 x 1000= 75000. Then 75000 x 3600 to convert the hours to seconds and got 270000000!! The right answer is .42 km so I would like to know how my answer is OBVIOUSLY wrong.

    2. A car going 12 m/s speeds up to 45 m/s in 5 seconds. How much distance did it cover in that time?

    I thought you were supposed to subtract 45-12=33 m/s and 33 becomes your accuracy and 12 and 45 were your initial and final velocity and plug all those into a distance formula. What I cameup with was 472.5 m but the correct answer is supposed to be 142 m!! What did I do wrong XD??!!
  2. jcsd
  3. Feb 5, 2007 #2
    If you include the units in your multiplication, it'll show you where you went wrong.

    You started with 750 km/hr and multiplied by1000?
    I'd say that you actually multiplied by 1!

    1000 meters = 1 kilometer, therefore, the fraction 1000m/1km has a numerator equal to the denominator, thus is equal to 1.

    Write it this way:
    [tex]\frac{750 km}{1 hour} * \frac{1000m}{1 km} [/tex]
    Note that when you multiply, the km cancels out since you have one on the top and one on the bottom. Thus, you've successfully converted 750km/hr to 750000m/hr.

    Now, be careful what you multiply by next. Do you want to multiply by [tex]\frac{3600sec}{1 hr}[/tex] or do you want to multiply by [tex]\frac{1 hr}{3600sec}[/tex]? Hint: you want the hour unit to cancel out.
  4. Feb 5, 2007 #3
    Sorry <=D,

    Do you multiply by 3600sec/1 hr??
  5. Feb 5, 2007 #4
    For the second problem--since you have initial velocity, final velocity, and time, you just need to find the acceleration right? So plug that into one of the three equations. Then take the acceleration and plug it the other equation for total distance.
  6. Feb 5, 2007 #5
    Oh!! Thankyou so much =D, Im such an airhead that never even crossed my mind <=P.:blushing:
  7. Feb 5, 2007 #6
    It will be a very long semester for you is you don't learn dimensional analysis. I would spend some extra time on it or get some tutoring. You might try leaving out the numbers at first and just examing the units to make sure that what you want to cancel does. For instance:

    km/hr * m/km * hr/min * min/s = m/s OK

    Then just put in the correct number for each term, like 60min/1s = 1 or 1hr/60min = 1.
  8. May 11, 2007 #7
    Hello there i am only a beginner too but i can see if i can help.(second problem)

    u=initial velocity .'. u=12
    v=final velocity .'. v=45
    t=time from u-v .'. t=5
    s=distance(what we must find) .'. s=?



    There you are i hope this helps.
    so what i did is used one of the two equations for the velocity average there is (v.ave=v+u/2) and (v.ave=s/t) in the first one(v.ave=v+u/2) we have all the information but in the second we only have one bit of information so if we can get v.ave from this equation(v.ave=v+u/2) we can use it in the second(v.ave=s/t) then we will have two parts of information in the second equation and can work out (S) which is the distance. so we workd out v.ave to be 28.5 so we take it around to the second equation(v.ave=v+u/2) to get the distance, this would look like this (28.5=s/5).
    By this point we move 5 over to the other side of the equation and it turns from a division to a multiplication. This would look like this (28.5*5=s).
    we take the calculations and it should give the answer.

    im not sure if this will help but goodluck.
    Last edited: May 11, 2007
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook