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Homework Help: Physics Help

  1. Sep 24, 2006 #1
    today is my first day here so if i violate any rule that apply in this forum plz forgive me..

    the question is

    Dr. Silverstein is driving a 1.95 * 10^3 kg car at 55.0Km/hr. He slams on the brakes and comes to rest in a distance of 17.0 m Assume that Dr. Silverstein has a mass of 75.0 kg

    a) what is the cars acceleration, assuming forward is positive?

    b) what is the force of friction acting on the wheels of the car? ( hint, what is the mass that is being stopped?)

    c) Assuming Dr.Silverstein is well buckled in, what is the force that his seat belt exerts on him?

    these three questions i don't understand...my professor give homework pretty easy questions..but not these..plz help me
    thank you in advance..
  2. jcsd
  3. Sep 24, 2006 #2

    Andrew Mason

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    What is the car's average speed (vavg) after the brakes are applied? (it starts at 55 km/hr and ends up at 0 after constant deceleration)

    Use [itex]a = \Delta v/\Delta t[/itex]. To find [itex]\Delta t[/itex] use [itex]d = v_{avg}\Delta t \rightarrow \Delta t = d/v_{avg} [/itex]

    Work that out and then see if you can do the last two parts (using F=ma)

    Last edited: Sep 24, 2006
  4. Sep 24, 2006 #3
    thank you for the help..but i need more easier explanation plz..i'm sorry i'm not good at physics..i'm looking at the book about similar problem..still can't get it..
    can understand concept of that formula you gave me..i pluged in the
    55.0 = 17/x so..i got x = 3.23 and i'm not sure i'm going right direction
    Last edited: Sep 24, 2006
  5. Sep 24, 2006 #4
    Since Andrew is offline I will just chip in with a few things I see that might help, cyks07.

    Firstly, cyks07, you should use the terms [tex]\Delta t[/tex] and [tex]\Delta v[/tex] so that you can easily know what you are working with, it looks like you were using x, which could confuse you if you aren't careful, just a tip. You said you were not very good at physics and simple things like that can help.

    You can use the equation [tex]v_{f}^{2}=v_{i}^{2}+2a\Delta d[/tex] to find the acceleration, simply plug in the final and initial velocities which you already know and solve for a. THis answers your first question.

    For question b, what is the force required to cause a negative acceleration that will stop him in 17.0m? Tell me if you can't get the answer and I will be glad to help. Try to figure it out on your own though. The best way to get good at physics is to do lots of physics problems, trust me.

    Remember, [tex]F=m\vec a[/tex]
  6. Sep 24, 2006 #5
    thank you for the advise..i'm trying to solve many problem but i get scared when i see those symbols.. :(
    anyway.. first i tried convert 55 km/hr to m/s
    so i got 15.3 m/s so i divided that 15.3 m/s from the 17 m and i got 1.11m/s... i don't know what i am doing..but i what i think the acceleration
    - 1.11 m/s
    plz reply..what i am doing it wrong
  7. Sep 24, 2006 #6
    give me a minute to work through the problem so that I know that I am doing it properly and I will be better able to help you out, one sec.
  8. Sep 24, 2006 #7
    ok..thank you for your help checkfate...
  9. Sep 24, 2006 #8
    cyks07, I think your math is where your problems lie. You will get better at it but you need to practice, you need to want to get better at it. I will gladly help you solve this problem, but I think that it would be beneficial for you to go over some algebra.http://www.wtamu.edu/academic/anns/mps/math/mathlab/beg_algebra/ Here is a website that will give you the basics.

    You might also find http://www.learner.org/resources/series66.html handy, simply register and watch the movies.

    Anyways, onto your question! Okay so, we want to take [tex]v_{f}^{2}=v_{i}^{2}+2a\Delta d[/tex] and solve for a, so that we know the acceleration and can get full marks for question a) .

    So, we do this.

    EVERY PROBLEM for now, you should find out what you know. We know that the initial velocity is 55.0km/h=15.3m/s. We know that the final velocity = 0m/s. We also know that the displacement is 17.0m SO

    [tex]\Delta d=17.0m[/tex]

    [tex]v_{f}^{2}=v_{i}^{2}+2a\Delta d[/tex]
    [tex](0m/s)^{2}=(15.3m/s)^{2}+2a(17.0m)[/tex] ( simply plugging in the known values )
    [tex]-234.09m^{2}/s^{2}=2a(17.0m)[/tex] (squaring the initial velocity and moving it to the left hand side, the final velocity of 0m/s disappears)
    [tex]a=\frac{-234.09m^{2}/s^{2}}{2(17.0m)}[/tex] (simply isolating a so we can solve for it)

    If you want to get good at physics, you need to know math, physics is built on math! It doesn't take that long to learn if you are dedicated, but trust me, it's fun once you know it! I swear! I am not some 40 year old parent telling you this, I am 19. lol

    For question b, find out what force is required to make a car of that mass accelerate at the rate we found here, using the f=ma formula above.
    Last edited: Sep 25, 2006
  10. Sep 24, 2006 #9
    learn to carry your units with you in your equations so you know that you have the right units at the end of it, too. Trust me, you'll learn quick if you try :)
  11. Sep 24, 2006 #10
    i really appreciate about your help checkfate..hm.also i kinda confuse which formula should i use for certain problem..i guess i have to keep practice.. thank u again..now i'm trying other two questions. :)
    also i checked two website link that you posted..its good website :)
    Last edited: Sep 24, 2006
  12. Sep 24, 2006 #11
    No prob, https://www.physicsforums.com/showthread.php?t=110015 was put together with Doc Al and others, it has a lot of good equations. Every question you come across, simply list the variables you know and the variable you need to know, then look for an equation that has all those variables in it and no others. You'll get it, cya around.
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