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Homework Help: Calculating the time of stopping distance

  1. Nov 29, 2008 #1
    well i'm struggling with a level physics and need some help with a new topic area we're studying

    the problem is

    a moving car t velocity of 20 ms(-1) brakes to a standstill in a distance of 40m, a child of mass 15kg is sitting in a forward facing child car seat fitted to the back-seat of the car

    calculate (1) the deceleration (2) the force on the child

    i always thought to calculate the deceleration is braking distance/ mass but the only mass given is that of the child and surely the car will way more than that, hence why i dont understand what formula to use, is it using suvat? if so i cant find a suvat formula with only s U and v, its driving me mad ( no pun intended)

    for the second part of the question I haven't been told any information on how to work it out but is the formula t= 2s/U + v (t is time, s is distance/displacement, U is initial velocity and v is final velocity
  2. jcsd
  3. Nov 29, 2008 #2


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    I don't know why you were thinking that distance/mass equals acceleration. Erase that thought. What about the kinematic equation vf^2 = vo^2 + 2as? You didn't do enough searching on your suavat's.
    How about using Newton's 2nd law?
  4. Nov 29, 2008 #3
    i think i have it now

    thanks (i found out newtons second law about 2 minutes before u posted, and the suvat question i just found,) thanks for providing me with support so i avoidied doubting myself
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