Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Force and newton's laws (airbag)

  1. Oct 1, 2003 #1
    Hi, we're starting our chapter on Newton's Laws and force and I wanted to know if I am approaching the following problem correctly:

    A car travels 53 km/h and hits a bridge abutment (what's that?). The person in the car moves forward 65 cm (w/respect to the road) while being brought to rest by an inflated air bag. What magnitude of force (assumed constant) acts on the passenger's upper torso, which has a mass of 41 kg?

    Well, I believe that the Vo of the car will also be the Vo of the person. So:

    Voc = Vop = Vo = 53 km/h = 14.72 m/s

    The distannce the person moved forward:

    Dp = X-Xo = 65 cm = 0.65 m

    Because the airbag brought the person to rest:

    Vf = 0 m/s

    We now have Vo, X-Xo, and Vf. We can find a:

    V^2 = Vo^2 + 2a(X-Xo) --> a = -166.676 m/s^2

    Using F=ma:

    F = ma = (41)(-166.676) = -6833.7 N

    Because they are looking for magnitude, I can take the abs value of F and get my answer: 6.8 x 10^3 N

    Was my approach correct? Is F negative because a was negative, meaning it was decelerating?
  2. jcsd
  3. Oct 2, 2003 #2


    User Avatar
    Science Advisor

    A "bridge abutment" is the pillar on one side of the road holding the bridge up. Not at all a nice thing to hit at 53 km/hr- they are large, heavy, and have a bad attitude.

    Your calculation is correct: you find the deceleration and then use f= ma.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook