Question:

Help with a Physics question.?

by  |  earlier

0 LIKES UnLike

I tried a hundred times and cant figure out how to do this problem without initial velocity. The driver of a car slams on the brakes when he sees a tree blocking the road. The car slows uniformly with acceleration -5.40 m/s2 for 4.30 s, making straight skid marks 62.5 m long ending at the tree. With what speed does the car then strike the tree?

 Tags:

   Report

2 ANSWERS


  1. The first working formula is

    S = Vo(T) + (1/2)(a)T^2

    where

    S = distance travelled = 62.5 m (given)

    Vo = initial velocity of car

    T = travel time of car = 4.3 sec (given)

    a = acceleration = -5.4 m/sec^2

    Substituting values,

    42.5 = Vo(4.3) + (1/2)(-5.4)(4.3)^2

    4.3 Vo = 42.5 + (1/2)(5.4)(4.3)^2

    Vo = (1/4.3)(42.5 + (1/2)(5.4)(4.3)^2)

    Vo = 26.15 m/sec.

    The second formula to be used is

    Vf^ - Vo = aT

    where

    Vf = velocity at which car will strike the tree

    and all the other terms have been previously defined.

    Substituting values,

    Vf - 26.15 = (-5.4)(4.3)

    Vf = 26.15 - 5.4(4.3)

    Vf = 2.93 m/sec


  2. s= 62.5m (displacement)

    t = 4.3s (time)

    a = -5.4ms^-2 (acceleration)

    v=? (final velocity)

    u=? (initial velocity)

    you are right you need initial velocity which can be found out using:

    s = ut + 0.5at^2

    62.5 = 4.3u - 2.7 x 4.3^2

    62.5 +2.7 x 4.3^2 = 4.3u

    u = (62.5 + 2.7 x 4.3^2)/4.3

    knowing u, sub this into v^2 = u^2 + 2as to find final velocity

Question Stats

Latest activity: earlier.
This question has 2 answers.

BECOME A GUIDE

Share your knowledge and help people by answering questions.