Homework Help: kinetics

Posted by dmkp on Wednesday, March 28, 2012 at 12:51pm.

a stationary car is hit from behind by another car travelling at 40km per hr. After collision both cars remain locked together. The masses of the stationary car and the moving car are 1500kg and 1300kg respectively (use g=9.8ms-2)
a) is this an elastic or inelastic collision
b) calculate the velocity of the two cars immediatly after the collision
c) If the brakes of the stationary cars are applied before impact and the coefficient of friction between the wheels and the road surface is 0.4 calculate the decelaration of the cars.the time taken for the cars to come to rest and the distance travelled by the cars

kinetics - Elena, Wednesday, March 28, 2012 at 3:27pm
a) This is the inelastic collision.
b)
v =40 km/h=11.11 m/s
M(st)•0+M(mov) •v=(M(st)+M(mov)) •V
V= M(mov) •v/(M(st)+M(mov))=1300•11.11/(1300+1500) =5.158 m/s.
c) m•a =F(fr) = k•m•g
a=k•g =0.4•9.8 =3.92 m/s.
s=v^2/2a = (11.11)^2/2•3.92 = 15.72 m
0 = v- at
t = v/a – 11.11/3.92 = 2.83 s.

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