Chapter 6 P42 Physics 210 Problems - My Solutions
Dr. Hulan E. Jack Jr.
Serway, Faughn and Vuille: College Physics 8th Edition , Thomson Brooks/Cole, Vol I(ISBN #) 978-049511374-3
THE PROBLEM STATEMENT
Ch 6 42. A1200-kg car traveling initially with a speed of 25.0 m/s in an easterly direction crashes
into the rear end of a 9000-kg truck
moving in the same direction at 20.0 m/s
(Fig. P6.42). The velocity of the car right
after the collision is 18.0 m/s to the east.
(a) What is the velocity of the truck right
after the collision?
(b) How much mechanical energy is lost in the collision? Account for this loss in energy.
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Physics 210 Problems - My Solutions
Dr. Hulan E. Jack Jr.
Ch 6 42. A1200-kg car traveling initially with a speed of 25.0 m/s in an easterly direction
crashes into the rear end of a 9000-kg truck
moving in the same direction at 20.0 m/s
(Fig. P6.42). The velocity of the car right
after the collision is 18.0 m/s to the east.
(a) What is the velocity of the truck right
after the collision?
(b) How much mechanical energy is lost in
the collision? Account for this loss in energy.
Basic Solution (Including BRAINSTORMING-Definitions, concepts , principles and Discussion)
Given: mcar = 1200 kg , mtruck = 9000 kg, vcarf = +18 m/s ,
East +
Find:
,
,
vcari = +25 m/s , vtrucki = +20 m/s .
vtruckf , Elost .
Conservation of Linear momentum pf = pi
SKETCH!
mcarvcarf +mtruckvtruckf = mcarvcari +mtruckvtrucki
Conservation of Energy
Ei = Ef + Elost
½ mcarvcari2 +½mtruckvtrucki 2 = ½mcarvcarf 2+½mtruckvtruckf 2
(a) Solve for vtruckf
From (1) mcarvcarf +mtruckvtruckf
Hence
(1)
+ Elost
(2)
= mcarvcari +mtruckvtrucki
vtruckf = (mcarvcari +mtruckvtrucki - mcarvcarf)/ mtruck
(3)
= (1200kg*25m/s +9000kg*20m/s-1200kgm/s*18m/s)/9000kg
= (2.1 x105 kg m/s- 1200kgm/s*18m/s)/9000kg = 20.9 m/s .
(b) Solve for , Elost
From Eq. (2) Elost = Ei – Ef = ½(mcarvcari2 +mtruckvtrucki 2)- ½(mcarvcarf 2+mtruckvtruckf 2) (4)
= ((1200kg*(25m/s)2 +9000kg*(20m/s)2) - ((1200kg*(18m/s)2 +9000kg*(20.9m/s)2)
= 8,00 x105 J
NOT NEED FOR GRADE
Calculation of momenta to draw Conservation of momentum picture.
pcari = mcarvcari = 1200kg* 25m/s =3.0x104 kg m/s
pcarf = mcarvcarf =
Pptrucki = mtruckvtrucki =9000kg*20m/s =1.80x105 kg m/s
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ptruckf = mtruckvtruckf