The driver of a 1,300 kg car is driving along a street at 42 km/h. Another drive
ID: 1568808 • Letter: T
Question
The driver of a 1,300 kg car is driving along a street at 42 km/h. Another driver, coming from directly behind (and not paying attention), is driving a 1,700 kg car at 61 km/h and hits the slower car. A) If both drivers slam the brakes at the moment of impact and their fenders catch on each other, with what combined speed do they begin to skid? B) how much did the kinetic energy of the system change? (Hint: Calculate the initial and final kinetic energies and then compare them.) Answers: A) 14.7 m/s B) The kinetic energy of the system decreased by 7,600 J.
Explanation / Answer
Let M1=1300kg
M2=1700kg
U1=42kmph
U2=61kmph
V=coommon velocity
We have V=M1U1+M2U2/(M1+M2)
V=1300*42+1700*61/(1300+1700)
= 52.76666667kmph
= 14.657 m/s
B) 42kmph=42*5/18m/s=11.667m/s
KE1=1/2 *M1U1^2= 88472.2Joule
KE2=1/2*M2U2^2= 244047Joule
Total K.E= 332519.2679Joule
Final K.E= 1/2 * (M1+M2) V^2
= 324135.47Joule
Loss= 332519.2679-324135.47 Joule
=8384Joule
=
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