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In the before part of Fig. 9-60, car A (mass 1100 kg) is

Chapter , Problem 56

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QUESTION:

In the before part of Fig. 9-60, car A (mass 1100 kg) is stopped at a traffic light when it is rear-ended by car B (mass 1400 kg). Both cars then slide with locked wheels until the frictional force from the slick road (with a low mk of 0.13) stops them, at distances dA ! 8.2 m and dB ! 6.1 m. What are the speeds of (a) car A and (b) car B at the start of the sliding, just after the collision? (c) Assuming that linear momentum is conserved during the collision, find the speed of car B just before the collision. (d) Explain why this assumption may be invalid.

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QUESTION:

In the before part of Fig. 9-60, car A (mass 1100 kg) is stopped at a traffic light when it is rear-ended by car B (mass 1400 kg). Both cars then slide with locked wheels until the frictional force from the slick road (with a low mk of 0.13) stops them, at distances dA ! 8.2 m and dB ! 6.1 m. What are the speeds of (a) car A and (b) car B at the start of the sliding, just after the collision? (c) Assuming that linear momentum is conserved during the collision, find the speed of car B just before the collision. (d) Explain why this assumption may be invalid.

ANSWER:

Step 1 of 4 a) We can write the change in kinetic energy of any of the two cars as. The final kinetic energy of both cars is zero because they both stop in the end. That is, The work done is, The normal force, So, The work done is nothing but the change in kinetic energy. That is, That is, Implies,\n The speed of car A at the start of sliding is, Given, The coefficient of friction, The distance travelled by car A is, So, The speed of car A at the start of sliding is .

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