Problem 6-25.

The answer is independent of the car's mass m because:

In terms of force - The frictional force is proportional to m. However, the mass has no effect on deceleration (due to frictional force) because a=F/m.

In terms of energy - Assuming the same initial speed, the car's initial kinetic energy is proportional to m. On the other hand, the frictional force is also proportional to m. Therefore it will take the same distance for the frictional force to do enough work to "remove" the kinetic energy from the car.

Notes:

This problem can also be solved by the kinematics equations we learnt in chapter 2.

So now we should realize the kinetic energy and work done relationship we learn in this section is simply the kinetic version of the well-studied kinematics equation in chapter 2:

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