Example 9.3 How Good Are the Bumpers? In a particular crash test, a car of mass

Question

Example 9.3 How Good Are the Bumpers? In a particular crash test, a car of mass 1,260 kg collides with a wall as shown in the figure. The initial and final velocities of the car are v,=-15.0 m/s and v- 2.60i m/s, respectively. If the collision lasts 0.100 s, find the impulse caused by the collision and the average net force exerted on the car. Before -15.0 m/s After 3 +2.60 m/s (a) The car's momentum changes as a result of its collision with the wall. (b) In a crash test, much of the car's initial kinetic energy is transformed into energy associated with the damage to the car. SOLVE IT Conceptualize The collision time is short, so we can imagine the car being brought to rest very rapidly and then moving back in the opposite direction with a reduced speed Categorize Let us assume that the force exerted by the wall on the car and friction from the ground is large compared with other forces on the car (such as air resistance). Furthermore, the gravitational force and the normal force exerted by the road on the car are perpendicular to the motion and therefore do not affect the horizontal momentum. Therefore, we categorize the problem as one in which we can apply the impulse approximation in the horizontal direction. We also see that the car's momentum changes due to an impulse from the environment. Therefore, we can apply the nonisolated system (momentum) model

Explanation / Answer

we may need to use the mass of the car equal to 1260kg tocalculate the time:

t = -mv/F = 1260kg * 21m/s /(3.0*10^4N) = 0.882s

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