# Newton's second law?

$F=ma$. A car strikes a wall at 60 mph. Its acceleration is zero at the time. The force of the car against the wall or vice versa is? To look at the car the force is not zero. Please explain.

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Can you clarify what you are asking? As soon as the car touches the wall it will start decelerating (as it's bodywork crumples) and the force between the wall and the car will be the deceleration multipled by the mass of the car. –  John Rennie Apr 3 '13 at 15:14
You seem to be assuming that you measure the instantaneous acceleration at a time before the the impact can be communicated to "the car". You can do that but this simply means that you are examining the car "before" the impact. As the impact develops both the force and the acceleration will grow rapidly (starting with the mass of the bumper in particular and propagating backwards at the speed of sound in the material of the car's frame) toward some maximum value. –  dmckee Apr 3 '13 at 16:04
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## 1 Answer

The minute the car touches the wall, it will decelerate rapidly (and crumple in the process). This deceleration/crumpling will be from the force the wall exerts on the car, which can be calculated from $F=ma$.

The car also exerts a force on the wall. However, the wall is attached to the ground, and the ground exerts a balancing force on the wall, keeping it in place. This means that the wall exerts an equal and opposite force on the ground. But, the ground (Earth) is so massive that we can just assume that the mass is infinite, giving us zero acceleration for the ground.

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