I've been trying to think of an easy way to illustrate the difference in the forces involved in the collisions without resorting to equations.
Basically the point to grasp is the concept of frames of reference as others have noted. Consider two cars travelling in the same direction. The front car travels at 99km/hr, the rear car at 100km/hr. When the rear car collides with the front car, they will experience a gentle bump (though admittedly probably lose control of their vehicles). This is because, to the driver of the rear car, he was travelling at 1km/hour relative to the object he hit. Not a serious bump.
Now consider that the front car travels at 90km per hour instead. The bump would be slightly larger as the car behind would be travelling at 10km/hour relative to the object that he hit (the front car).
Try 50km per hour (still in the same direction). The rear car now runs into the front car at a speed of 50km/hour relative to the car in front.
Now consider that the front car is moving at 0km/hour (ie a brick wall). The rear car would slam into it at 100km per hour relative to the object he hit.
The same mathematics applies when the velocities go into the negatives. Lets say the rear car is still going 100km per hour, but now the car in front is reversing at 1km per hour. When they collide, the rear car was travelling at 101km/hr relative to the car he hit, experiencing a slightly larger force than when he hit the stationary object.
This could continue if the car in front was reversing at 50 km/hr making the rear car crash travelling at 150km/hr relative to the object he hit.
Now extend to the front car reversing at 100km per hour and slamming into the rear car travelling at 100km/hour. The driver of the rear car would have been travelling at 200km per hour relative to the object he hit.
The only other illustration of frames of reference I can think of without resorting to equations is to consider throwing an orange inside your car. If you throw it at the dashboard whilst driving at 100km per hour, the orange travels slightly faster than 100km per hour (say 101km/hour). When it hits the dash though, the dash is moving at 100km per hour so the force experienced by the orange is equivalent to that of a collision with a stationary object when the orange travels at 1km per hour.
Now consider (don't actually do it) throwing the orange out the window at a stationary street sign. Vastly different forces because the the object the orange hits is moving at 101 km/hr relative to the orange.
Now obviously I realise that hitting a car with crumple zones and only 4x20 square centimetre patches of rubber attaching it to the road will be different to hitting a wall with no crumple zones and securely anchored to the ground but the physical concept asked about in the beginning of the thread is illustrated here. I'm sure there'll still be those I can't convince though.
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