If everything must decelerate before stopping or changing direction?

Question

When does a car going 40 mph decelerate when hitting a train going 40 mph head on? Wouldn't the car just switch directions without decelerating?

Answer 1

As an example...
If you were to throw a ball at the wall, it comes back almost instantly.
However in a small fragment of time,
The ball impacts the wall
The ball deforms and "squishes" or flattens.
In this period the energy is stored in the process.
The ball then will start to go back to it's original shape.
The ball in doing so uses the squished energy to propel itself away from the wall and back to you.
Now with the car the energy driving the car forward is not stored but changed. The car absorbs the energy and will destroy itself upon impact. If the car did store the energy it would come back or switch directions but it was not designed to do that as in the example of the ball the period of time is small but it does take time and with a normal view it would be as if it happened instantly but it didn't.
If the car had a glancing blow it would change direction but also slow down.

Answer 2

I would not call it "deceleration", that is more for when the magnitude of the acceleration is lessened but still has the same sign (for example going from 4 m/s^2 to 2 m/s^2). In this case, it would be "acceleration in the negative direction" since the acceleration is no longer ‘positive’; it is not in the opposite direction of travel. Just because acceleration is negative does not mean that the velocity cannot still be positive.

When a car traveling at 40 mph hits a train traveling at 40 mph in the opposite direction (we assume thr train has significantly more momentum) then the car accelerates in the "negative" direction in a very short amount of time...but it does take time.

If you were to use a very high speed camera, or somehow slow time down so you could better watch the collision occur, as the car hits the train...the car continues to travel forward as it crumples up and is compressed by the train, then for an instant, the car is motionless, afterward, the car begins to accelerate in the opposite direction as it was previously traveling.

Answer 3

The deceleration of the car crashing on the train take a few milliseconds, while the whole car structure collapses and bend.
Then the train will push the remains of the car in the other direction.
The movement is constant velocity, abrupt deceleration (and deformation), stop (a near non-measurable instant), acceleration in the other direction.
You have to look at it in terms of forces, mass and acceleration (remember F = m.a).

Answer 4

No. To change direction or speed instantaneously would require an infinite force. That's impossible. But of course, you can decelerate so quickly that to a human, it feels like instantaneously.

For example, suppose a projectile traveling with a speed of 400 meter per second hits a thick steel plate and leaves a scratch only 1mm deep, it may have taken it something like 50 microseconds to decelerate, and that is xero for most practical purposes :-)

Answer 5

The deceleration occurs on impact, in a fraction of a second. It then will accelerate in the other direction: let's face it, a car hasn't the momentum to stop a train...

Answer 6

In Yahoo Browser, Type, Negative Acceleration. Then Press Go. Then press item #2

Answer 7

No, the car does decelerate. Very very very quickly!

Answer 8

Watch a humming bird for a while and reconsider your question.