Car collision

[rov1200]

If a 1-ton car is travelling at a constant 60mph what force is exerted if it hit a stationary object? I always thought that Force=mass x acceleration but in this case 'a' = 0, therefore F = 0.

[curry]

think momentum

[rojash]

Acceleration is rate of change of velocity. The car was doing 60 and now it's doing 0. That can only happen if there is a change in its velocity.

[chakka35]

'a' is not zero. The car has decelerated from 60mph to zero in a very short time. That is a large acceleration, albeit a negative one, causing the car to slow and stop rather than increase velocity.

[TCL-MUMPING]

The acceleration is a change in the rate of velocity but without knowing the new velocity or how long it took the car to come to rest there is not enough info to work out what that is. Without knowing the acceleration, how can you work out the force?

[Dodger666]

If the car weighed a metric ton(1000kgs) and came to a stop in 1 second the force would be -26824 Newtons. If it came to stop in 0.5 seconds the force is -536448 Newtons.

The - represents decelaration.

If it came to a stop instantly the force would be -infinity.

I look forward to other suggestions.

[rojash]

The calculations are much more difficult if the car hits a stationery object. In the case of it hitting a piece of paper, you are almost into the quantum realm, although filing cabinets (especially full ones) are easier to deal with.

[heathfield]

The whole of the car does not stop instantaneously. If it did, it would be, as Dodger says, an infinite force.. It comes to rest with what is known as an 'Impulse of Force' collision. The time for negative acceleration has to be taken into account...

http://hyperphysics.p...du/HBASE/Impulse.html

[factor-fiction]

Did it hit a fly, as some have suggested on AB that would stop a train so on that basis it would definitely stop a car.

[Dodger666]

I have dozens of dead flies on the front of my car and they certainly didn't stop me, so it's hard to believe one would stop a train. Someone's having you on

[vascop]

Nobody pointed out to rov that what he was doing when he said a=0, therefore F=0, was calculating the force on the car while it was poodling along at constant velocity.
But of course the car has momentum and also kinetic energy and if it hits a stationary object this energy must go somewhere. Hence the smashed up car and broken wall.