Force of a car crash basic physics.

[InsideJoke]

Hello all.
Firstly, I want to note that I'm sure you've all heard this question multiple times but I don't know what search phrases I would use to find it - so I'm sorry for asking a boring basic question.

OK,
I'm learning extremely basic introductory physics and I was having a look at Newton's law f=ma
for an assignment on car safety.

I was just wondering what would happen if a car hit, say, a wall while at a constant speed and not accelerating? I mean, that would mean the force would be zero but it seems to me that in a car crash the car exerts huge amounts of force forward into whatever it's hitting.

I've heard that the accelerating in this type of question is actually the negative acceleration of the car as it comes to a sudden stop but what puzzles me is wouldn't that meant the force would be transferred backward, not forward into the wall? If the force was created by negative acceleration wouldn't that mean it would have to be a negative force (very basic physics knowledge here, so if it does in fact mean negative force please explain)? (f= say, 500kg * -90km/ph^2 = -45000)
Sorry for certain errors in maths and units ect but I'm sure you get my gist.

Thanks!

 

[RichardParker]

You can look at it in a different light:

Force is also defined as the change in momentum (mass * velocity) of the object with respect to time. That is

F = mv/t

When the car crashes to a wall then the time it takes for the velocity to change is very small. So F = mv/ 0.0001 seconds is a large force.

 

[tiny-tim]

Welcome to PF!

Hello InsideJoke! Welcome to PF!

I've heard that the accelerating in this type of question is actually the negative acceleration of the car as it comes to a sudden stop but what puzzles me is wouldn't that meant the force would be transferred backward, not forward into the wall? If the force was created by negative acceleration wouldn't that mean it would have to be a negative force (very basic physics knowledge here, so if it does in fact mean negative force please explain)?

You're confusing the force on the car with the force from the car.

In F = ma, F is the total force on the body whose acceleration is a.

The car's acceleration is negative, so the force on it is negative.

The force from the car on the wall is equal and oppostie (good ol' Netwon's third law ), ie positive.

 

[InsideJoke]

oh!
that makes sense!
thanks very much for that :D