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

 

[rwisz]

I understand your curiosity about the force of a car crash and how it relates to Newton's law of f=ma. Let me explain the basic physics behind it.

Firstly, it's important to note that in a car crash, the car is not at a constant speed. It may seem like it is, but in reality, the car is decelerating rapidly as it comes into contact with the wall or another object. This deceleration is what causes the force to be exerted.

According to Newton's second law, force (F) is equal to mass (m) multiplied by acceleration (a). In the case of a car crash, the force is caused by the deceleration (negative acceleration) of the car. This deceleration is a result of the car's kinetic energy being converted into other forms of energy, such as heat and sound, upon impact.

So, in your example, the negative acceleration of the car would indeed result in a negative force. However, this negative force is actually acting in the direction of the impact, not backwards. This is because the force is being exerted by the car on the object it is hitting.

In summary, the force of a car crash is a result of the deceleration of the car upon impact. This deceleration is caused by the conversion of the car's kinetic energy into other forms of energy. I hope this helps clarify your understanding of the basic physics behind a car crash.