# Car and wall

1. Jan 16, 2017

### denzel

I've been trying to study for my final and I can't seem to figure out how is this suppose to work.

1. The problem statement, all variables and given/known data

A wall (m infinite, v=0) hits a car (m=2600 kg; v=142 km/h). The car becomes deformed and the crush zone (0.7 m) is compressed. Calculate the corresponding acceleration (assuming a constant value). Within which time interval does that compression happen? Try to find out, how fast each part of the airbag system therefore has to operate. Imagine the crush-zone is replaced by a spring. Calculate the required spring constant.

2. Relevant equations

3. The attempt at a solution
I think it has to do with newtons second law but I dont know what to do with that velocity.

2. Jan 16, 2017

### PeroK

What else do you know about kinematics? Netwon's second law is a start.

3. Jan 16, 2017

### denzel

I know the Kinematic Equations and how they work.

4. Jan 16, 2017

### PeroK

Then you can apply them here!

5. Jan 16, 2017

### denzel

Can you give me a hint? I know that I'm over looking something very basic but I can't seem to find what?

6. Jan 16, 2017

### PeroK

Car - initial velocity given - uniform acceleration assumed - stopping distance given - find force and stopping time.

That's the problem in a nutshell, isn't it?

7. Jan 16, 2017

### denzel

So that 0.7 can be taken as the distance?

8. Jan 16, 2017

### PeroK

Yes.

9. Jan 16, 2017

### denzel

Thanks a lot!

So I should assume that the final velocity is 0 and use the kinematic equation to get acceleration?

10. Jan 16, 2017

### PeroK

You need to think on your feet more. If the car is still moving, then you know too little to solve the problem.

11. Jan 16, 2017

### denzel

I'm at a loss again then, is average velocity the same as a constant velocity?

12. Jan 16, 2017

### PeroK

That question doesn't make a lot of sense.

13. Jan 16, 2017

### denzel

Can you please show me the formulas I'm suppose to be taking into account?

14. Jan 16, 2017

### PeroK

We can only help so much. We can't teach you physics from the beginning. One formula you might like to consider, for constant acceleration is: $a = \frac{\Delta v}{\Delta t}$.

15. Jan 16, 2017

### denzel

V = d/t

then

a = v/t

I think I got it?