Modelling a crumple zone. (1 Viewer)

[Twickel]

Hi
I have to model a crumple zone, to do this I thought of using my model car and adding a aluminium can to the front of the car and testing its effects. To see what actually happens I will be placing an egg in the drivers seat.

I plan to push the car into a wall several times at the same distance but with different forces with and without the crumple zone ( aluminium can). Is that ok?

Well the egg will not fit, so I put a mini logo man inside.

All I need is to figure out a way to calculate the acceleration

 

[Aerath]

Well, get a stop watch, and you'll know the distance, therefore, speed will be d/t. Once you get the speed, treat it as final velocity, and sub it into v = u + at. It's probably not as accurate as you would ilke it to be because v = d/t calculates average velocity, instead of instantaneous velocity, but unless you have a radar gun or something....

 

[Riet]

Record it with a video camera (since I'm guessing you don't have an accelerometer), and see how many frames the crash takes. Then divide by the frame rate to get time.

Oh, and put too lines of tape on the ground, a known distance apart. Then in the video you can once again see how long it takes to get from one two the other. It's actually a lot like how a lot of speed cameras work...

 

[Twickel]

I will be pushing a model car in a wall, I need to figure out the acceleration, so the initial velocity will always be 0. The final velocity will be the speed of the car just as its about to hit the wall. If I find the avg velocity can I use that as the final velocity and then find the acceleration.

EG
AVG velocity of the car in a 170 cm distance is 45cm s or ms over 5 seconds.

Can I use 45-0/5 to then find the acceleration?

 

[lolokay]

Re: Physics Question

if you're assuming the acceleration is constant, then s/t = (v+u)/2
so if s = 1.7, t = 5, u = 0, then final velocity is 0.68m/s, and the acceleration is this divided by time (5 seconds) so 0.136ms^-2

I'm not too sure that I get exactly what you have to do for the assignment though. Could you post up the question or any other explanations?

 

[Twickel]

Re: Physics Question

Hi
Well I have to model a car safety device and I chose to do a crumple zone. This is the question im asking.

calculations and discussions of
Velocity, acceleration and force
momentum and impulse
kinetic energy and work

See, I need to find the acceleration of the car each time I push it into the wall.

So the initial velocity will always be 0. But I dont know how to find the instantaneous velocity just before the car will hit the wall. So im asking if I use the avg velocity using V=S/T as final velocity will that be ok?

Maybe this is worded better

The formula is v-u/t The initial velocity in my case will ALWAYS be 0. But I have no equipment to calculate the instantaneous velocity just as the car is going to hit the wall. So if I find the average velocity by using Speed= Distance/Time can I use the average velocity as the final velocity to calculate the acceleration?

 

[Twickel]

Re: Physics Question

s = ut + 1/2at^2

Therefore,

a = (2s - 2ut)/t^2

Since u = 0,

a = 2s/t^2

Credit supermodified YAY.

So the car travelled 170cm in 4seconds. Do I have to convert the cm to m or can I use it like that.?

 

[Twickel]

Re: Physics Question

Hold on the 2s is 2 times speed right. So 1.7/4 times 2 divide 16 I get an acceleration of
0.0531 ms is that right?

 

[Aerath]

I don't see why not.

 

[Twickel]

Ok here. 2.3 seconds taken to travel 1.7m

a= 0.643ms

if I want to find the distance travelled using the a I times a by times taken. How come I dont get 1.7m I get 1.48?

 

[Riet]

Re: Physics Question

s = ut + 1/2at^2

Therefore,

a = (2s - 2ut)/t^2

Since u = 0,

a = 2s/t^2

Credit supermodified YAY.

So the car travelled 170cm in 4seconds. Do I have to convert the cm to m or can I use it like that.?

Slava didn't post here. OMG PARABOX.

Ok here. 2.3 seconds taken to travel 1.7m

a= 0.643ms

if I want to find the distance travelled using the a I times a by times taken. How come I dont get 1.7m I get 1.48?

Because acceleration isn't linear and instantaneous and average velocity aren't the same. And I just realised that I already told you this on GFaqs lol.

 

[Twickel]

So it still works right?
its still correct if I use 2s/t^2.

Are you craylomatic?

 

[Twickel]

When I use 2s/t^2 im not using final velocity as avg velocity so the formula will work for me. I hope.

 

[lolokay]

Ok here. 2.3 seconds taken to travel 1.7m

a= 0.643ms

if I want to find the distance travelled using the a I times a by times taken. How come I dont get 1.7m I get 1.48?

acceleration is the change in velocity over time, not the change in displacement over time. 1.48m/s is the final velocity.

 

[Twickel]

If I use 1.48 as final velocity and figure out acceleration its 0.64ms

If I use 2s/t^2 I get the same a=0.64

 

[lolokay]

yeah.. that's because they're equivalent equations.

If you're going to be pushing the car you won't be getting a constant acceleration, so why not let the car roll down a slope? (one with minimal friction)

 

[Twickel]

Here is what planned to do, push the car on level surface and calculate acceleration using 2s/t^2. Then let car roll down a slope again calculate acceleration using 2s/t^2, keep changing the angle of the slope and once again use 2s/t^2 to calculate the accleration.

 

[Twickel]

so 2s/t^2 will work right.

 

[lolokay]

so 2s/t^2 will work right.

if you know that acceleration is moreorless constant

 

[Twickel]

Hoe else am I going to do it?