 # Force involving crumple zone

I am trying to figure if 2 bikes collided one weighing 65kg and the other 75kg with a crumple zone of 0.5m came to a complete rest before seperating what would be force of each and what formaula should i use also what would be the combined force appreciate any help thankyou

Do you want to assume that except for weight each is an exact mirror image of the other? Will the crumple be even or will it vary? No riders? If riders then they and their differences need to be considered. There is a reason they use crash test. Computations are just too limited and not very live like.

Motorcycles have “crumple zones”? Shoot your best shot at what formula do you think should be used, you must have some idea.

Well, I’m not real clear on exactly what you’re after.
What kind of a collision is involved, head on, rear end or broadside?
Using the conservation of momentum formulas one can deduce pre-crash speeds of both vehicles. There is a transfer of momentum (force) at impact.
Are you looking for the speed before collision?

There is no simple equation for this. Check out this website for a better understanding of determining your answer. http://spiff.rit.edu/classes/phys311.old/lectures/impulse/impulse.html

In general, the velocities of both bikes will determine the linear momentum of each. At the point of collision, a large impluse will be applied to each to reduce their forward momentum. The compression of the ‘crumple zone’ will increase the amount of time of the impulse by a fraction, which reduces the maximum force applied by a fraction. However, this will create an integral equation, since the force applied will change over time as the collision occurs. I’m assuming your looking for maximum of the force applied.

Typically, a simple equation for one bike would be the calculate the velocity before the collision and after. Multiply each velocity by the total bike’s mass (bike & rider) to get momentum before collision (Pi) and momentum after (Pf). The impulse is the change in momentum (I = Pf - Pi), and impules is also force times time. The time is the actual time of the collision event. To get Force, you need the actual time the collision event, when the bikes first touch to the bikes separate.

I was asking the OP what he was looking for,trying to determine what level he was at

Sorry guys for the delay in responding I did not realise the site was so quick to respond I am mightly impressed To clarify my question First I was told to watch a short film of a motorcycle accident.There were passengers involved but the total weight of each bike was 60kg and 75kg. Yes it was a head on accident and the crumple zone was an even .5 meters per bike. This is my first year foundation and am new to this but have looked into first solving the kinetic energy then using a formula Favgd = =.5xmv2 would this be correct Many thanks to you all so far for your replies forgot to add speed of each bike was 13.4112 m/s

Hello Bustedknuckles Hope this is not to long winded but the actual assignment is as follows:
Given that 2 motorcycles weigh 65kg and 75kg and are travelling around 30 mph,have a 0.5 crumple zone and that they come to a complete rest before seperating perform the following
a. Calculate the force of each bike
b. calculate the combined force of the two bikes
c. calculate the change in momentum of each bike
d. calculate the change in energy of each bike
e. construct a velocity time graph of the motion of both the bikes
f. construct an acceleration time graph of the motion of the 2 bikes
g. investigate and explain the g force on the riders of the bikes
h give a detailed explanation of why the modelling of momentum and energy in accordance with newtons laws is not appropriate for this example many thanks

I see two problems using kinetic energy. One, your formula assumes all the kinetic engery is used at the time of collision. And two, doesn’t take into account the conservation of momentum. If both bikes are different mass, but same speed, one has more momentum than the other. When they collide, there is a transfer of momentum, and the heavier bike will have more. Both momentum and kinetic energy are vectors, meaning they have magnitude and direction. At the point of collision, the heavier bike should still have momentum, and that means there is additional kinetic energy transferred to the lighter bike. Your simple equation will overestimate for the heavy bike, and underestimate for the lighter one.

There are people who like to play with math and physics who, I’m sure, would love to help you. Go to www.physicsforums.com Tell’em CarTalk sent you.

Thanks for that just curious if you read the assignment above many thanks

All of my crash reconstruction formulas are not in metric, so you would have to do your own conversions. Velocity, for instance, is figured in feet per second, derived from miles per hour. You will need to convert all velocity and weights.

You are indeed wanting to use linear momentum in a head on or rear end crash. A broadside crash will use vector momentum’s, and the on scene evidence is much more complicated, needing the post impact departure angles of both vehicles.

Most of the time I was interested in determining the vehicles pre-crash speeds for litigation of criminal and civil trials. You are going more for the physics end of it, but the same formulas will solve some of your assignment.

The following site will have most of your information:
http://www.mcasco.com/p1lmc.html

Thankyou so much all I will look into the sites provided. Just to say I think I figured out the force of each bike and the change in momentum I am now stuck on the change of energy of each bike I have an kenetic energy of bike (a) of 5845.4593kj and bike (b) 6744.7607kj but am unsure how to figure the change. I thank you all ps sorry for delays in replying I am from the united kingdom different time zones