Wave speed of the collision process in a line of cars

Click For Summary

Discussion Overview

The discussion revolves around modeling the wave speed of collisions in a line of cars at a stoplight, focusing on the dynamics of elastic collisions and the propagation of disturbances through the line of vehicles. Participants explore equations related to wave speed and the implications of car lengths and collision types.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant seeks to derive an equation for wave speed in a line of cars, noting the lack of time information and specific limits for wave speed as distance approaches zero and infinity.
  • Another participant suggests assuming elastic collisions and questions the speed of the first bumped car after a collision, as well as the time until the next collision.
  • A participant asserts that the first bumped car travels at the initial speed v0, but their calculations for total distance and time do not meet the required limits.
  • Concerns are raised about the relationship between car length and time for disturbances to pass through, with a suggestion that longer cars should affect the time due to the disturbance's propagation speed.
  • One participant argues against the assumption that the shock wave would travel at the same speed as the final car speed, pointing out that cars would become compressed unrealistically and emphasizing the incompressibility of cars in elastic collisions.

Areas of Agreement / Disagreement

Participants express differing views on the effects of car length on collision dynamics and the nature of wave propagation in this context. There is no consensus on the implications of these factors or the correct approach to modeling the situation.

Contextual Notes

Participants note limitations in their assumptions regarding the nature of collisions (elastic vs. inelastic), the treatment of car lengths, and the mathematical relationships involved in wave speed calculations.

snickersnee
Messages
30
Reaction score
0

Homework Statement



I need to write an equation for the at rest at a stoplight. Each car bumps into the one in front of it until the first car in line gets bumped.

Homework Equations


I found these equations for wave speed:
V=√(T/(m/L)), where T is tension, m is mass, L is length
and of course V=λf

The Attempt at a Solution


l8atx.jpg


But both those equations have a time factor, and I wasn't given any time information. All I know is that wave speed needs to approach infinity as x → 0, and wave speed approaches vo as x→∞
Any hints would be appreciated.
 
Physics news on Phys.org
snickersnee said:
I found these equations for wave speed:
V=√(T/(m/L)), where T is tension, m is mass, L is length
You don't have a string here.

I think you are supposed to assume that all collisions are either perfectly inelastic or elastic. The latter is easier to study: what is the speed of the first bumped car after the collision? How long does it take to make the next collision?
 
Yes we're assuming elastic collisions. The first bumped car is traveling at v0 because the car behind it transferred all its energy. The time until the next collision is
upload_2015-9-7_19-16-47.png
. I was thinking the total distance would be (4L+3x) and total time would be 3v0/(L+x) but then (total distance)/(total time) gives (4L^2+7xL+3x^2)/3v0 but that doesn't satisfy the limits required.
 
Why should the time change for longer cars (larger L) if the distance between the cars does not change?

Also, your fraction does not have units of time, it has units of inverse time.
 
mfb said:
Why should the time change for longer cars (larger L) if the distance between the cars does not change?

Also, your fraction does not have units of time, it has units of inverse time.

Thanks for your help. I think the time should change for longer cars because the disturbance takes longer to pass through that car, assuming the velocity of the disturbance is constant. And you're right, I should flip the fraction so it becomes
upload_2015-9-8_19-0-9.png
 
If the shock would travel at a speed of v (the same speed as the final car speed), your cars would get completely compressed to a single "disk" in the process - certainly not realistic.
Assume that the cars are incompressible (they cannot get deformed permanently in elastic collisions anyway) - as soon as the back of a car moves, the front moves as well.
 

Similar threads

  • · Replies 18 ·
Replies
18
Views
3K
  • · Replies 19 ·
Replies
19
Views
2K
  • · Replies 2 ·
Replies
2
Views
1K
  • · Replies 8 ·
Replies
8
Views
1K
  • · Replies 4 ·
Replies
4
Views
7K
  • · Replies 6 ·
Replies
6
Views
3K
Replies
3
Views
4K
  • · Replies 9 ·
Replies
9
Views
845
  • · Replies 16 ·
Replies
16
Views
2K
  • · Replies 18 ·
Replies
18
Views
2K