Go Back   Physics Help Forum > High School and Pre-University Physics Help > Kinematics and Dynamics

Kinematics and Dynamics Kinematics and Dynamics Physics Help Forum

LinkBack Thread Tools Display Modes
Old Mar 6th 2010, 05:56 AM   #1
Junior Member
Join Date: Mar 2010
Posts: 1
the problem of modeling vehicle brakes

Hi there.
I have a problem with modeling brakes. How can something so seemingly simple as brakes rubbing on a wheel be so difficult to simulate (for me, at any rate!)? Call me thick if you like but this has been bugging me for years.

I am trying to write software (in Visual Basic) to simulate the velocity of a vehicle (e.g. a train).
It is a 'real-time' simulation so I am using a software 'timer' to iterate and update the calculation of the vehicle's momentum (from which I derive the velocity, v) every second, and display the value of v on a gauge. All works well until I try to simulate the brakes.

Before I run the iterative timer code, I start by assigning values to the basic variables:
m = mass of train (pounds)
P = max propulsive force acting on the driving wheels (down-force in poundals on driving axles x coeff of friction)
t = time period of iteration (seconds) (in this case 1 second, the timer period)
R = max brake force (down-force in poundals on braking axles x coeff of friction) (initially = 0)

So with brake force, R initially set to zero, and P set to a suitable value I start my train. On each 1-second time increment I re-calculate the velocity, v (feet/sec) of the train using the following equations:

[start timer loop]
mv = mv + (P - R)t
hence v = mv / m
[increment t and loop ...]

This works fine. I see the velocity rise nicely. Next, once I've got some speed up, I reduce (in real-time) the value of P to zero (power off). The velocity remains constant which is fine, too (I'm ignoring wind and frictional running resistance at this stage). Then I increase braking force, R and watch the velocity fall, and I leave the brakes 'on'. Very nice.

BUT the velocity falls through zero and now the brakes (R) are driving my train backwards as mv increases negatively. So, this equation cannot be simulating the way brakes work at all. Am I not right in thinking that brakes can only reduce existing momentum or KE, or prevent momentum or KE from being increased? They cannot create it themselves! (No matter how hard I press the brakes in my car it goes nowhere!)

How, then, can I represent brakes in a simulation such as this? What equation can I use that reflects the action of brakes?


Last edited by stuart; Mar 6th 2010 at 06:00 AM.
stuart is offline   Reply With Quote

  Physics Help Forum > High School and Pre-University Physics Help > Kinematics and Dynamics

brakes, modeling, problem, vehicle

Thread Tools
Display Modes

Similar Physics Forum Discussions
Thread Thread Starter Forum Replies Last Post
Finding velocity of vehicle njuice8 Kinematics and Dynamics 0 Jan 21st 2013 08:07 PM
Modeling a adsorption/desorption cycle jason_85 Thermodynamics and Fluid Mechanics 0 Aug 31st 2010 11:48 PM
Objects Thrown From Moving Vehicle Ruler of Hell Kinematics and Dynamics 4 Oct 8th 2009 01:42 PM
System modeling pinsky General Physics 4 Mar 10th 2009 04:26 AM
Euler and a vehicle stopping revolution2000 Kinematics and Dynamics 6 Mar 8th 2009 04:45 PM

Facebook Twitter Google+ RSS Feed