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Science assignment help
speedyxjs - 20/3/09 at 10:47 AM

I have just a couple of questions left and my brain wont work.

How do you calculate the braking efficiency of a car when all you have is the speed before braking, the deceleraction and the distance taken to stop?

omega0684 - 20/3/09 at 10:55 AM

im a Virology man, sorry, different kinda science, lol, sorry no help what so ever!

001Ben - 20/3/09 at 11:16 AM

I don't know if it will help but try here

hughpinder - 20/3/09 at 11:19 AM

calc deceleration from v = u -2as
v = 0 (stopped)
u = initial speed m/s
s = stopping distance in meters.

Efficiency depends on what braking is maximal - if its 1g, efficiency in percent

= ( a(from above) *100) / 9.81

Regards
Hugh

Added: That link that appeared just before my post is better!

[Edited on 20/3/09 by hughpinder]

tegwin - 20/3/09 at 11:19 AM

You have enough information there to work out the power dissapation required by the brakes to stop the car...

Id be buggered if I can remeber the formulas though... Pretty sure you need to work out the mass of the car and then take it from there!

matt_gsxr - 20/3/09 at 11:29 AM

dear mr Hugh

v = u - 2 as
is wrong.

just look at the units!!!

v.v = u.u - 2 a s

where in this case v = 0.


so

a = v*v / ( 2 * s)

This will yield "a" or the acceleration.

I'm not sure what you mean by efficiency. You could convert to braking force.

F = m a (Force = mass * acceleration) can be rerranged to tell you the force that you are decellerating with.

Note on units:

m in kg
v in m/s (1 m/s = 2.16mph, so to get speed in m/s divide mph by 2.16)


Matt

Dangle_kt - 20/3/09 at 11:32 AM

Eh?

Hasn't this forum got an english lanaguage only rule?

Sounds like a load of voodoo to me!

02GF74 - 20/3/09 at 12:16 PM

quote:
Originally posted by speedyxjs
How do you calculate the braking efficiency of a car when all you have is the speed before braking, the deceleraction and the distance taken to stop?


It is Friday aftefnoon so I ain't gonna think to hard about this but I don't think you can. As ^^^ say, you almost certainly would need to know mass of car.

I also don't think you need distance taken to stop since you have deceleration.

hughpinder - 20/3/09 at 01:12 PM

Matt
I am ashamed. Must think more carefully.
The 'braking efficiency' thing was in the original post.
Thanks
Hugh

mr henderson - 20/3/09 at 02:03 PM

Excuse me chaps, aren't we all (except me ) forgetting something?

We're not in a newtonian 'body with no forces acting on it' situation here, we are in a car whcih would slow down quite quickly if the engine was turned off situation.

Air resistance, the tyres and other sources of friction all need to be considered. If one knew how long the car would take to stop if the engine was turned off but the brakes were not applied then we would be in a good position to start calculating the brake efficiency for when they are applied

John

Tinks1 - 20/3/09 at 03:36 PM

if V=0 then surely

U*U = 2as

a = U*U/2s

[Edited on 20/3/09 by Tinks1]