# The Naked Scientists Forum

### Author Topic: Was the other driver speeding? Help please!  (Read 2215 times)

#### xuder

• First timers
• Posts: 1
##### Was the other driver speeding? Help please!
« on: 05/04/2006 22:20:52 »
I need the help of somebody who remembers their A level Physics better than I did.

A taxi drove into the side of my car today.  I'm trying to work out how fast the taxi was travelling when it hit me.

My car weighs 1250kg (including passengers).

The taxi decelerated to a dead stop after impact in 2.5 metres.

The car was pushed in a straight line in the taxi's direction of motion for 10.5 metres after impact, before it came to a stop.

I think the typical coefficent of friction between car tyres and road surface is 0.72.

I was thinking, that if I could out how much energy was required to push my car 10.5 metres, then I could work out the force the taxi was travelling at, and there for the initial velcity before impact.

Can anybody help?

#### daveshorts

• Moderator
• Neilep Level Member
• Posts: 2583
• Physics, Experiments
##### Re: Was the other driver speeding? Help please!
« Reply #1 on: 08/04/2006 01:00:41 »
if all wheels are locked your car and the taxi will be decellerating at the coefficient of friction * g if you are sliding the coefficient of friction will be less than .72 but I will stick with it for now
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`v^2-u^2=2asso -u^2 =2asu^2 = -2as    = .72 * 9.81ms-2 * 10.5m    = 74u=8.6ms-1a taxi weighs about 1800kgso assuming that momentum is conservedm_taxi * v_taxi = (m_taxi + m_car) * v_bothv_taxi = (1800 + 1250)kg * 8.6ms-1         -------------------------              1800kg       = 14.6ms-1       = 32mph`although this is assuming a probably slightly high value of the coefficient of friction, so I would have said that when he hit you he was doing a bit below 30mph. Of course if the taxi stopped braking when he hit you it would have just been your car slowing you both down so he could hav ebeen going even slower.

Really you need the proper police calculations involving skid marks.
« Last Edit: 08/04/2006 01:06:36 by daveshorts »

#### The Naked Scientists Forum

##### Re: Was the other driver speeding? Help please!
« Reply #1 on: 08/04/2006 01:00:41 »