Physics Help Forum Friction

 Kinematics and Dynamics Kinematics and Dynamics Physics Help Forum

 Apr 29th 2008, 11:50 PM #1 Junior Member     Join Date: Apr 2008 Location: Australia Posts: 3 Friction ok, to get things started, i DO NOT, i repeat DO NOTwant the answers. i would just like an example, SHOWING how i would do this. Even if you can, give me what equation i would have to use, and HOW to use it. THNX!!! physics is hard lol. The average Coefficient of friction between the tyres of an MGF Trophey and the ground is 0.95. How long will it take the trophey to brake to a stope to a top speed of... A) 60 Km/h? B)100Km/h?
Apr 30th 2008, 04:27 AM   #2

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 Originally Posted by firemonkey ok, to get things started, i DO NOT, i repeat DO NOTwant the answers. i would just like an example, SHOWING how i would do this. Even if you can, give me what equation i would have to use, and HOW to use it. THNX!!! physics is hard lol. The average Coefficient of friction between the tyres of an MGF Trophey and the ground is 0.95. How long will it take the trophey to brake to a stope to a top speed of... A) 60 Km/h? B)100Km/h?
First sketch a Free-Body Diagram. (See attachment below. I am taking the car to be moving in the +x direction.)

Apply Newton's 2nd Law in each coordinate direction. Since we know that the car is decelerating, there is an acceleration in the x direction.
$\displaystyle \sum F_x = -f = ma$

$\displaystyle \sum F_y = N - w = 0$

You can get the normal force from the y equation, and then the acceleration from the x equation.

Then this is just a motion under constant acceleration problem. You know the initial speed and the acceleration, and you want the time at which the speed is 0 m/s. (Remember to do the unit conversion on the initial speed!)

That should give you a good start. If you have any more questions on this, feel free to ask.

-Dan
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 Apr 17th 2018, 12:09 AM #3 Junior Member   Join Date: Apr 2018 Posts: 20 What is an MGF Trophey?
Apr 17th 2018, 12:23 AM   #4
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 Originally Posted by firemonkey ok, to get things started, i DO NOT, i repeat DO NOTwant the answers. i would just like an example, SHOWING how i would do this. Even if you can, give me what equation i would have to use, and HOW to use it. THNX!!! physics is hard lol. The average Coefficient of friction between the tyres of an MGF Trophey and the ground is 0.95. How long will it take the trophey to brake to a stope to a top speed of... A) 60 Km/h? B)100Km/h?
Neither (a) nor (b) since those are speeds and not times.

Apr 17th 2018, 12:56 AM   #5
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 Originally Posted by Pmb Neither (a) nor (b) since those are speeds and not times.
Well spotted. +1

Apr 17th 2018, 02:10 AM   #6
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 Originally Posted by Pmb Neither (a) nor (b) since those are speeds and not times.
I think the OP means that it's a two-part question:

a) How long will it take the trophey to brake to a stop from a speed of 60 km/h?

b) How long will it take the trophey to brake to a stop from a speed of 100 km/h?

In which case, following Topsquark's advice is a good start. You'll end up with an equation that gives you the braking duration for a given initial speed. Then you can plug in the values and compare the two durations. What you should find is that the duration obtained for part b) is much longer than expected because the braking time is proportional to energy which is proportional to velocity squared. This is why speeding is so dangerous.

Last edited by benit13; Apr 17th 2018 at 02:13 AM.

Apr 17th 2018, 02:56 AM   #7
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 Originally Posted by benit13 I think the OP means that it's a two-part question: ...
Yeah. I figured it was something like that. But he wanted to limit our responses, Hence to <expletive> eating grin.

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