# Friction

#### 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 (Sweating)(Worried) 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?

#### topsquark

Forum Staff
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 (Sweating)(Worried) 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

#### Attachments

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#### Monoxdifly

What is an MGF Trophey?

#### Pmb

PHF Hall of Fame
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 (Sweating)(Worried) 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.

1 person

#### studiot

Neither (a) nor (b) since those are speeds and not times.
Well spotted. +1

#### benit13

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.

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#### Pmb

PHF Hall of Fame
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.