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Old Mar 23rd 2015, 06:12 PM   #1
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Join Date: Mar 2015
Posts: 3
Friction

But these are the most confusing to me....


003 (part 1 of 2) 10.0 points
A car is traveling at 31.3 mi/h on a horizontal
highway.
The acceleration of gravity is 9.8 m/s
2
.
If the coefficient of friction between road
and tires on a rainy day is 0.086, what is the
minimum distance in which the car will stop?
(1 mi = 1.609 km)
Answer in units of m.



004 (part 2 of 2) 10.0 points
What is the stopping distance when the surface
is dry and Ádry = 0.652?
Answer in units of m.
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Old Sep 26th 2015, 01:18 PM   #2
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Join Date: Sep 2015
Posts: 5
You forgot to specify the mass of the car.

Anyway, this is a possible approach, having the mass:
1) Draw a free body diagram.
2) Convert the velocity from mph to m/s.
3) Find the deceleration of the car.
This deceleration is due to the sliding friction force (look at the FBD) which you can easily find:
Ff = μN

And N in this case is simply mg (you can see this in your FBD).
After you have found Ff, you can find the deceleration applying Newton's 2nd Law:
F = ma

4) At this point you have the initial velocity, the final velocity (0m/s, since the car stops), and the deceleration. You can apply kinematic equations to find the distance traveled.

In the second part you just have a different coefficient of friction. You can repeat the whole process from step 3, and find the distance.

Last edited by Roller; Sep 26th 2015 at 01:23 PM.
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