Finding length of skid marks using f k

In summary, to find the length of skid marks on wet concrete, use the formula Fg = mg = weight = normal force and fk = µkFN to calculate the friction force. Then, use the standard constant acceleration equations, such as x = x0 + vt, to find the distance of the skid marks. Finally, use Newton's second law, F = ma, to find the acceleration.
  • #1
physjeff12
13
0
Finding length of skid marks using fk

Homework Statement



A 1000kg car traveling at a speed of 40m/s skids to a halt on wet concrete where µk = 0.60. How long are the skid marks?

(i know this an easy problem, but i am stuck when trying to find the length of the skid marks.)

Homework Equations



Fg = mg = weight = normal force

fk = µkFN

The Attempt at a Solution



Fg = 1000(9.8) = 9800 N, therefore the normal force = 9800N
µk = .60

fk = .60(9800)
fk = 5800 N

v0 = 40m/s
fk = 5880 N

Where do i go from here to find the distance of the skid marks?

i was thinking maybe using the formula, x = x0 + vt, but i am not sure
 
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  • #2
Welcome to PF!

Hi physjeff12! Welcome to PF! :wink:
physjeff12 said:
fk = 5880 N

Where do i go from here to find the distance of the skid marks?

i was thinking maybe using the formula, x = x0 + vt, but i am not sure

From fk, find the acceleration, and then use one of the standard constant acceleration equations. :smile:
 
  • #3


tiny-tim said:
Hi physjeff12! Welcome to PF! :wink:From fk, find the acceleration, and then use one of the standard constant acceleration equations. :smile:

thanks :smile:

and how do i find the acceleration using fk? i don't know any formulas that connect acceleration with friction force.

EDIT: nvm, i am stupid. i found out how. F = ma :smile:
 
  • #4
Friction is like any other force …

just use good ol' Newton's second law. :wink:
 

1. How does "f k" help in finding the length of skid marks?

"f k" refers to the coefficient of kinetic friction, which is a measure of the resistive force between two surfaces in contact. By using this value, along with other factors such as the vehicle's weight and speed, the length of skid marks can be calculated using equations of motion.

2. What is the significance of finding the length of skid marks?

Finding the length of skid marks can provide important information about the dynamics of a vehicle during an accident. It can help determine the initial speed of the vehicle, the type of braking system used, and the condition of the road surface. This information can be useful in accident reconstruction and determining liability.

3. How accurate is the calculation of skid mark length using "f k"?

The accuracy of the calculation depends on several factors, such as the accuracy of the coefficient of kinetic friction measurement, the accuracy of the vehicle's weight and speed measurements, and the assumptions made in the equations used. However, with proper measurements and calculations, the result can be accurate within a reasonable margin of error.

4. Can "f k" be used to calculate skid mark length for all types of vehicles?

The coefficient of kinetic friction is affected by various factors, including the weight and speed of the vehicle, the type of tires, and the condition of the road surface. Therefore, it may not be suitable for all types of vehicles. However, with proper adjustments and considerations, it can be used for most vehicles.

5. Are there any limitations to using "f k" in calculating skid mark length?

One limitation is that "f k" assumes the vehicle is skidding in a straight line. In reality, the vehicle may have been turning or swerving, which can affect the accuracy of the calculation. Additionally, the coefficient of kinetic friction may vary due to factors such as weather conditions and tire wear. Therefore, it is important to consider these limitations and other factors when using "f k" to calculate skid mark length.

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