Solve Physics Problem: Stopping a Car with Skid Marks

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SUMMARY

The problem involves calculating the initial speed of a car that skids to a stop over a distance of 40 meters, with a coefficient of friction of 0.50 for rubber tires on asphalt. Using the kinematic equation v² = u² + (2*a*s), where v is the final velocity (0 m/s), s is the distance (40 m), and a is the deceleration derived from the frictional force, the initial velocity (u) can be determined. The deceleration is calculated as a = -coefficient * g, leading to a final formula for u based on the given parameters.

PREREQUISITES
  • Understanding of kinematic equations, specifically v² = u² + (2*a*s)
  • Knowledge of frictional force calculations, including Ff = coefficient * Fn
  • Basic physics concepts such as acceleration due to gravity (g = 9.8 m/s²)
  • Familiarity with the concept of deceleration in motion
NEXT STEPS
  • Study the derivation and application of kinematic equations in physics problems
  • Learn about the principles of friction and how to calculate frictional forces in various scenarios
  • Explore the effects of different coefficients of friction on stopping distances
  • Practice solving similar physics problems involving motion and forces
USEFUL FOR

Students studying physics, particularly those focusing on mechanics and motion, as well as educators looking for effective problem-solving strategies in teaching kinematics.

runningirl
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Homework Statement



Driving in a car on a level road, you hit the brakes hard, lock up the wheels and skid to a stop. You measure the skid marks and find them to be 40 m long. Use this information to determine the speed of the car at the moment you hit the brakes. The coefficient of friction for rubber tires skidding on asphalt is 0.50.

Homework Equations



f=ma

The Attempt at a Solution



a=(9.8)(sin theta)
Ff=coefficient(Fn)
9.8(sintheta)(.5)=Ff

i honestly wasn't sure where to go with this problem
 
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Can you write the expression for frictional force?
 
v^2 = u^2 + (2*a*s)
The 3rd kinematical equation...
.
You have the following...
v = 0m/s (the final velocity, since your car has stopped)
s = 40m (distance traveled)
u = initial velocity, (that's what we want to find)...
If we only have a (the acceleration, in our case deceleration)
.
Ff = coefficient(Fn)-------(1)
Fn = mg as you're driving in a car on a level road... --------(2)
Ff = ma --------(3)
.
Substituting 2 and 3 in eq 1
a = (-1*coefficient*g) ...(negative since its a deceleration)
.
Put it all in the first equation...
 
Hi Carl!

That was very well put :-) But what we try to do here on Physics Forums is not give away the formulas and let the person plug and chug and be happy. The idea is to guide the student through the solution, so that next time they have a similar problem (say in the exam), they can solve it themselves.
 
Sure thing...
New here, so still have to get used to things...
In future if you catch me going outta the regular way, let me know...
Thanks
:-)
 

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