Concept explainers
In the braking test of a sports car, its velocity is reduced from 70 mi/h to zero in a distance of 170 ft with slipping impending. Knowing that the coefficient of kinetic friction is 80 percent of the coefficient of static friction, determine (a) the coefficient of static friction, (b) the stopping distance for the same initial velocity if the car skids. Ignore air resistance and rolling resistance.
(a)
The co-efficient of static friction.
Answer to Problem 12.122RP
Explanation of Solution
Given information:
At a braking test,
The velocity is reduced from
The stopping distance is equal to
Co-efficient of kinematic friction is
For a uniformly accelerated motion,
In the above equation,
The static friction force is defined as,
In the above equation,
Calculation:
Convert,
For a uniformly accelerated motion,
For the force balance in upwards direction,
Therefore,
Apply Newton’s second law of motion,
Then,
Therefore,
Conclusion:
The co-efficient of static friction is equal to
(b)
The stopping distance if a car skids.
Answer to Problem 12.122RP
Stopping distance if car skids is
Explanation of Solution
Given information:
At a braking test,
The velocity is reduced from
The stopping distance is equal to
Co-efficient of kinematic friction is
For a uniformly accelerated motion,
In the above equation,
The kinematic friction force is defined as,
In the above equation,
Calculation:
Convert,
According to the given information,
Apply Newton’s second law of motion,
Therefore,
Substitute,
Now,
Rearrange to find the stopping distance,
Substitute,
Conclusion:
For skidding, the stopping distance is equal to
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Chapter 12 Solutions
Vector Mechanics For Engineers
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