Stopping distance (work & energy)

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Homework Help Overview

The problem involves calculating the stopping distance of a vehicle on a level dry concrete road during an emergency stop, given that the friction force is approximately 80% of the vehicle's weight. The vehicle's initial speed is specified, and gravitational acceleration is provided for calculations.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • Participants discuss various equations related to work and energy, including the work-energy theorem and kinetic energy. There are attempts to apply these equations, but confusion arises regarding their application and relevance to the problem.

Discussion Status

Some participants have expressed difficulty in applying the equations correctly, with hints provided to guide their thinking towards the work-energy theorem. There is a recognition of the relationship between kinetic energy and the work done by friction, but clarity is still lacking among participants.

Contextual Notes

Participants are navigating through the constraints of the problem, including the assumption that all wheels lock during braking and the specific conditions of the road. There is also mention of confusion regarding the relevance of potential energy in this context.

Chica1975
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Homework Statement



in an emergency stop on a level dry concrete road the magnitude of the friction force when sliding is approx 80% of the weight of the vehicle. What stopping distance is required for a vehicle traveling at 88km/h (24.444 m.s)?

Assume that all the wheels lock when the brakes are applied

Use 9.8 m/s2 for gravitational acceleration

Homework Equations



W= F*X
.5mvE2
SumF = ma

The Attempt at a Solution



I have tried a number of combinations using the above equations - I can't figure this out.

Help anyone!
 
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Show what you tried. Hint: Use the work-energy theorem.
 
basically I tried using each of the above and nothing worked I even tried .5mvfE2 -.5mv0E2 - nothing works
 
Use my hint! (You need another formula that combines your first two.)
 
is it potential energy? PE = mgh?
 
a bit lost
 
Chica1975 said:
is it potential energy? PE = mgh?
No. Gravitational PE is not relevant here since the height doesn't change.

How does the initial KE of the car relate to the work done by friction in stopping the car?
 
to be honest I don't know - it must reduce kinetic energy or change it in some way becoz friction is going in the opposite direction?

I am completely lost.
 

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