Solving a Car Coasting Downhill Problem: Average Retarding Force

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Homework Help Overview

The discussion revolves around a physics problem involving a car coasting down a hill, focusing on calculating the average retarding force due to friction. The scenario includes a car with a specified mass, initial conditions, and distance traveled along a grade, leading to questions about the forces acting on the car and energy considerations.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning, Problem interpretation

Approaches and Questions Raised

  • Participants explore energy balance by considering kinetic energy at the bottom and the impact of friction. Others suggest analyzing forces acting on the car, including gravity and friction, while questioning the relevance of certain forces. There are inquiries about alternative methods to solve the problem without relying on kinetic energy concepts.

Discussion Status

The discussion is active, with participants offering different perspectives on how to approach the problem. Some have proposed energy methods, while others are seeking clarification on force analysis. There is no explicit consensus on a single method, but various lines of reasoning are being explored.

Contextual Notes

Participants note constraints such as the lack of knowledge regarding certain concepts, like kinetic energy, and the absence of specific values, such as the coefficient of friction. These factors are influencing the discussion and the approaches being considered.

TeeNaa
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A 1200 kg car start from rest and coast down a uniform grade, at the bottom of which ti has a speed of 20 m/s. If the car has traveled 800 m along the grade and has descended 35 m, what is the average retarding force(friction) encountered by the car? How would I solve this problem? I know Fx = ma = Ff - mgsintheta Fy = 0 = N - mgcostheta Ff = muN I don't have mu though. Thanks
 
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I would consider energy balance.

What is the kinetic energy at the bottom?
What would be the kinetic energy in the absence of friction?
What happened to the difference?
 
Is there any other way to solve this problem beside KE? I haven't learn that yet. Thanks
 
TeeNaa said:
Is there any other way to solve this problem beside KE? I haven't learn that yet. Thanks

Sum of forces along the ramp = ma
One force is gravity.
Second force is friction.
(Third force is force pushing up against the car. Why is this irrelevant?)

Get a from ramp length s and final velocity v.
Hint: dv/ds = dv/dt dt/ds = a/v.
 
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