Deceleration without knowing the time

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Homework Help Overview

The discussion revolves around a physics problem involving the calculation of the net force required to stop a car of mass 1000 kg traveling at an initial speed of 15 m/s over a very short distance, specifically the diameter of a dime (1.8 cm). The challenge lies in determining the necessary deceleration without knowing the time taken for this change.

Discussion Character

  • Exploratory, Assumption checking, Conceptual clarification

Approaches and Questions Raised

  • Participants explore the relationship between acceleration, distance, and initial velocity, questioning how to incorporate distance into the calculations. Some mention the need to consider constant deceleration and energy principles.

Discussion Status

Several participants have provided insights into the problem, suggesting different approaches to relate acceleration and distance. One participant indicates a realization of their previous misunderstanding, while another expresses a sense of having overcomplicated the situation. There is an ongoing exploration of the problem without a clear consensus on the final solution.

Contextual Notes

Participants are grappling with the constraints of the problem, particularly the lack of time information and the very short stopping distance, which complicates the application of standard equations of motion.

BOAS
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Homework Statement


What net force would be necessary to stop an automobile of mass 1000kg traveling initially at a speed of 15m/s in a distance equal to the diameter of a dime, which is 1.8cm ?

Homework Equations

The Attempt at a Solution



The force required to stop the car is given by F = dp/dt.

Change in momentum = mv - mv0

As far as I can see, it's not possible for me to determine a time in which this change in momentum takes place. I have been staring at it for ages and not making any head way.

Thanks for any help you can give.
 
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You have to introduce distance. What is the relationship between acceleration and distance travelled?
 
Acceleration is the second derivative of displacement with respect to time.
 
BOAS said:
Acceleration is the second derivative of displacement with respect to time.
That is correct, but not the important point.

There are two approaches:
(a) assuming constant deceleration, what is the relationship between initial velocity, acceleration and distance traveled in the stopping process?
(b) Work with the energy of the car.
 
I was overcomplicating this... a = (v^2 - u^2)/2s

I have solved the problem.
 
mfb said:
That is correct, but not the important point.

There are two approaches:
(a) assuming constant deceleration, what is the relationship between initial velocity, acceleration and distance traveled in the stopping process?
(b) Work with the energy of the car.

I just realized my mistake before reading your post. I moved on to the next problem and the answer to this one was staring me in the face.
 

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