Motion at constant acceleration

AI Thread Summary
To estimate the speed of a car just before braking, the problem involves a car that leaves 92m skid marks while decelerating at 7.00m/s². The initial attempt to solve the problem incorrectly assumed that dividing distance by deceleration yields time, leading to an erroneous calculation of speed. The correct approach utilizes the equation vf² - vi² = 2ad, which allows for the calculation of initial speed based on the known deceleration and distance. The final calculation shows that the initial speed is approximately 35.889 m/s. Understanding the correct application of motion equations is crucial for solving such problems accurately.
steve snash
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Homework Statement


In coming to a stop, a car leaves skid marks 92m long on the highway. Assuming a deceleration of 7.00m/s^2 estimate the speed of the car just before braking.


Homework Equations



deceleration=7m/s^2, distance=92m

The Attempt at a Solution


I found there was not enough information to solve the problem, but i had a go, assuming dividing the distance by deceleration you would get time. 92/7 =13.14 seconds

I substituted this into the equation of motion x=x0+v0t+1/2at^2
=52.99m/s. Is this right??

NEED TO KNOW BY TOMORROW
 
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Why would distance divided by deceleration give you time? Why would x=x0+v0t+1/2at^2, which is an equation for displacement, give you speed?

Are you familiar with the equation vf^2-vi^2=2ad?
 
so the answer should be vi^2=2(7)(92)
=square root 1288=35.889 m/s??
 

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