Deceleration distance over time

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AI Thread Summary
The discussion revolves around calculating the distance a car travels while decelerating from an initial velocity of 20 m/s at a rate of 5 m/s² over 20 seconds. The initial attempt at the formula resulted in confusion due to a negative value, prompting a reevaluation of the deceleration process. Participants clarify that the car's speed decreases over time, eventually reaching zero after four seconds, at which point it would not continue to a negative speed. The importance of using the correct equations and understanding the implications of deceleration on distance traveled is emphasized. Ultimately, the conversation highlights the need for accurate calculations in physics problems involving motion and deceleration.
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Homework Statement

:[/B]
A person drives a car for 20 seconds in a straight line with an initial velocity of 20m/s. During the entire course of the journey they applied breaks causing the car to decelerate at 5m/s2. How far will it be from the starting point after the given time?

Homework Equations

:[/B]
distance = 20 * 20 + 0.5 * -5m/s^2 * 20^2

The Attempt at a Solution

: [/B]
However since I acquire a negative value, I'm convinced that my working is incorrect. What am I misunderstanding about the deceleration?
 
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Hello Plebian, :welcome:

So what happens to the speed of the car ? As a function of time I mean.
 
I forgot to mention that I attempted the deceleration over time 20 * 20 + 0.5 * -5m/s^2 * 20 this rendered a better result that looks like it could be correct. I am on the correct track here?
 
No ! it's not as if you can solve this by trial and error juggling the expressions. The dimensions don't even fit.
Now read post #2 and react to it: what happens if you slam the brakes while moving at 20 m/s and thus reduce your speed with 5 m/s per second ?
 
BvU said:
No ! it's not as if you can solve this by trial and error juggling the expressions. The dimensions don't even fit.
Now read post #2 and react to it: what happens if you slam the brakes while moving at 20 m/s and thus reduce your speed with 5 m/s per second ?
The velocity of 20m/s would slow down by 5m/s per second.
 
Yes, that's what I wrote. So after 1 second your speed is .., after 2 ..., after 3 ... and after 4 it is zero. Would it then go negative if you keep braking ?
 
BvU said:
Yes, that's what I wrote. So after 1 second your speed is .., after 2 ..., after 3 ... and after 4 it is zero. Would it then go negative if you keep braking ?
It would remain stationary at that point.
 
Does the expression you use in post #1 reflect that ?
 
BvU said:
Does the expression you use in post #1 reflect that ?
Not at all, which was why I initially posted this query.
 
  • #10
So for how long can you use that expression ?
 
  • #11
BvU said:
So for how long can you use that expression ?
Point taken.
 

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