Deceleration problem

  • Thread starter cerberus9
  • Start date
  • #1
26
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I reeeeeeeeally need help with thiss. thanks :)

Homework Statement


A track runner finishes a race and begins to slow down in a uniform manner. In 3.50 seconds, she comes to a complete stop, covering 12.0 meters while doing so. What was her speed when she finished the race?


Homework Equations



v=d/t
d=vot+ 1/2 at2

The Attempt at a Solution



I don't really know how to find the original speed so i did v=d/t
12.0/3.50=3.43 m/s
 

Answers and Replies

  • #2
rl.bhat
Homework Helper
4,433
7
I reeeeeeeeally need help with thiss. thanks :)

Homework Statement


A track runner finishes a race and begins to slow down in a uniform manner. In 3.50 seconds, she comes to a complete stop, covering 12.0 meters while doing so. What was her speed when she finished the race?


Homework Equations



v=d/t
d=vot+ 1/2 at2

The Attempt at a Solution



I don't really know how to find the original speed so i did v=d/t
12.0/3.50=3.43 m/s
The car is retarding. So the acceleration is negative.
Rewrite the relevant equation with negative a.
 
  • #3
8
0
You are on the right lines with the second formula you posted. Do you know a rearrangement of it? You need to find a formula with initial and final velocities, time and displacement.
 
  • #4
rock.freak667
Homework Helper
6,230
31
You should assume that she stated at rest.
 
  • #5
26
0
You should assume that she stated at rest.
but it starts when she finishes the race. so i should have her initial velocity be the speed she finishes the race at, no?
 
  • #6
26
0
You are on the right lines with the second formula you posted. Do you know a rearrangement of it? You need to find a formula with initial and final velocities, time and displacement.
i don't. >.<
 
  • #7
8
0
but it starts when she finishes the race. so i should have her initial velocity be the speed she finishes the race at, no?
Correct.

Do you have a text book with a section on kinematics? Or even have a look here...
http://en.wikipedia.org/wiki/Equations_of_motion#Motion_equation_1"

Remember, you're trying to get something with no a term. In earlier physics, kinematics equations are usually presented "as is" but it will be useful for you to understand how you can move between them to help you solve problems. If you can understand that, you can get away with forgetting one in an exam, as you can derive it at the time.

Pick one you think looks right and plug in the information. Have a go and then we'll compare answers ;-)
 
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