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Kinematics Eq'n Problem

  • Thread starter harujina
  • Start date
  • #1
77
1

Homework Statement


A student riding a bicycle begins to go downhill and accelerates at a rate of 1.8m/s2. If the acceleration lasts for 2.4s, and the final speed of the bicycle is 10.2m/s, at what speed was he initially travelling?

a = 1.8
t = 2.4
vf = 10.2

vi = ?

Homework Equations


a = vf - vi / t


The Attempt at a Solution


I first isolated vi to solve for the problem and got : vi = vf-a/t and then plugged in the variables but it didn't work.
 

Answers and Replies

  • #2
SteamKing
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a = (vf - vi) / t is the correct equation. Parentheses make a difference. Also, using correct algebra.

However, this equation is good only over short time intervals.
 
  • #3
collinsmark
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Homework Statement


A student riding a bicycle begins to go downhill and accelerates at a rate of 1.8m/s2. If the acceleration lasts for 2.4s, and the final speed of the bicycle is 10.2m/s, at what speed was he initially travelling?

a = 1.8
t = 2.4
vf = 10.2

vi = ?

Homework Equations


a = vf - vi / t
Be careful with your parenthesis, they make a difference.

the correct equation for uniform acceleration is

a = ( vf - vi )/t

The Attempt at a Solution


I first isolated vi to solve for the problem and got : vi = vf-a/t and then plugged in the variables but it didn't work.
There's an algebra mistake in there somewhere. It shouldn't contain the term a divided by t. It's something else.

[Edit: SteamKing beat me to the response.]
 
  • #4
77
1
Ok, then is this correct?:

a = (vf - vi)/t
at = vf - vi
at + vi = vf
vi = vf - at
 
  • #5
SteamKing
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Looks Good
 
  • #6
77
1
Ohh, I was so confused since I thought I had to divide t from both sides since it was a*t...
Anyways, thank you!!
 

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