Finding Initial Velocity from Changes in Time and Distance

[insomniac950]

Homework Statement

Driving along a crowded freeway, you notice that it takes a time t to go from one mile marker to the next. When you increase your speed by 8.4 mi/h, the time to go one mile decreases by 13 s. What was your original speed? So we know the question is asking for velocity.

Homework Equations

vi=vf+a*t

The Attempt at a Solution

I just can't seem to get the correct setup to solve this problem.

 

[ideasrule]

Acceleration is neither given nor requested, so vi=vf+a*t doesn't need to be used. What you need is only d=vt. Write out d=vt for both the initial speed and the final speed (of course, you'll have to substitute the appropriate variables for d, v, t), solve the two equations, and you'll be done.

 

[insomniac950]

what am i supposed to put as my initial speed? I rearranged the equation and did t=v/d and got .615 for the speed given to me. Is my initial speed zero?

 

[ideasrule]

what am i supposed to put as my initial speed? I rearranged the equation and did t=v/d and got .615 for the speed given to me. Is my initial speed zero?

The question asks for the initial speed, so it's an unknown. Call it "x", or "v".

By the way, t=d/v, not v/d. You also can't get 0.615 or any other value for t because v isn't given; you have to solve for it.