Relative velocity of two cars

  1. How long does it take an automobile traveling in the left lane at 60.0 km/hr to pull along side a car traveling in the right lane at 40.0 km/hr if the cars' front bumpers are initially 100 m apart?

    I know the velocity of the car in the left is 60 and the velocity of the car on the right is 40. I also know it's imperative to find the velocity of the left car in relation to the right car in order to compare their distances. I found this to be 20 km/hr. What I don't know, however, is how to find out how far the car on the right goes before the car on the left reaches it. I know time is distance divided by velocity, so I figure that if I can find this distance and divide it by 20 km/hr, I'll have the time.

    I also know the answer in the back of the book .. 18.0 seconds. The question is, how do I get it?
  2. jcsd
  3. catchup time in hrs = 100 m/(20 km/hr) = .1 km/(20 km/hr) = ?

    catchup time in secs = ?
  4. HallsofIvy

    HallsofIvy 40,201
    Staff Emeritus
    Science Advisor

    You don't need to know that. The whole point of finding the speed of one car relative to another is to be able to use the distance between them as the distance- and you are given that. You just have to be careful to convert from km to m and from hr to sec.
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