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Math Help - Direct Proportions

  1. #1
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    Direct Proportions

    I'm stuck on this math problem:

    "The stopping distance d of an automobile is directly proportional to the square of its speed v. A car required 75 feet to stop when its speed was 60 miles per hour. Find a mathematical model that gives the stopping distance d in terms of its speed v"

    I'm not sure how to approach this problem. I tried plugging converting the 60 mph to ft (318,800ft) and then plugging it into D=KV^2, and then solving for K to get the constant term needed to write the equation.

    Whenever I solve K, I get "7.37946E-10", and plug it into D=(7.37946E-10)(V^2), which is apparently wrong according to the program I am using. Any help?
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  2. #2
    Forum Admin topsquark's Avatar
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    Quote Originally Posted by JoeF107 View Post
    I'm stuck on this math problem:

    "The stopping distance d of an automobile is directly proportional to the square of its speed v. A car required 75 feet to stop when its speed was 60 miles per hour. Find a mathematical model that gives the stopping distance d in terms of its speed v"

    I'm not sure how to approach this problem. I tried plugging converting the 60 mph to ft (318,800ft) and then plugging it into D=KV^2, and then solving for K to get the constant term needed to write the equation.

    Whenever I solve K, I get "7.37946E-10", and plug it into D=(7.37946E-10)(V^2), which is apparently wrong according to the program I am using. Any help?
    Well, mph is "miles per hour." The speed you want is in ft/s, not feet. I'm getting your v to be 88 ft/s. That should fix things up for you.

    -Dan
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  3. #3
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    Quote Originally Posted by topsquark View Post
    Well, mph is "miles per hour." The speed you want is in ft/s, not feet. I'm getting your v to be 88 ft/s. That should fix things up for you.

    -Dan
    Hmmm...

    I got d=(v^2)(.009685), which the program is still telling me is wrong O_o.

    However, I was able to use that exact same formula to correctly solve for a distance given a velocity 104 ft/s. do you think I am still doing something wrong, or do to think there may be a glitch in the program I'm using.
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