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Physics: distance, rate, time problem

  1. Oct 15, 2006 #1
    Hello, having trouble figuring this one out. Its a general physics question.

    question: Julie drives 100 mi to Grandmother's house. On the way to Grandmother's Julie drives half the distance at 40 mph and half the distance at 60 mph. On her return trip, she drives half the time at 40 mph and half the time at 60 mph.

    a. What is Julie's average speed on the way to Grandmothers house?
    b. What is her average speed on the return trip?

    answers: a:48mph b: 50 mph

    my work: i've tried to come up with these answers and i'm not sure how to get the answer for a.

    for b, all i did was use this: d = r*t
    so, 100 = ((40 + 60)t)/2
    which gave me 50. so 50mph?

    i'm confused, i'm not conviced that i'm attempting these correctly, but i know there not challenging when you set them up right.

    any help please. Thankyou.
     
  2. jcsd
  3. Oct 15, 2006 #2
    For a) You do need to know that average speed = (Total distance) divided by (total time taken). To find the total time taken, You do need to find the individual time taken for each section of the journey. Total distance is fixed at 100. To find total time.. Find the time taken while julie is travelling at 40 mph and at 60 mph then add them together.

    Hope this helps.
     
    Last edited: Oct 15, 2006
  4. Oct 16, 2006 #3
    okay i understand now. just as long as i solved the part b above correctly also.

    i just did this: 50 = 40t
    so trip is 1.25 hrs long

    and 50 = 60t
    so trip is .8333333.. hrs long

    and then used the total trip time equation: 100 = r(1.25 + .83333..)

    and got r = 48.00000... =D

    thanks.
     
    Last edited: Oct 16, 2006
  5. Oct 16, 2006 #4
    yep.that's correct now.

    However, i do not really understand part b, what does the question mean by "half the time"? I think you would need to consult ur lecturer on that.
     
    Last edited: Oct 16, 2006
  6. Oct 16, 2006 #5
    I think half the time mean for ## miles she driving at 40mph is half the time of the total time and another ## miles at half the time of total time in 60mph.

    for instant if the trip is 100 miles and to complete that trip it takes a total of let say 60 minute so for 30 minute she drove 40 mph for a certain amount of distance and then for the next 30 minute she drove 60 mph to complete that 100 miles and 60 minute. thats what i think but not very sure.
     
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