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Homework Help: Displacement and Average Velocity of a Whale

  1. Jan 24, 2009 #1
    1. The problem statement, all variables and given/known data
    A whale swims due east for a distance of 8.0 km in a time of 2.50 hours. It then turns around and moves west for 2.0 km on a time of 0.50 h. It turns around again and heads 4.0 km due east in a time of 1.00h.

    2. Relevant equations
    a. What is the total distance travelled by the whale?
    b. What is the total displacement?
    c. Calculate the average speed for the entire trip.
    d. Calculate the average velocity for the entire trip.

    3. The attempt at a solution
    I only got a and c correct, for a I put 14 Km and for c I put 3.5 km/ hr. I tried to do the question again and I drew out the whale swimming, and guessed that the displacement is 8 Km. This was a previous test, and the teacher didn't go over it! I'm trying to study for my exam ;). Thanks.
  2. jcsd
  3. Jan 24, 2009 #2
    The total displacement is the final position minus the initial position. So,

    Total displacement = 8 km - 2 km + 4 km.

    Think of the displacements as vectors then add these vectors.

    The average velocity is the total displacement divided by the total time traveled.
  4. Jan 24, 2009 #3
    Chrisk, it wouldn't be 8 km?

    Because the whale goes east 8 and then west 2 and then east 4.

    But, I could be wrong! I don't understans displacement all that well. So, can you clarify? TY!
  5. Jan 24, 2009 #4
    Think of total displacement as the final stopping point minus the starting point. For example, you tell someone who is a room with you that you are going to the store down the street. That person leaves the room before you go to the store. You arrive back from the store empty handed and find the person is not in the room. A little while later, the person enters the room and asks if you went to the store. You say you did but that person says you didn't go anywhere because you are at the starting point. So, your total displacement is zero! If this person happened to see you standing just outside the room talking to someone after you went to the store, that person would think you only moved from the room to just outside the room.
  6. Jan 24, 2009 #5

    8-2+4 is ur answer
    Please explain how are you getting 8? It will tell us what your confusion is.

    displacement: final position - initial position
    Nothing in between matters.

    You are at your house and you are going to get coffee from starbucks. You want the displacement from your house to starbucks.
    Its also easier to look at them on a number line

    -5 -4 -3 -2 -1 0 1 2 3 4 5
    Let say your house is your initial (starting) and its located at 1
    Starbucks is your final and its located at 3
    As you can see starbucks is very close to your house since they are everywhere
    You get out of your house and you head west for 2 miles and you reach starbucks. You're there

    Now the math 3-1=2 miles
    2 miles is your answer.

    On the other hand let say you are going to starbucks. You go 3 miles East to pick up your 1st friend, You go another 1 mile to pick up your other friend and then you all go to starbucks together.

    -5 -4 F F -1 0 H 2 S 4 5

    the 3 and the 1 miles you went to drive your friend dont matter. Its still final - initial
    3-1 and your displacement is still 2 miles.

    Hope that helps out a little bit. I should warn you though. The negatives on the number line are not always on the left.

    it could be backwards

    5 4 3 2 1 0 -1 -2 -3 -4 -5
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