Hey there! Sign in to join this conversationNew here? Join for free
x Turn on thread page Beta
    • Thread Starter
    Offline

    3
    ReputationRep:
    At 2pm the coastguard spots a rowing dinghy 500m due South of his observation point. The dinghy has constant velocity(2\mathbf{i}+3\mathbf{j}ms^{-1}

    a)Find in terms of t, the position vector of the dinghy t seconds after 2pm
    I got 2t\mathbf{i}+(-500+3t)\mathbf{i}


    b)Find the distance of the dinghy from the observation point at 2:05pm

    Help pls on part b no idea what to do talk me through what to do.
    • Very Important Poster
    Offline

    21
    ReputationRep:
    Very Important Poster
    (Original post by thefatone)
    At 2pm the coastguard spots a rowing dinghy 500m due South of his observation point. The dinghy has constant velocity(2\mathbf{i}+3\mathbf{j}ms^{-1}

    a)Find in terms of t, the position vector of the dinghy t seconds after 2pm
    I got 2t\mathbf{i}+(-500+3t)\mathbf{i}


    b)Find the distance of the dinghy from the observation point at 2:05pm

    Help pls on part b no idea what to do talk me through what to do.
    Draw a diagram and see if it becomes clearer then :borat:
    • Thread Starter
    Offline

    3
    ReputationRep:
    (Original post by SeanFM)
    Draw a diagram and see if it becomes clearer then :borat:
    this has done

    absolutely nothing for me i'm not seeing anything
    • Very Important Poster
    Offline

    21
    ReputationRep:
    Very Important Poster
    (Original post by thefatone)
    this has done

    absolutely nothing for me i'm not seeing anything
    What does your diagram look like?

    Can you see any way of finding the distance between where the dinghy isand the observation point?
    • Thread Starter
    Offline

    3
    ReputationRep:
    (Original post by SeanFM)
    What does your diagram look like?

    Can you see any way of finding the distance between where the dinghy isand the observation point?
    • Very Important Poster
    Offline

    21
    ReputationRep:
    Very Important Poster
    (Original post by thefatone)
    What lovely paper

    You have fallen into a trap - look carefully at the units in the question and then the time it asks you where the dingy is in the question.

    But with that diagram you can still work out the method of finding the distance between the position and the origin.

    How do you find the distance between the origin and the position? (You can think of them as x and y co-ordinates if you like).

    Hint:
    Spoiler:
    Show
    The distance is represented by a line that you already have on your diagram.
    • Thread Starter
    Offline

    3
    ReputationRep:
    (Original post by SeanFM)
    What lovely paper

    You have fallen into a trap - look carefully at the units in the question and then the time it asks you where the dingy is in the question.

    But with that diagram you can still work out the method of finding the distance between the position and the origin.

    How do you find the distance between the origin and the position? (You can think of them as x and y co-ordinates if you like).

    Hint:
    Spoiler:
    Show
    The distance is represented by a line that you already have on your diagram.
    no idea, 100% clueless
    • Very Important Poster
    Offline

    21
    ReputationRep:
    Very Important Poster
    (Original post by thefatone)
    no idea, 100% clueless
    Okay, let's look at an example. If you had the point (3,4) on a graph and I asked you to find the distance between that and the origin, how would you do it?

    Hint:
    Spoiler:
    Show
    You can say that the origin has co-ordinates (0,0) if that helps.
    • Thread Starter
    Offline

    3
    ReputationRep:
    (Original post by SeanFM)
    Okay, let's look at an example. If you had the point (3,4) on a graph and I asked you to find the distance between that and the origin, how would you do it?

    Hint:
    Spoiler:
    Show
    You can say that the origin has co-ordinates (0,0) if that helps.
    pythagoras
    • Very Important Poster
    Offline

    21
    ReputationRep:
    Very Important Poster
    (Original post by thefatone)
    pythagoras
    Correct :borat: so, how does that help with anything?
    • Thread Starter
    Offline

    3
    ReputationRep:
    (Original post by SeanFM)
    Correct :borat: so, how does that help with anything?
    it doesn't
    • Very Important Poster
    Offline

    21
    ReputationRep:
    Very Important Poster
    (Original post by thefatone)
    it doesn't
    There is not much more I can say without giving away the answer so I'll go over what I've said before:

    1. Remember to look at the units in the question and correct your position vector at 2:05.

    2. Try and find the distance between the position vector and the origin using ...
    • Thread Starter
    Offline

    3
    ReputationRep:
    (Original post by SeanFM)
    There is not much more I can say without giving away the answer so I'll go over what I've said before:

    1. Remember to look at the units in the question and correct your position vector at 2:05.

    2. Try and find the distance between the position vector and the origin using ...
    where's the 90° angle?
    • Thread Starter
    Offline

    3
    ReputationRep:
    (Original post by SeanFM)
    There is not much more I can say without giving away the answer so I'll go over what I've said before:

    1. Remember to look at the units in the question and correct your position vector at 2:05.

    2. Try and find the distance between the position vector and the origin using ...
    i don't understand the time aspect of things, do i count time in hours or seconds?
    • Very Important Poster
    Offline

    21
    ReputationRep:
    Very Important Poster
    (Original post by thefatone)
    where's the 90° angle?
    Well, where is the 90 degree angle between (0,0) and (3,4)?

    For your time question, look at the question and see if you can work it out.

    (I am deliberately giving you as little as possible so you can work it out for yourself )
    • Thread Starter
    Offline

    3
    ReputationRep:
    (Original post by SeanFM)
    Well, where is the 90 degree angle between (0,0) and (3,4)?

    For your time question, look at the question and see if you can work it out.

    (I am deliberately giving you as little as possible so you can work it out for yourself )
    between the x-axis and line parallel to the y-axis but i'm not seeing how this helps me do this question :/ i don't know if it's a right angled triangle.

    no idea, it's literally a guess i have no idea what i count time as maybe 5? 0.5? 0.005? i really don't know
    • Very Important Poster
    Offline

    21
    ReputationRep:
    Very Important Poster
    (Original post by thefatone)
    between the x-axis and line parallel to the y-axis but i'm not seeing how this helps me do this question :/ i don't know if it's a right angled triangle.

    no idea, it's literally a guess i have no idea what i count time as maybe 5? 0.5? 0.005? i really don't know
    (3,4) means that from (0,0) you go 3 to the right and 4 up. So you can draw a line 3 units to the right then 4 units up from that point, and then a straight line from (0,0) to (3,4) and there is your right angled triangle. The straight line represents the distance between the two points.

    I'll give you another hint for the time - 2:05 is 5 minutes after 2 o'clock, and your velocity vector is measured in metres per second.
    • Thread Starter
    Offline

    3
    ReputationRep:
    (Original post by SeanFM)
    (3,4) means that from (0,0) you go 3 to the right and 4 up. So you can draw a line 3 units to the right then 4 units up from that point, and then a straight line from (0,0) to (3,4) and there is your right angled triangle. The straight line represents the distance between the two points.

    I'll give you another hint for the time - 2:05 is 5 minutes after 2 o'clock, and your velocity vector is measured in metres per second.
    i see so the time t is 300 then? so i sub in 300 as t? .-.



    sooooo

    i get
    600\mathbf{i}+400\mathbf{j}?
    • Very Important Poster
    Offline

    21
    ReputationRep:
    Very Important Poster
    (Original post by thefatone)
    i see so the time t is 300 then? so i sub in 300 as t? .-.



    sooooo

    i get
    600\mathbf{i}+400\mathbf{j}?
    Correct. :borat: (as long you understand why, if you don't just say )

    Okay, you're almost there. What next?
    • Thread Starter
    Offline

    3
    ReputationRep:
    (Original post by SeanFM)
    Correct. :borat: (as long you understand why, if you don't just say )

    Okay, you're almost there. What next?
    good point what next? i have no idea we have distance... final distance, time, constant velocity so this means a=0

    soooo i'm done? 600\mathbf{i}+400\mathbf{j} is my final answer? no... pythagoras

    so \sqrt{600^2 +400^2} = 721 m thanks a ton
 
 
 
Reply
Submit reply
Turn on thread page Beta
Updated: April 24, 2016
Poll
Are you going to a festival?
Useful resources

Make your revision easier

Maths

Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

Equations

How to use LaTex

Writing equations the easy way

Student revising

Study habits of A* students

Top tips from students who have already aced their exams

Study Planner

Create your own Study Planner

Never miss a deadline again

Polling station sign

Thinking about a maths degree?

Chat with other maths applicants

Can you help? Study help unanswered threads

Groups associated with this forum:

View associated groups

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Write a reply...
Reply
Hide
Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.