Hey there! Sign in to join this conversationNew here? Join for free
    • Thread Starter
    Offline

    14
    ReputationRep:
    Hello

    any tips about how i can start this? or at least how i can visualise it?

    Name:  bmatqqq.PNG
Views: 60
Size:  7.7 KB


    Thanks
    • Community Assistant
    Offline

    15
    ReputationRep:
    Community Assistant
    Half a minute to travel 1 km, how fast is that if the courier is the only thing moving?
    3 minutes to travel 1 km, how fast is that if the courier is the only thing moving?
    The average of these speeds will be the speed of the courier. I think
    • Thread Starter
    Offline

    14
    ReputationRep:
    (Original post by an_atheist)
    Half a minute to travel 1 km, how fast is that if the courier is the only thing moving?
    3 minutes to travel 1 km, how fast is that if the courier is the only thing moving?
    The average of these speeds will be the speed of the courier. I think
    Thank you for taking part.
    it would lead to the answer, but didn't it say constant speed?
    what do you think?
    Offline

    14
    ReputationRep:
    Daniel Atieh
    This is relative motion.
    Let v_{A} be the velocity of the convoy and v_{B} the velocity of the courier.
    When the couriers travels in the opposite direction to the convoy, the speed of the convoy relative to the courier is |v_{A}|+|v_{B}| and when the courier travels in the same direction the speed of the convoy relative to the courier is |v_{B}|-|v_{A}|

    Note:
    |v_{A}| is the speed of the convoy and |v_{B}| is the speed of the courier.

    In both instances one kilometre is travelled.

    Hope this helps.
    Offline

    14
    ReputationRep:
    Hence we can deduce the following *in spoiler*
    Spoiler:
    Show
    |v_{A}|+|v_{B}|=120
    |v_{B}|-|v_{A}|=20
    2|v_{B}|=140
    \therefore |v_{B}|=70
    • Community Assistant
    Offline

    15
    ReputationRep:
    Community Assistant
    (Original post by Daniel Atieh)
    Thank you for taking part.
    it would lead to the answer, but didn't it say constant speed?
    what do you think?
    That's why you average.
    It returns the same value as the previous poster
    Or, he travels 2km in 3.5 minutes, find the speed from that
    Offline

    14
    ReputationRep:
    (Original post by an_atheist)
    That's why you average.
    It returns the same value as the previous poster
    Or, he travels 2km in 3.5 minutes, find the speed from that
    This method would give you an incorrect answer.
    Offline

    3
    ReputationRep:
    The average method doesn't work for an interesting reason. Because the courier spends much longer catching up with the front of the convoy, he will be travelling at the slower speed (relative to the convoy) for a longer period of time. But when you average the two speeds, you're implicitly giving both legs an equal time-weighting.

    E.g.1 I spend 1 hour walking at 2 mph, then I spend 1 hour running at 5 mph. Average speed = (2+5)/2. It is okay to average here because both speeds lasted the same amount of time.

    E.g.2 I walk 1 mile at 2 mph, then run 1 mile at 5 mph. Average speed =/= (2+5)/2 ! It is not okay to average - even though the distance travelled is the same, the time spent doing each activity is not. Correct answer, work it out the long way: time spent walking = 0.5 hours; time spend running = 0.2 hours; total time spent = 0.7; average speed = total distance / total time = 2/0.7 = ~= 2.8 mph. Which is less than 3.5 mph (the average of 2 and 5).

    Be wary of shortcuts!
 
 
 
  • See more of what you like on The Student Room

    You can personalise what you see on TSR. Tell us a little about yourself to get started.

  • Poll
    Would you like to hibernate through the winter months?
    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
  • See more of what you like on The Student Room

    You can personalise what you see on TSR. Tell us a little about yourself to get started.

  • 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

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