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

    0
    ReputationRep:
    A car starts from rest the bottom of a slope. It accelerates up the slope for 8 seconds at 1.5ms^-2, then disengages the engine and coasts. If its acceleration is now -1ms^-1, find the time between the car's leaving the bottom of the slope and returning to it.

    I'm so confused, thanks for the help :confused:
    Offline

    20
    ReputationRep:
    What I have done is drawn a diagram (which you should always do);

    1st - draw a diagonal line going up and label the beginning A and the other end B.
    So, A --> B

    a = 1.5
    t = 8
    u = 0
    V = ?
    S = ?

    V=u+at
    v = 0 + 1.5(8)
    V = 12

    V2=U2+2as
    144 = 0 +2(1.5)s
    S = 48

    (See if you can work the rest out for yourself before opening spoiler)

    Spoiler:
    Show

    B --> A

    a = -1
    t = ?
    s = 48
    u = 0
    V = ?

    S = ut + 1/2at2
    48 = -t2
    48 = t2
    t = Sqrt 48

    t = 8+sqrt48

    I think this is right, however, try and find the answer first before taking my work for it. It also wouldn't hurt to get as second opinion on this. =l
    • Thread Starter
    Offline

    0
    ReputationRep:
    48 = -t2
    48 = t2
    How did you get from here to here?
    • Thread Starter
    Offline

    0
    ReputationRep:
    But according to the answer its wrong ? :/
    Offline

    0
    ReputationRep:
    Think of it as two situations, where there is constant acceleration, the car going up the slope and the car sliding due to g. I would form two us ate equations


    Posted from TSR Mobile
    Offline

    20
    ReputationRep:
    What's the answer?
    • Thread Starter
    Offline

    0
    ReputationRep:
    The answer says 35.5s
    • Study Helper
    Offline

    16
    ReputationRep:
    Study Helper
    (Original post by 3.1415927)
    The answer says 35.5s
    Do you know if that's a rounded answer or supposed to be exact?

    I get 35.4919 seconds approximately.
    Offline

    20
    ReputationRep:
    (Original post by davros)
    Do you know if that's a rounded answer or supposed to be exact?

    I get 35.4919 seconds approximately.
    Working?
    • Study Helper
    Offline

    16
    ReputationRep:
    Study Helper
    (Original post by Scienceisgood)
    Working?
    There may be a more optimum way, but I broke the motion into 3 stages:

    Stage 1: the car goes up the slope for time t1 = 8 seconds with acc 1.5ms^-2 for a distance s1, say.

    Stage 2: the car carries on going up the slope but is being decelerated at 1ms^-2 until it comes to rest. This takes a further t2 seconds, say, and covers an extra distance s2, say, upwards.

    Stage 3: the car starts from rest and is now sliding back down the slope with acceleration 1ms^-2 until it has covered the combined distance s1 + s2. This takes a further time t3, say.

    Our total time is now 8 + t2 + t3
    • Thread Starter
    Offline

    0
    ReputationRep:
    Yeah its probably rounded
 
 
 
  • 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?
  • 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.