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    (Original post by uberteknik)
    There are two clues.

    The first is the word 'average'. It means you don't need to do anything complicated and all the information you need to solve is included at the start with the description. That is also iterated by the question leading us to the 'centre of mass' and also the dimensions referenced to the centre of mass. So we cannot do anything without using that information.

    Conceptually, the diver jumps from the board and must accelerate under the force of gravity. He will achieve a maximum vertical speed up to the point he hits the water. At that instant, the water upthrust acts to decelerate him. He will then decelerate until at some depth below the surface, his downwards velocity is arrested.

    The key here stated in the question is 'Force' and immediately you know that force = mass x acceleration. Upthrust acts in opposition to gravity.

    The second clue is that word 'average'. which means use the centre of mass as the reference for all calculations.


    You should have by now realised that this is nothing more than a momentum question which needs SUVAT to solve:

    1) find the final velocity of the diver at the point his centre of mass hits the water. (centre of mass because the question says to use the average upthrust).

    2) use that calculated velocity as the initial velocity in calculating the deceleration experienced by the divers centre of mass within the distance he stops under the surface of the water. THIS IS THE PART OF THE SOLUTION THAT YOU SHOULD HAVE FIRST CONCEIVED AS NEEDING TO SOLVE. i.e. because we though of upthrust as f=ma and to get to the depth stated, there needs to be a deceleration. That immediately takes us to SUVAT and an appropriate equation needs the initial velocity. Hence to get there we need to solve part 1 above.

    3) Once you know that deceleration, we can use that force = mass x (-ve)acceleration equation to calculate the 'average' force required to stop him. i.e, that force is the average upthrust.

    See how you get on and come back.
    Ok so suvat for the final velocity when he touches the water

    a=9.8 s=3.2 u=0 v=v

    v²=u²+2as
    [/latex]v= \sqrt 19.6 \times 3.2[/latex]
    v=7.9m/s

    suvat for the bit under the water to find the acceleration under the water

    a=a s=1.6 v=0 u=7.9

    v²=u²+2as
    [/latex]0=7.9^2 +3.2a[/latex]

    a=-19.6 bc y'know you slow down in water ^-^

    then i did f=ma

    F=72x19.6(ignore negative)
    =1411.2N which is ecf in the answer but not the true correct answer where did i go wrong?
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    (Original post by thefatone)
    Ok so suvat for the final velocity when he touches the water

    a=9.8 s=3.2 u=0 v=v

    v²=u²+2as
    [/latex]v= \sqrt 19.6 \times 3.2[/latex]
    v=7.9m/s

    suvat for the bit under the water to find the acceleration under the water

    a=a s=1.6 v=0 u=7.9

    v²=u²+2as
    [/latex]0=7.9^2 +3.2a[/latex]

    a=-19.6 bc y'know you slow down in water ^-^

    then i did f=ma

    F=72x19.6(ignore negative)
    =1411.2N which is ecf in the answer but not the true correct answer where did i go wrong?
    Can you post the answer given by the mark scheme please?
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    (Original post by uberteknik)
    Can you post the answer given by the mark scheme please?
    2116.8N or 2100N
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    you can succeed by not believing you are failing. persevere, practice papers and get your teacher to mark them and give you feedback on improvements
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    (Original post by gloria1)
    you can succeed by not believing you are failing. persevere, practice papers and get your teacher to mark them and give you feedback on improvements
    I'm not trying to cast away your advice here

    but it's sorta hard not to get demotivated and depressed when you consistently get like U grades on every physics test. A big fat U in front of you when you get your test handed back..... the evidence is there, being ignorant towards that won't do me any good.

    If you read my responses which have all been the same since everyone told me to do this, it's i can't do papers since i don't understand how to do the questions, the teacher is getting his soul ripped out by my ineptitude and stupidness and he just can't make me understand. The mark scheme sometimes doesn't make any sense to me so i can't really do much about that.
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    (Original post by thefatone)
    2116.8N or 2100N
    Ahhhh. OK. It makes things difficult without a diagram. The horizontal distance comes into play methinks. I will have another look and get back.
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    (Original post by uberteknik)
    Ahhhh. OK. It makes things difficult without a diagram. The horizontal distance comes into play methinks. I will have another look and get back.
    Suvat makes much more sense to me really not what the model answers here are.....

    in any case if you wanted to know here's what the mark scheme says


     GPE_{lost} = Work\ Done\ against\ water





mg\Detla h= fs





f=\dfrac{mg\Delta h}{s}





f=\dfrac{72 \times 9.8 \times [3.2+1.6]}{1.6}





f=2117N

    3.2+1.6 is the total distance fallen through by the person
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    (Original post by thefatone)
    I'm not trying to cast away your advice here

    but it's sorta hard not to get demotivated and depressed when you consistently get like U grades on every physics test. A big fat U in front of you when you get your test handed back..... the evidence is there, being ignorant towards that won't do me any good.

    If you read my responses which have all been the same since everyone told me to do this, it's i can't do papers since i don't understand how to do the questions, the teacher is getting his soul ripped out by my ineptitude and stupidness and he just can't make me understand. The mark scheme sometimes doesn't make any sense to me so i can't really do much about that.
    perhaps try a new way of learning such as looking at websites or watching how some the equations are being solved on youtube. Ask people in your class how they go about answering the questions.
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    (Original post by gloria1)
    perhaps try a new way of learning such as looking at websites or watching how some the equations are being solved on youtube. Ask people in your class how they go about answering the questions.
    the same way the teacher explains to me which doesn't help .-.

    youtube and website don't help me much, i generally look at it find it boring then procrastinate so it's best for me not to do that.
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    (Original post by thefatone)
    Ok so suvat for the final velocity when he touches the water

    a=9.8 s=3.2 u=0 v=v

    v²=u²+2as
    [/latex]v= \sqrt 19.6 \times 3.2[/latex]
    v=7.9m/s

    suvat for the bit under the water to find the acceleration under the water

    a=a s=1.6 v=0 u=7.9

    v²=u²+2as
    [/latex]0=7.9^2 +3.2a[/latex]

    a=-19.6 bc y'know you slow down in water ^-^

    then i did f=ma

    F=72x19.6(ignore negative)
    =1411.2N which is ecf in the answer but not the true correct answer where did i go wrong?

    Apologies for the delay, I had a previous engagement last night and could not stay online any longer.

    Back to the question.

    What you have done so far is completely correct but there is an omission:

    The force you calculated (1411.2N) has neglected the force due to gravity which continues to act on the object even after the diver enters the water.

    So the total upthrust force must include the effect of that continued downwards gravitational acceleration which must also be overcome to stop the diver sinking.

    Hence, mass x gravitational acceleration must be added to our final calculation.

    i.e. mg = 72 x 9.81 = 706.32N must be added to 1411.2N to give 2117.52N


    All of the information you already know. That last bit on gravitational acceleration is something which catches a lot of students - you are not alone!

    Now that you know what went wrong, the learning here is to be vigilant and check to see if all forces are accounted for in the answer. Take comfort (and be confident) that you are not missing anything vital in your understanding with this one other than being vigilant about the full 'picture' when answering.

    Next example.
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    (Original post by thefatone)
    I'm getting a U if i don't pull my briny pants up and get smarter.

    But how?

    The teacher writes the answer and puts plenty of detail into them but i don't understand his answer, i don't understand WHY.

    what can i do?

    I can be ignorant and only remember so many answer and only so many way to do questions but i can't do it forever.
    Read the question, but this time think smart, I used to think dumb now I think smart I am now Quantum Mechanic
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    (Original post by Quantum Mechanic)
    Read the question, but this time think smart, I used to think dumb now I think smart I am now Quantum Mechanic
    Implying i'm dumb is
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    Show
    a correct assumption unfortunately :/

    however just reading the question is no good because i don;'t understand what i'm supposed to do.
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    (Original post by uberteknik)
    Apologies for the delay, I had a previous engagement last night and could not stay online any longer.

    Back to the question.

    What you have done so far is completely correct but there is an omission:

    The force you calculated (1411.2N) has neglected the force due to gravity which continues to act on the object even after the diver enters the water.

    So the total upthrust force must include the effect of that continued downwards gravitational acceleration which must also be overcome to stop the diver sinking.

    Hence, mass x gravitational acceleration must be added to our final calculation.

    i.e. mg = 72 x 9.81 = 706.32N must be added to 1411.2N to give 2117.52N


    All of the information you already know. That last bit on gravitational acceleration is something which catches a lot of students - you are not alone!
    Right i see that's why i got it wrong .-. oh well next time then...
 
 
 
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