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    Can someone please explain this entire question??
    So for part a) why would you use the crate (mass, resistant etc) and not the whole lift when the questions says the lift??
    And for part b) why would you not include the resistance of the crate (743N) when you've included the mass of the crate?
    And why do you ONLY use the crate for part a but then use the entire thing for part b when both questions ask about the lift??

    So stuck and confused.. 😭😭😭
    Please help
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    (Original post by CrystalPark)
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    Can someone please explain this entire question??
    So for part a) why would you use the crate (mass, resistant etc) and not the whole lift when the questions says the lift??
    Note that the crate and lift must have the same acceleration, and that the tension in the cable is unknown, so it's best to compute the acceleration of crate instead. Also note that your answer ignored the fact that there was an upwards tension force from the cable, so your equation was incorrect anyway.

    That said, if you fix it by subtracting T from the LHS then...

    And for part b) why would you not include the resistance of the crate (743N) when you've included the mass of the crate?
    And why do you ONLY use the crate for part a but then use the entire thing for part b when both questions ask about the lift??

    So stuck and confused.. 😭😭😭
    Please help
    ...You can now work out T using the corrected version of your equation from (a).

    The reason why is because you have two unknowns - the acceleration of the lift, and the tension exerted on the lift by the cable - so in order to find these, you need two equations. You use the crate to find the acceleration from the first equation because it has nothing to do with T (part (a)), then you use the acceleration to find the tension by considering the lift (part (b)).
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    (Original post by CrystalPark)
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    Can someone please explain this entire question??
    So for part a) why would you use the crate (mass, resistant etc) and not the whole lift when the questions says the lift??
    And for part b) why would you not include the resistance of the crate (743N) when you've included the mass of the crate?
    And why do you ONLY use the crate for part a but then use the entire thing for part b when both questions ask about the lift??

    So stuck and confused.. 😭😭😭
    Please help
    a) Both the lift and the crate are moving with the same acceleration so you could resolve for either the lift or the crate and get the correct acceleration.

    The problem is that if you resolve for the lift, you have to include tension, which is unknown. You haven't included tension which is where your mistake lies.

    Since tension is unknown, you have to resolve for the crate instead to find the acceleration.


    b) This time you're looking for a force exerted on the lift so you have to resolve for the lift.

    When you say the resistance of 743N, do you actually mean a normal reaction of 473N from the lift on the crate? Since for b) you're resolving forces acting on the lift, you don't include this one.


    For lift and similar questions, you should always draw two seperate force diagrams, one for the particle and one for the lift. Then decide on which diagram to use depending on the question.

    EDIT : A few seconds too late!
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    (Original post by notnek)
    a) Both the lift and the crate are moving with the same acceleration so you could resolve for either the lift or the crate and get the correct acceleration.

    The problem is that if you resolve for the crate, you have to include tension, which is unknown. You haven't included tension which is where your mistake lies.

    Since tension is unknown, you have to resolve for the crate instead to find the acceleration.


    b) This time you're looking for a force exerted on the lift so you have to resolve for the lift.

    When you say the resistance of 743N, do you actually mean a normal reaction of 473N from the lift on the crate? Since for b) you're resolving forces acting on the lift, you don't include this one.


    For lift and similar questions, you should always draw two seperate force diagrams, one for the particle and one for the lift. Then decide on which diagram to use depending on the question.

    EDIT : A few seconds too late!
    Thank yo so much!! That was a perfect explanation👌🏻👌🏻
    For part b) when they ask for the force acting on the lift does that mean the Tension??
    Also when I use those numbers and take the acceleration to be 1.2 I got an answer of 2043... Not 2000 nor 2040 (which are the given answers)
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    (Original post by Farhan.Hanif93)
    Note that the crate and lift must have the same acceleration, and that the tension in the cable is unknown, so it's best to compute the acceleration of crate instead. Also note that your answer ignored the fact that there was an upwards tension force from the cable, so your equation was incorrect anyway.

    That said, if you fix it by subtracting T from the LHS then...


    ...You can now work out T using the corrected version of your equation from (a).

    The reason why is because you have two unknowns - the acceleration of the lift, and the tension exerted on the lift by the cable - so in order to find these, you need two equations. You use the crate to find the acceleration from the first equation because it has nothing to do with T (part (a)), then you use the acceleration to find the tension by considering the lift (part (b)).
    But when I use those numbers and the acceleration as 1.2 I get the tension to be 2043... Not 2000 nor 2040..
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    (Original post by CrystalPark)
    Thank yo so much!! That was a perfect explanation👌🏻👌🏻
    For part b) when they ask for the force acting on the lift does that mean the Tension??
    Yes.
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    (Original post by CrystalPark)
    But when I use those numbers and the acceleration as 1.2 I get the tension to be 2043... Not 2000 nor 2040..
    2043 is correct for b).
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    (Original post by notnek)
    2043 is correct for b).
    Ahh that's a relief
    Thank you so much!!
    How comes it says 2000 or 2040 though??
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    (Original post by CrystalPark)
    Ahh that's a relief
    Thank you so much!!
    How comes it says 2000 or 2040 though??
    The answer uses g and so you must round to 3sf or 2sf to get the marks.

    This is written at the start of M1 papers that most students forget (like I just did!):

    "Whenever a numerical value of g is required, take g = 9.8 and give your answer to either two significant figures or three significant figures."

    The model answers posted on TSR had 2043 as the answer - this would have lost 1 mark.
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    Ohh I remember doing this question, by considering the whole life for a, you're making your life difficult, work with what you are given Name:  image.jpg
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    (Original post by CrystalPark)
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    Can someone please explain this entire question??
    So for part a) why would you use the crate (mass, resistant etc) and not the whole lift when the questions says the lift??
    And for part b) why would you not include the resistance of the crate (743N) when you've included the mass of the crate?
    And why do you ONLY use the crate for part a but then use the entire thing for part b when both questions ask about the lift??

    So stuck and confused.. 😭😭😭
    Please help
 
 
 
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