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    "A freely falling object on Earth has a speed of 5.0 m/s.
    After falling a further 20 m its speed is""

    The answer is supposed to be +25.
    But Im confused about the use of positive/negative in the equations....

    Should I take the initial velocity as -5 because velocity is a vector and negative is the convention for down/left. If I were to take the 5 as positive, wouldn't I then have to take "a" as 9.8 and not -9.8?

    Id appreciate if someone could do this problem with the correct signs and get 25 ( or if you take the initial velocity as negative, then please show me how you got -25 )



    Also, suppose we had a book lying on the ground. Could you state the two Newton pairs of forces acting on the book?

    thanks!
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    In these questions, there is no hard and fast rule about which direction is positive and which is negative. You can set it yourself, depending on the situation, to make the question easier to answer. If something is being dropped, down is the obvious positive. If it is being thrown upwards, then up is the obvious positive.

    In this case, the question says that the object is falling down. Take down as positive. So u = 5 m s-1, and as you say, a = 9.8 m s-2 (positive). s= 20 m. We can therefore work out v.

    I don't make the answer 25 m s-1, however, unless you meant to say that it falls a further 30 m rather than 20 m.
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    (Original post by princejan7)
    "A freely falling object on Earth has a speed of 5.0 m/s.
    After falling a further 20 m its speed is""

    The answer is supposed to be +25.
    But Im confused about the use of positive/negative in the equations....

    Should I take the initial velocity as -5 because velocity is a vector and negative is the convention for down/left. If I were to take the 5 as positive, wouldn't I then have to take "a" as 9.8 and not -9.8?

    Id appreciate if someone could do this problem with the correct signs and get 25 ( or if you take the initial velocity as negative, then please show me how you got -25 )



    Also, suppose we had a book lying on the ground. Could you state the two Newton pairs of forces acting on the book?

    thanks!
    Okay dokey.

    V^2=U^2+2AS

    U=5
    A=9.81
    S=20

    Hope this helps!
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    Yeah as long as you are consistent with the +ve and -ve directions, it shouldn't matter. To clarify you could put your answer like this:

    -20m/s i.e. 20m/s downwards (or upwards) depending on which way you took the -ve to be.

    Often when considering falling objects, it's easier to consider down as +ve, and objects that take a projectile motion, consider up as +ve.
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    The answer is not 25 as the person above said

    v^2=u^2+2as

    v^2=417

    v=20.42ms^-1
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    I wasn't providing an answer; I was commenting on his use of direction.
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    (Original post by princejan7)
    "A freely falling object on Earth has a speed of 5.0 m/s.
    After falling a further 20 m its speed is""

    The answer is supposed to be +25.
    But Im confused about the use of positive/negative in the equations....

    Should I take the initial velocity as -5 because velocity is a vector and negative is the convention for down/left. If I were to take the 5 as positive, wouldn't I then have to take "a" as 9.8 and not -9.8?

    Id appreciate if someone could do this problem with the correct signs and get 25 ( or if you take the initial velocity as negative, then please show me how you got -25 )



    Also, suppose we had a book lying on the ground. Could you state the two Newton pairs of forces acting on the book?

    thanks!
    Hi, I'd just thought I'd add. The question says find the speed, not velocity.
    Velocity is sign dependent, speed is always positive as it's only a scalar quantity.
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    (Original post by l4ith)
    I wasn't providing an answer; I was commenting on his use of direction.
    I meant the person above you
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    (Original post by Freier._.lance)
    Hi, I'd just thought I'd add. The question says find the speed, not velocity.
    Velocity is sign dependent, speed is always positive as it's only a scalar quantity.
    But I should still treat displacement and acceleration as vectors, right? Should they be negative?
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    (Original post by princejan7)
    But I should still treat displacement and acceleration as vectors, right? Should they be negative?
    Yes treat them as vectors. When they ask for "speed" they mean just the value of the velocity (magnitude) not the direction
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    (Original post by princejan7)
    But I should still treat displacement and acceleration as vectors, right? Should they be negative?
    Yep! Exactly what Soup ^^ (above this post) said.
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    (Original post by Pangol)
    In these questions, there is no hard and fast rule about which direction is positive and which is negative. You can set it yourself, depending on the situation, to make the question easier to answer. If something is being dropped, down is the obvious positive. If it is being thrown upwards, then up is the obvious positive.

    In this case, the question says that the object is falling down. Take down as positive. So u = 5 m s-1, and as you say, a = 9.8 m s-2 (positive). s= 20 m. We can therefore work out v.

    I don't make the answer 25 m s-1, however, unless you meant to say that it falls a further 30 m rather than 20 m.
    Precisely this.
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    Well you gotta remember that everything is relative to your frame of reference

    theres no right answer, if you choose that going towards the earth is negative, then its negative, but if you choose it to be +ve then your answer is positive.

    usually we use the frame of reference of the earth, in which case it'd be -ve.

    you could always just say 20 m/s ( or whatever) down which also solves the problem
 
 
 
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