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Math Help - Resultant Force

  1. #1
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    Resultant Force

    One force of 20 pounds and one force of 15 pounds act on a body atthe same point so that the resultant force is 19 pounds. Find, to the nearest degree, the angle between the two original forces.

    What exactly is the resultant force anyway?

    My teacher stated that this question can be solved without the use of vectors.

    Can someone explain this in detail?
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  2. #2
    Rhymes with Orange Chris L T521's Avatar
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    Quote Originally Posted by magentarita View Post
    One force of 20 pounds and one force of 15 pounds act on a body atthe same point so that the resultant force is 19 pounds. Find, to the nearest degree, the angle between the two original forces. To solve this, we would need to apply the Law of Cosines. We can set up a triangle that is not a right triangle, where all three sides are given.

    What exactly is the resultant force anyway? A resultant force is a single force that can be used to represent a number of different forces acting at the same point on an object. Resultant forces are used in engineering and physics [mechanics] to simplify calculations when analyzing forces using free body diagrams [force analysis diagrams].

    My teacher stated that this question can be solved without the use of vectors.

    Can someone explain this in detail?
    If you set up the problem as such, where the forces act at a same point P:


    you can use law of cosines to find \alpha, \beta, and then ultimately find \varphi=\alpha + \beta

    Do you recall how to use the law of cosines?

    --Chris
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  3. #3
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    let me ask..........

    Quote Originally Posted by Chris L T521 View Post
    If you set up the problem as such, where the forces act at a same point P:


    you can use law of cosines to find \alpha, \beta, and then ultimately find \varphi=\alpha + \beta

    Do you recall how to use the law of cosines?

    --Chris
    I have two questions:

    How does one use the Law of Cosines when no angle is given?

    What do math textbooks and math test prep books mean by "Not Drawn to Scale."

    Thanks
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  4. #4
    Rhymes with Orange Chris L T521's Avatar
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    Quote Originally Posted by magentarita View Post
    I have two questions:

    How does one use the Law of Cosines when no angle is given? The Law of Cosines is used when we have 3 given sides, and no given angles. In this case, we know the sides.

    Note that there are 3 different variations of the law of cosines for sides A, B, C, and the respective opposite angles \alpha \beta, and \gamma :


    A^2=B^2+C^2-2BC\cos\alpha
    B^2=A^2+C^2-2AC\cos\beta
    C^2=A^2+B^2-2AB\cos\gamma

    What do math textbooks and math test prep books mean by "Not Drawn to Scale." They mean that the measurements in the diagram they gave you aren't portrayed properly [most likely because they need to fit this into a tiny section of the page allotted for the question]. For example, in the diagram I drew, the 19 lb resultant force was a bit larger than the 20 lb force. It would look better if the resultant force was a tad smaller than the 20 lb force. In other diagrams, you may see lengths like 100 ft and 20 ft appearing to be the same size.

    Thanks
    --Chris
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  5. #5
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    resultant force

    How does one figure out the resultant force if only the 2 forces are given? eg 20lbs and 15lbs, how does one figure out the resultant of 19lbs?
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