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16 December, 07:16

At one instant in time, a rock falling off a cliff experiences an upward force from air resistance at a magnitude of 10 N. The rock itself has a mass of 7 kg. What is the magnitude and direction of the acceleration of the rock at this instant? (Take the magnitude of acceleration due to gravity to be 9.8 m/s2 and indicate direction by using a (-) value for downward and a (+) value for upward.)

(Hint: What is the net force acting on the rock?)

Note: Be careful entering your units! Follow the instructions in the guide posted in the homework section!

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  1. 16 December, 07:45
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    a=-8.4 m/s^2

    Explanation:

    We can use (-) value for downward and a (+) value for upward

    The weight of the rock is calculated as follows:

    W=mass*gravity

    W=7kg * (-9.8m/s^2)

    W=-68.8N

    The air resistance force makes sense opposite to the weight of the rock:

    Fr=+10N

    The total force on the rock is:

    Ft = Fr+W

    Ft=10N-68.8N

    Ft=-58.8N

    We also know that force is the product of mass by acceleration:

    Ft = m*a

    Thus:

    a = Ft/m

    a=-58.8N/7kg

    a=-8.4 m/s^2 (downward)
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