Serena drops a 5-kg book down the stairwell. What is the book's final velocity vf after it falls freely for 4 seconds

39.2m/s

Explanation:

Given parameters:

Mass of book  = 5kg

Time taken for fall  = 4s

Unknown:

Final velocity of the book  = ?

Solution:

Serena dropped the book from rest therefore, the initial velocity of the book is 0.

 Let us find the appropriate motion equation to solve this problem;

        V  = U + gt

V is the final velocity

U is the initial velocity

g is the acceleration due to gravity  = 9.8m/s²

t is the time taken

  Insert the given parameters and solve;

       V = 0 + 9.8 x 4  = 39.2m/s

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c. newton's third law of motion