A 2.5-kg book slides horizontally and falls from a shelf 3.0 m above the floor. How much work does the force of gravity do on the book as it falls 3.0 m?

1 Answer
May 1, 2018

I tried this:

Explanation:

We know that work is:

#W=vecF*vecd=Fdcos(theta)#

where:

#vecF# is the force;
#vecd# is the displacement.
#theta# is the angle between these two vectors.

During the horizontal motion the force (gravitational pull=weight) and the horizontal displacement form an angle of #270^@# so that the cossine will give us zero and, consequently, zero contribution to the work.

Only during the fall where force and displacement have the same direction and so #theta=0# we will have a cossine equal to one and for this a contribution to the work.
We finally get:

#W=mg*dcos(0)=2.5*9.8*3=73.5J#

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