Answer to Question #310782 in Electricity and Magnetism for Mark Gonzales

Question

A man uses a rock as a fulcrum and lifts a 300-kg boulder with a metal bar 6.00 m long. Calculare the force necessary to lift the boulder if the fulcrum is:

(a) 1.00 m from where the bar contacts the rock.

(b) 3.00 m from where the bar contacts the rock

Answer 1

Answer

At the equilibrium the total torque around the pivot point (fulcrum) is zero, so

$F_1 d_1=F_2 d_2$

or

$Fd_1=mgd_2$

Thus

$F=mg d_2/d_1$

$d_2=1 m, \\d_1=l-d_2=6-1=5 m\\ F=300×9.8×1/5=588 N \\d_2=3 m, \\d_1=l-d_2=6-3=3 m \\F=300×9.8×3/3=2940 N$