The effort required to lift a load W on a screw jack with helix angle $$\alpha $$ and angle of friction $$\varphi $$ is equal to
A. $${\text{W}}\tan \left( {\alpha + \varphi } \right)$$
B. $${\text{W}}\tan \left( {\alpha - \varphi } \right)$$
C. $${\text{W}}\cos \left( {\alpha + \varphi } \right)$$
D. $${\text{W}}\sin \left( {\alpha + \varphi } \right)$$
Answer: Option A
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