Question

A weightless ladder, $20ft$ long rests against a frictionless wall at an angle of ${60}^o$ with the horizontal. A $150pound$ man is $4ft$ from the top of the ladder. A horizontal force is needed to prevent it from slipping. Choose the correct magnitude from the following (in $lb$)

• A. $17.3$
• B. $100$
• C. $120$
• D. $150$

Answer 1

The correct option is A $17.3$

To prevent slipping taking the moment about A we have

$F\ast OA=weight\ast MN$

$F\ast 20sin{60}^o=150\ast 4cos{60}^o$

$F=17.3lb$