Question

A man pushes an $80N$ crate a distance of $5.0m$ upward along a frictionless slope that makes an angle of ${30}^o$ with the horizontal. The force he exerts is parallel to the slope. If the speed of the crate is constant, then the work done by the man is:

• A. $-140J$
  
• B. $140J$
  
• C. $200J$
  
• D. $-200J$
  

Answer 1

The correct option is C $200J$

The net result is a negative acceleration of 1.5 so...

let F be the man's force

mg sin 30 is the component of gravity parallel to the slope

$80/9.8$is the crate's mass

$F-gravity=ma$

$F-mgsin30=80/9.8$

$F-80sin30=80/9.8$

$F=40+80/9.8$

Then work is just F*d since the force is parallel to the displacement

$W=(40+80/9.8)$

$W=200J$