A Variable Force of Magnitude $F ( x )$ Moves a Body of Mass

Multiple Choice

A variable force of magnitude $F ( x )$ moves a body of mass $m$ along the $x$ -axis from $x _ { 1 }$ to $x _ { 2 }$ . The net work done by the force in moving the body from $x _ { 1 }$ to $x _ { 2 }$ is $W = \int _ { x _ { 1 } } ^ { x _ { 2 } } F ( x ) d x = \frac { 1 } { 2 } m _ { 2 } { } ^ { 2 } - \frac { 1 } { 2 } m v _ { 1 } ^ { 2 }$ , where $v _ { 1 }$ and $v _ { 2 }$ are the body's velocities at $x _ { 1 }$ and $x _ { 2 }$ . Knowing that the work done by the force equals the change in the body's kinetic energy, solve the problem.
-How many foot-pounds of work does it take to throw a baseball 20 mph? A baseball weighs 5 oz, or 0.3125 lb.

A) 2 ft-lb
B) 63 ft-lb
C) 4 ft-lb
D) 131 ft-lb