Answer
(a) The pitcher does $124~J$ of work on the baseball.
(b) A pitcher would have to throw 10,300 fastballs in order to burn off a 1520 kilocalorie meal.
Work Step by Step
(a) We can find the kinetic energy of the ball:
$\frac{1}{2}mv^2 = \frac{1}{2}(0.153~kg)(40.2~m/s)^2 = 124~J$
The pitcher does $124~J$ of work on the baseball.
(b) This energy is only 20% of the chemical energy that is used to throw the baseball. We can find the amount of chemical energy $E_c$ that is used:
$0.20~E_c = 124~J$
$E_c = \frac{124~J}{0.20}$
$E_c = 620~J$
We can convert this energy to units of kilocalories:
$E_c = 620~J \times \frac{1~kcal}{4184~J} = 0.148~kcal$
We can find the number of fastballs a pitcher would have to throw in order to burn off a 1520 kilocalorie meal:
$\frac{1520~kcal}{0.148~kcal/fastball} = 10,300~fastballs$
A pitcher would have to throw 10,300 fastballs in order to burn off a 1520 kilocalorie meal.
