How many foot-pounds of work does it take to throw a baseball 90 mph? A baseball weighs 5 oz, or 0.3125 lb.

How many foot-pounds of work does it take to throw a baseball 90 mph? A baseball weighs 5 oz, or 0.3125 lb.

HINT: You'll need to convert some units. To find the mass (not the weight!) of the baseball, divide its weight by 32 ft/s^2, the acceleration due to gravity. Also, remember that the energy formula needs speed in ft/s!

Thank you!

Really this is a unit conversion problem mixed with energy

m = 5 oz = 5/16 lb = 5/16 lb * 1/(32 ft/s^2) <--- (slug)
90 mph = 90 miles/hour * (5280 ft / mile) * (1/3600 hour/sec) = 132 ft/s

Work = change in energy = Change in Kinetic energy + change in potential energy

Assume the baseball starts from rest and no loss of height

Work = Change in kinetic energy + 0 = 1/2*m*(vf^2 - vi^2) = 1/2*m*vf^2 - 0

Work = 1/2 * (5/16 lb * 1/(32 ft/s^2)) * (132 ft/s)^2 = 85.1 ft*lb

To show the units
Work = lb * s^2 / ft * (ft / s)^2 = lb * s*s * ft * ft / (s *s *ft)

So, the s's cancel and (1) of the ft

Work = ft*lb