Potential energy and conservation of energy problem

[magnolia1]

Homework Statement

An outfielder throws a baseball with an initial speed of 79.0 mi/h. Just before an infielder catches the ball at the same level, its speed is 65 ft/s. How much of the ball's mechanical energy is dissipated by air drag? (The weight of a baseball is 9 oz.)

Homework Equations

ke=1/2mv²
ug=mgh
Emec = ΔU + ΔK

ANSWER IS: 80.4 ft*lb

The Attempt at a Solution

Converting knowns to metres/second:
79.0 mi/h (0.44704) = 35.32 m/s
65 ft/s (0.3048) = 19.81 m/s

When they say "weight" do they mean
fg=9 oz. = 2.5 N
or
mass= 9 oz. = 0.255 kg

ΔKE=1/2mvf²-1/2mvi²
=1/2(0.255)(19.81)²-1/2(0.255)(35.32)²
=50.0356-159.0565
=-109.02 J

This is where I wasn't sure how to continue..
Any help is appreciated, thanks!

 

[tiny-tim]

hi magnolia1!

(The weight of a baseball is 9 oz.)
…
When they say "weight" do they mean
fg=9 oz. = 2.5 N
or
mass= 9 oz. = 0.255 kg

in the ft-lb system, the pound (and the ounce) is a force

so "weight" is used instead of mass (and has the g already in it)

(the unit of mass is the slug)

anyway, why have you converted to metres etc when the required answer is in ft-lbs ??

start again ​