40. Since the speed is constant ∆K = 0 and Eq. 8-31 (an application of the energy conservation concept)
implies
W
applied
= ∆E
th
= ∆E
th (cube)
+ ∆E
th (floor)
.
Thus, if W
applied
= (15)(3.0) = 45 J, and we are told that ∆E
th (cube)
= 20 J, then we conclude that
∆E
th (floor)
= 25 J.