Work and Energy Problems
A 50 kg diver steps off a 10 m high diving board
and drops straight down into the water. If the
diver comes to rest 5 m below the surface of the
water, determine the average resistance force
exerted on the diver by the water.
#37,part 1
Work required to stop
the diver
W = Fdcosq
W = (F)(5)(cos180)
W = -5F
5m
#37, part 2
W = DTE
Set PE=0 at bottom, KE= 0 at bottom
ΔTE = 0 - PE at top of tower
PEg = (50)(9.8)(15) = 7350 J
ΔTE = -7350
W = -7350
-5F = -7350
F = 1470 N
In a circus performance, a monkey on a sled is
given an initial speed of 4 m/s up a 25º incline.
The combined mass of the monkey and the sled
is 20 kg, and the coefficient of kinetic friction
between the sled and the incline is 0.20. How far
up the incline does the sled move?
#39
vi = 4 m/s
m = 20 kg
m =
0.2
d0
h = d sin 25
25
o
#39, part 2
W = ΔTE = PEg - KE
PEg = (20)(9.8)(dsin25) = 82.8 d
KE = ½ (20)(4)2 = 160J
WFf = Ff(d)(cos180)
Ff = µmgcosθ
WFf =(0.2)(20)(9.8)(cos25)(d)(-1)= -35.5d
#39, part 3
W=ΔTE
WFf =PEg – KE
-35.5d =82.8d – 160
-118.3 d = -160
d = 1.4 m