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Chapter 5 Review
Work is force x distance
Only the force parallel to the
displacement applies to the work
Fpull = 20N
Pulling a box
N = 137.2N
f = -16.5N
Fpully = sin30°
Fpull = 20N
20N = 10N
30°
angle = 30°
Fpullx = cos30°(20N) = 17.3N
mass = 15 kg
mg = -147.2N
Label and calculate all forces
pulled 8 m
Calculate the work done by the pull
m = .12
and friction
Wpull = Fxd = 138.4J
Wf = fd = -132J
Potential Energy is energy of position
GPE = mgh
affected by height
EPE = ½kx2
affected by distance
squeezed or pulled
k is the spring constant
Kinetic Energy is energy of motion
KE = ½mv2
affected by velocity
if I triple the speed of something
what happens to it’s KE?
Mechanical Energy is PE + KE
MEi = MEf
if there is no friction
if there is friction, some KE
is changed to heat and lost
40 J of work is done to
raise the ball of a pendulum
parallel to the ground
30°
½h
If it is released how much KE,
PE and ME will it have after 30°?
½h
At its lowest point, how much KE,
PE and ME does it have?
3 m/s
What will the wagon’s speed
be at the bottom of the hill?
KEi + PEi = KEf
10 m
½mv2i + mghi = ½mv2f
v2f = v2i + 2ghi
vf = 14.3 m/s
A 5 kg block is pushed 20 cm
5 kg
against a spring with a spring
constant of 240 N/m
What is the stored elastic potential energy?
EPE = ½kx2 = ½(240N/m)(.2 m)2 = 4.8 J
If released, what is the velocity of the block, as it comes
off the spring?
KE = EPE = 4.8 J = ½mv2
v = 1.39 m/s
If 4 N of frictional force is acting between the floor and
block, what distance will the block go?
W = DKE = f d
d = DKE / f = 4.8 J / 4 N
= 1.2 m