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8 Potential Energy
Gravitational Potential Energy:
U=mgh
SI unit: J
Consider gravitational force:
hf
h
W
hi
Wg =ΔK= - ΔU; E=Kf+Uf=Ki+Ui
Conservative force: work done by these force
to move an object between any two points is
independent of the path taken.
woke done by conservative forces, Mechanical
Energy is conservative.
K+U=constant
Slide m=80 kg, vi=0
Find the speed the person at the bottom of slide?
10 m
Calculating a home run
A player hits a 0.15-kg baseball over the outfield fence. The
ball leaves the bat with a speed of 36 m/s, and a fan in the
bleachers catches it 7.2m above the point where it was hit.
Assuming frictional force can be ignored, find (a) the kinetic
energy of the ball when it is caught and (b) its speed when
caught.
Skateboard exit ramp
A 50-kg skateboard enters a ramp moving horizontally with
a speed of 8.0 m/s and leaves the ramp vertically height with
1.4 m. Find final velocity vf, assuming no energy loss to
frictional forces
Potential energy of a spring, the displacement x,
U=1/2kx2
Energy of spring
E=K+U=1/2mv2+1/2kx2
Problem:
K=4000 N/m, m=60kg, How the height?
0.5m
Ex.
M=1.7kg, k=955N/m, x=4.6cm
Find vi
(1) Conservative forces, such as gravity and
spring force.
(2)Work done by Non conservative forces, such
as friction, air resistance.
Work-energy
Wtotal=ΔK
Wgravity+Wspring+Wnc=ΔK
Wgravity+Wspring= - ΔU
Wnc=ΔU+ΔK=ΔE
Ex.
Falling leaf
Find air resistance force on leaf?
Vi=0
5.3 m
Vf=1.3 m/s