Unformatted text preview: only by amounts of
energy that are transferred to or
from the system.
• The total energy E of an
isolated system cannot change
isolated system cannot change. NYC – SF by train. On a regular schedule
It takes 4 days for 1 way trip
One train per day; starts at 1 pm in NYC
and arrives in 4 days at 1 pm to SF.
Quickly unload – load and go back.
(a)
Calculate the work of the engine when the train of the mass
10,000 kg
accelerates to v = 72 km/h from zero at the departure from NYC.
72
NYC
(b)
Calculate the work done by the breaks (friction force) when the train slows
down from v = 72 km/h to v = 0 arriving to SF.
(c)
How many other trains will our train meet during one way trip?
Lecture 9 Andrei Sirenko, NJIT 29 Lecture 9 Andrei Sirenko, NJIT 30 Potential Energy Curve
Sample Problem 84 1D Motion • Turning Points
• Equilibrium Points A plot of U(x), the potential energy function
U( th
of a system containing a particle confined to
move along the x axis. There is no friction, so
mechanical energy is conserved.
Lecture 9 Andrei Sirenko, NJIT – Neutral
Equilibrium
– Unstable
Equilibrium
Equilibrium
– Stable Equilibrium 31 Lecture 9 A 61.0 kg bungeecord jumper is
61 kg bungee cord jumper is
on
on a 45.0 m bridge above a river.
The elastic bungee cord has a
relaxed length of L = 25.0 m.
Assume that the cord obeys
Hooke’s
Hooke’s law, with a spring
constant of 160 N/m. If the
jumper stops before reaching the
jumper stops before reaching the
water,
water, what is the height h of her
feet above the water at her lowest
feet above the water at her lowest
point?
point?
Andrei Sirenko, NJIT
32 Sample Problem 72
Sample Problem 88 Lecture 9 A circus beagle of mass
m = 6.0 kg runs onto the left
kg runs onto the left
end
end of a curved ramp with
speed v0 = 7.8 m/s at height
y0 = 8.5 m above the floor. It
then slides to the rig...
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 Fall '08
 KEN
 Physics, Energy, Kinetic Energy, Potential Energy, Work, Andrei Sirenko

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