Chapter 11: Work. • How many kinds of energy are there? • Under what conditions is energy conserved •How does a system gain or lose energy?. Stop to think 11.1 P304 Stop to think 11.2 P306 Stop to think 11.3 P310 Stop to think 11.4 P312 Stop to think 11.7 P327. Example 11.1 P307
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• How many kinds of energy are there?
• Under what conditions is energy conserved
•How does a system gain or lose energy?
Force F does work as the particle moves from
Si to Sf
Question: 1 Kg ball free fall from 5m high place to the ground, how much work is
Done by the gravitational force?
The work done by a force depends on the angle θ between the force and displacement, not on the direction the particle is moving. The work done on all four particles in the figure is the same, despite the face that they are moving in four different direction.
( we chose equilibrium position x(e) as origin )
work, always make potential energy decrease.
Independent of path
For example, friction.
• If there are only conservative forces
because Wc = ΔK =-ΔU→ ΔK + ΔU =0
( total mechanical energy is conserved)
• If there is nonconservative force
ΔK + ΔU = Wnc or
m1 and table. f(k) = μ(m1g)
Ei – f(k)h = Ef
m2 gh -μ(m1g)h = ½(m1+m2)V2
P11.56 The spring shown in Fig. is compressed 50cm and used to launch a 100 Kg students. The track is frictionless until it stars up the incline. The student’s coefficient of kinetic friction on the 30o incline is 0.15
at bottom. After that student goes to incline with friction.
We can apply the energy and work equation.
With h2 = s sin(30o) and friction
F(k) = μ mg cos(30o)
is defined as:
unit of power is Watt.
When work as the source of energy trasfer