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Lecture

Phys 111


Department
Physics
Course Code
PHYS111
Professor
David G.Hawthorn

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This week we learnt:
Projectiles Using Energy
Conservative System: Ei = initial mechanical energy = KEi + PEi
Ef = KEf + PEf
=
mvi2 + mgh
=
mvi2
Conservation: Ef = Ei
mvi2 =
mvi2 + mgh
Conservative Systems with Springs:
Hooke’s Law =
spring = -k
Wsp
sp
Fext = -Fsp
= kx
Wsp =
kx2 = PE stored by spring
Non-Conservative Systems (Friction):
Wext =
ext = Fext x
Frictionless: Wext = ∆KE + ∆PE
Add Friction: Wext = ∆KE + ∆PE + ∆TE
TE: thermal energy dissipated by the system
Alternatively, Wext - |Wf| = ∆PE + ∆KE
TE = |Wf| Wf = Fkx
Power:
- the rate at which energy is transferred from one system to another
Wext = Fextx
Power = 

IF an external force is applied giving external work Wext over a time interval, then
the average power is Pavg = 

If Wext = Fextx, Pavg = 
 = Fext 
 = Fext Vavg
Instantaneous Power: Pinst =
ext
inst =
ext
Momentum & Impulse:
- collisions between objects involve complex forces that act over short time
scales and are typically not constant
- consider a large force exerted over a small interval of time
o impulse force
o parameterized by the area under the force-time graph = Impulse
Impulse =
=
avgt = area under force-time graph
Momentum: p = mV
I = Favgt
= 
 ∆t
I = Pf Pi = ∆P
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