Download 1 PHYSICS 231 Lecture 11: How much energy goes into problems?

PHYSICS 231
Lecture 11: How much energy goes into
problems?
Remco Zegers
Walk-in hour: Thursday 11:30-13:30 am
Helproom
PHY 231
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Previous lecture
 Work: W=Fcos()x
 Power: P=W/t
 Potential energy (PE)
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Energy transfer
Rate of energy transfer
Energy associated with
position.
Gravitational PE: mgh
Energy associated with
position in grav. field.
Kinetic energy KE: 1/2mv2 Energy associated with
motion
Conservative force:
Work done does not depend on path
Non-conservative force:
Work done does depend on path
Mechanical energy ME:
ME=KE+PE
 Conserved if only conservative forces are present
KEi+PEi=KEf+PEf
 Not conserved in the presence of non-conservative forces
(KEi+PEi)-(KEf+PEf)=Wnc
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Conservation of mechanical energy
A) what is the speed of m1 and m2
when they pass each other?
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work
How much work is done by the
gravitational force when the masses
pass each other?
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Friction (non-conservative)
The pulley is not completely frictionless.
The friction force equals 5 N. What is the
speed of the objects when they pass?
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A spring
Fs=-kx
+x
k: spring constant (N/m)
Fs(x=0)=0 N
Fs(x=-a)=ka
Fs=(0+ka)/2=ka/2
Ws=Fsx=(ka/2)*(a)=ka2/2
The energy stored in a spring
depends on the location of the
endpoint: elastic potential
energy.
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PINBALL
The ball-launcher spring has a
constant k=120 N/m. A player
pulls the handle 0.05 m. The
mass of the ball is 0.1 kg. What
is the launching speed?
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Ball on a track
A
h
end
B
h
end
In which case has the ball the highest velocity at the end?
A) Case A
B) Case B
C) Same speed
In which case does it take the longest time to get to the end?
A) Case A
B) Case B
C) Same time
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Race track
KE PE TME NC
With friction
KE PE TME NC
KE PE TME NC
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KE PE TME NC
KE PE TME NC
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A swing
30o
L=5m
If relieved from rest, what is
the velocity of the ball at the
lowest point?
h
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A running person
While running, a person dissipates about
0.60 J of mechanical energy per step per
kg of body mass. If a 60 kg person develops
a power of 70 Watt during a race, how
fast is she running (1 step=1.5 m long)
What is the force the person exerts on the
road?
W=Fx
P=W/t=Fv
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