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8/13/2019 Fisika Modern Fisdas2
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FISIK MODERN
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Two Postulates Leading to Special
Relativity (1)
1. Observers can
never detect their
uniform motion,
except relative toother objects.
This is equivalent to:
The laws of physics are the same for all
observers, no matter what their motion, as
long as they are not accelerated.
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Einstein’s Special Relativity
1,000,000 ms-1
0 ms-1
300,000,000 ms-1
Both spacemen measure the speed of the approaching ray of
light.
How fast do they measure the speed of light to be?
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Special Relativity
• Stationary man
– 300,000,000 ms-1
• Man travelling at 1,000,000 ms-1
– 301,000,000 ms-1?
– Wrong!
• The Speed of Light isthe same for all observers
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Two Postulates Leading to Special
Relativity (2)
2. The velocity oflight, c , isconstant andwill be the
same for allobservers,independent oftheir motion
relative to thelight source.
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Here Come The
Consequences!
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ddl l
x x
x = v Dtl = c Dtd = c Dt’
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(a) Sumber merah dan
biru sama-sama dalam
kerangka diam
(b) Sumber merah diam
dan sumber biru
bergerak dgn kecepatan
tertentu
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Basics of Special Relativity (2)Now, recall that the velocity of light, c , is
the same for all observers.
The times Dt and Dt’ must be different!
Then, the Pythagorean Theorem gives:
(cDt)2 = (cDt’)2 + (vDt)2
or
Dt’ = (Dt)/gwhere g = 1/(1 – [v/c]2)1/2
is the Lorentz factor.
This effect is called time dilation.
v Dt
c Dtc Dt’
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Time Dilation
• Observers measure moving clocks torun slow
• The faster the speed, the slower the
time
Length Contraction
• Length scales on a rapidly moving
object appear shortened.
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Contoh Soal
• Jarak antara dua titik A dan B di permukaanbumi adalah 600 km. Sebuah pesawatsuper cepat bergerak dengan kelajuan 0,8 cmelintasi kedua titik tersebut.
(a) Tentukan jarak A ke B menurut pilotpesawat.
(b) Tentukan waktu yang diperlukan
pesawat untuk melintasi kedua titik tersebutmenurut orang di bumi dan menurut pilotpesawat.
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Doppler Effect
• The pitch of the siren:
– Rises as the ambulance approaches
– Falls once the ambulance has passed.
• The same applies to light!
– Approaching objects appear blue (Blue-shift)
– Receding objects appear red (Red-shift)
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Other Effects of Special Relativity
• The mass of moving object:
221 cv
m
m o
m0 = rest mass
• The energy of a body at rest is not 0.
Instead, we find
E0 = m0 c2