Fisika Modern Fisdas2

8/13/2019 Fisika Modern Fisdas2

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FISIK MODERN



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.



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?



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



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.



Here Come The

Consequences!



ddl l

x x

x = v Dtl = c Dtd = c Dt’



(a) Sumber merah dan

biru sama-sama dalam

kerangka diam

(b) Sumber merah diam

dan sumber biru

bergerak dgn kecepatan

tertentu



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’



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.



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.



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)



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

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