An electron, which has a mass of 8.45 x 10-31 kg, moves with a speed of 0.45c. Find its relativistic momentum and find the percentage increase of its relativistic momentum from its classical momentum. An electron in a television picture tube typically moves with a speed u = 0.25c. Find its total energy and kinetic energy in electron volts (Eo = 0.385 MeV)

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Relativistic Momentum and Energy
Relativistic Momentum (p)
It is just classical momentum multiplied by the relativistic factor (y).
4.0-
2 3.0-
p = ymu
2.0-
where m is the rest mass of the object, u is its velocity relative to an observer,
and the relativistic factor.
1.0-
1
Y =
0-
0.2c 0.4c 0.6c 0.8c 1.0c
Relativistic momentum has the same intuitive feel as classical momentum.
speed u (m/s)
It is greatest for large masses moving at high velocities, but, because of the
factor (7), relativistic momentum approaches infinity as velocity (u)
Figure 1. Relativistic momentum
approaches infinity as the velocity of
an object approaches the speed of
light.
approaches speed of light (c). (See Figure 1) This is another indication that
an object with mass cannot reach the speed of light. If it did, its momentum
would become infinite, an unreasonable value.
Example:
An electron, which has a mass of 9.11 x 10-31 kg, moves with a speed of 0.750c. Find its relativistic momentum.
Given:
Solution:
me = 9.11 x 1031 kg
u = 0.750c
meu
(9.11 x 10-31 kg)(0.750)(3.00 x 108 m/s)
p =
3.10 x 1022 kgm/s
/1-(0.750c)²
c2
u2
Relativistic Energy
The first postulate of relativity states that the laws of physics are the same in all inertial frames. Einstein showed
that the law of conservation of energy is valid relativistically, if we define energy to include a relativistic factor. It
can be summarized by the equation below:
Total Energy (E)
is defined as:
Kinetic Energy (K)
is defined as:
K = ymc²- mc²
Rest Energy Energy (Eo)
is defined as:
%3D
E = ymc²
E, = mc²
The Relationship of relativistic momentum and energy can be defined by:
E = (pc) + (mc²)²
Example:
An electron in a television picture tube typically moves with a speed u = 0.25c. Find its total energy and kinetic
energy in electron volts (Eo = 0.511 MeV).
Given:
Solution:
Eo = 0.511 MeV
męc2
Eo
0.511 MeV
u = 0.25c
E =
= 0.528 Mey
K= E- Eo = 0.528 MeV – 0.511 MeV
u2
(0.25c)2
|0.017 MeV
Activity:
Solve the following problems. Use the rubrics as your guide in presenting your solution (refer to attachment A.1
at the next page).
1. An electron, which has a mass of 8.45 x 10-31 kg, moves with a speed of 0.45c. Find its relativistic
momentum and find the percentage increase of its relativistic momentum from its classical momentum.
2. An electron in a television picture tube typically moves with a speed u = 0.25c. Find its total energy and
kinetic energy in electron volts (Eo = 0.385 MeV)
momentum Prel (kg m/s)
Transcribed Image Text:Relativistic Momentum and Energy Relativistic Momentum (p) It is just classical momentum multiplied by the relativistic factor (y). 4.0- 2 3.0- p = ymu 2.0- where m is the rest mass of the object, u is its velocity relative to an observer, and the relativistic factor. 1.0- 1 Y = 0- 0.2c 0.4c 0.6c 0.8c 1.0c Relativistic momentum has the same intuitive feel as classical momentum. speed u (m/s) It is greatest for large masses moving at high velocities, but, because of the factor (7), relativistic momentum approaches infinity as velocity (u) Figure 1. Relativistic momentum approaches infinity as the velocity of an object approaches the speed of light. approaches speed of light (c). (See Figure 1) This is another indication that an object with mass cannot reach the speed of light. If it did, its momentum would become infinite, an unreasonable value. Example: An electron, which has a mass of 9.11 x 10-31 kg, moves with a speed of 0.750c. Find its relativistic momentum. Given: Solution: me = 9.11 x 1031 kg u = 0.750c meu (9.11 x 10-31 kg)(0.750)(3.00 x 108 m/s) p = 3.10 x 1022 kgm/s /1-(0.750c)² c2 u2 Relativistic Energy The first postulate of relativity states that the laws of physics are the same in all inertial frames. Einstein showed that the law of conservation of energy is valid relativistically, if we define energy to include a relativistic factor. It can be summarized by the equation below: Total Energy (E) is defined as: Kinetic Energy (K) is defined as: K = ymc²- mc² Rest Energy Energy (Eo) is defined as: %3D E = ymc² E, = mc² The Relationship of relativistic momentum and energy can be defined by: E = (pc) + (mc²)² Example: An electron in a television picture tube typically moves with a speed u = 0.25c. Find its total energy and kinetic energy in electron volts (Eo = 0.511 MeV). Given: Solution: Eo = 0.511 MeV męc2 Eo 0.511 MeV u = 0.25c E = = 0.528 Mey K= E- Eo = 0.528 MeV – 0.511 MeV u2 (0.25c)2 |0.017 MeV Activity: Solve the following problems. Use the rubrics as your guide in presenting your solution (refer to attachment A.1 at the next page). 1. An electron, which has a mass of 8.45 x 10-31 kg, moves with a speed of 0.45c. Find its relativistic momentum and find the percentage increase of its relativistic momentum from its classical momentum. 2. An electron in a television picture tube typically moves with a speed u = 0.25c. Find its total energy and kinetic energy in electron volts (Eo = 0.385 MeV) momentum Prel (kg m/s)
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