AP Physics 2 : Principles of Special Relativity

Study concepts, example questions & explanations for AP Physics 2

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Example Questions

Example Question #1 : Principles Of Special Relativity

Determine the observed length of a \(\displaystyle 3\textup{ m}\) rod traveling along it's long axis at \(\displaystyle 0.75\textup{ c}\) in relation to an observer.

\(\displaystyle \textup{c}=3.00*10^8\ \frac{\textup{m}}{\textup{s}}\)

Possible Answers:

\(\displaystyle 1.98\textup{ m}\)

\(\displaystyle 3.98\textup{ m}\)

\(\displaystyle 2.5\textup{ m}\)

\(\displaystyle 2.36\textup{ m}\)

\(\displaystyle 3\textup{ m}\)

Correct answer:

\(\displaystyle 1.98\textup{ m}\)

Explanation:

Using the formula for length contraction:

\(\displaystyle L=L_0\sqrt{1-\frac{v^2}{c^2}}\)

Where \(\displaystyle L_0\) is the rest length,

\(\displaystyle v\) is the velocity of the object

\(\displaystyle c\) is the speed of light

\(\displaystyle L\) is the observed length

Plugging in values

\(\displaystyle L=3\sqrt{1-\frac{(.75c)^2}{c^2}}\)

\(\displaystyle L=1.98m\)

Example Question #1 : Principles Of Special Relativity

Determine the observed length of a \(\displaystyle 7\textup{ m}\) rod traveling along it's long axis at \(\displaystyle 0.86\textup{ c}\) in relation to an observer.

Possible Answers:

\(\displaystyle 3.57\textup{ m}\)

\(\displaystyle 6.68\textup{ m}\)

\(\displaystyle 2.54\textup{ m}\)

None of these

\(\displaystyle 8.44\textup{ m}\)

Correct answer:

\(\displaystyle 3.57\textup{ m}\)

Explanation:

Use the following equation:

\(\displaystyle L=L_0\sqrt{1-\frac{v^2}{c^2}}\)

Where \(\displaystyle L_0\) is the rest length,

\(\displaystyle v\) is the velocity of the object

\(\displaystyle c\) is the speed of light

\(\displaystyle L\) is the observed length

Plugging in values

\(\displaystyle L=7\sqrt{1-\frac{(.86c)^2}{c^2}}\)

\(\displaystyle L=3.57\textup{ m}\)

Example Question #11 : Quantum And Nuclear Physics

\(\displaystyle 3m\) long rod is traveling at \(\displaystyle .75c\) in relationship to an observer along it's long axis. Determine the observed length.

Possible Answers:

None of these

\(\displaystyle 3.98m\)

\(\displaystyle 2.98m\)

\(\displaystyle 4.5m\)

\(\displaystyle 1.98m\)

Correct answer:

\(\displaystyle 1.98m\)

Explanation:

Using

\(\displaystyle L=L_0\sqrt{1-\frac{v^2}{c^2}}\)

Plugging in values

\(\displaystyle L=3*\sqrt{1-\frac{(.75c)^2}{c^2}}\)

\(\displaystyle L=1.98m\)

Example Question #1 : Principles Of Special Relativity

\(\displaystyle 363ft\) tall rocket is traveling at \(\displaystyle .57c\) in relationship to an observer along it's long axis. Determine the observed length.

Possible Answers:

\(\displaystyle 175ft\)

\(\displaystyle 401ft\)

None of these

\(\displaystyle 320ft\)

\(\displaystyle 298ft\)

Correct answer:

\(\displaystyle 298ft\)

Explanation:

Using

\(\displaystyle L=L_0\sqrt{1-\frac{v^2}{c^2}}\)

Plugging in values

\(\displaystyle L=363*\sqrt{1-\frac{(.57c)^2}{c^2}}\)

\(\displaystyle L=298ft\)

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