2.6: Exercises

1. In the ``game'' of Russian roulette, the player inserts a single cartridge into the drum of a revolver, leaving the other five chambers of the drum empty. The player then spins the drum, aims at his/her head, and pulls the trigger.

1. What is the probability of the player still being alive after playing the game N times?
2. What is the probability of the player surviving N-1 turns in this game, and then being shot the th time he/she pulls the trigger?
3. What is the mean number of times the player gets to pull the trigger?

 

2. Suppose that the probability density for the speed  of a car on a road is given by

$$\begin{equation}P(s)=A s \exp \left(-\frac{s}{s_{0}}\right)\end{equation}$$

where $$\begin{equation}0 \leq s \leq \infty\end{equation}$$. Here, $$\begin{equation}A \text { and } s_{0}\end{equation}$$ are positive constants. More explicitly, $$\begin{equation}P(s) d s\end{equation}$$ gives the probability that a car has a speed between $$\begin{equation}s \text { and } s+d s\end{equation}$$.

 

1. Determine  in terms of .
2. What is the mean value of the speed?
3. What is the ``most probable'' speed: i.e., the speed for which the probability density has a maximum?
4. What is the probability that a car has a speed more than three times as large as the mean value?

3. An radioactive atom has a uniform decay probability per unit time $$\begin{equation}w\end{equation}$$: i.e., the probability of decay in a time interval $$\begin{equation}d t \text { is } w d t . \text { Let } P(t)\end{equation}$$ be the probability of the atom not having decayed at time , given that it was created at time . Demonstrate that

$$\begin{equation}P(t)=\mathbf{e}^{-w t}\end{equation}$$

What is the mean lifetime of the atom?

Contributors

• Richard Fitzpatrick (Professor of Physics, The University of Texas at Austin)

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