Math Problem Statement

Suppose you are standing in a field near a straight section of railroad tracks just as the locomotive of a train passes the point nearest to​ you, which is one sixth mi away. The​ train, with length two fifths ​mi, is traveling at 26 ​mi/hr. If you start running in a straight line across the​ field, how slowly can you run and still catch the​ train? In which direction should you​ run?

Solution

To solve this problem, let's consider the geometry and physics of the situation. We'll calculate your minimum running speed and the direction you should run to catch the train. Here are the steps:


Step 1: Train's position and motion

  • The train passes the point nearest to you when it begins at a distance of 16\frac{1}{6} mi.
  • The train has a length of 25\frac{2}{5} mi and is traveling at 26mi/hr26 \, \text{mi/hr}.

The train takes time to fully pass a specific point, ttraint_{\text{train}}: ttrain=Train lengthTrain speed=2526hours.t_{\text{train}} = \frac{\text{Train length}}{\text{Train speed}} = \frac{\frac{2}{5}}{26} \, \text{hours}.


Step 2: Your running path

You start at a distance 16mi\frac{1}{6} \, \text{mi} from the train tracks and can run in any direction. Let’s denote:

  • vyv_y: your running speed (to be determined),
  • dyour pathd_{\text{your path}}: the total distance you must cover to reach the train.

The direction to run minimizes the time to reach the train's end, so you must run diagonally toward the train's motion.


Step 3: Relative motion and geometry

Using Pythagoras:

  • If you run directly towards the train's path at angle θ\theta, your total path length to the nearest train point is: d=(16)2+(x)2.d = \sqrt{\left(\frac{1}{6}\right)^2 + (x)^2}.]### Solution Process

To determine how slowly you can run and still catch the train, we need to analyze the relative motion and geometry of the problem.


Step 1: The Train’s Motion

  • Train speed: vtrain=26mi/hrv_{\text{train}} = 26 \, \text{mi/hr},
  • Train length: Ltrain=25miL_{\text{train}} = \frac{2}{5} \, \text{mi}.

The train takes time to completely pass a fixed point: ttrain=Ltrainvtrain=2526=2130hours.t_{\text{train}} = \frac{L_{\text{train}}}{v_{\text{train}}} = \frac{\frac{2}{5}}{26} = \frac{2}{130} \, \text{hours}.

This simplifies to: ttrain=165hours.t_{\text{train}} = \frac{1}{65} \, \text{hours}.


Step 2: Geometry of Your Path

You are 16mi\frac{1}{6} \, \text{mi} away from the train’s nearest point on the tracks. To catch the train, you need to minimize the time it takes to run to a point where the train is still reachable. The direction you run will depend on an angle θ\theta, which determines the diagonal distance to the tracks and the effective distance you run along the tracks.

Let:

  • ddirectd_{\text{direct}}: the straight-line distance from your starting position to the track at an angle θ\theta,
  • dalongd_{\text{along}}: the distance you run along the tracks to catch the train.

By geometry: ddirect=(16)2+x2.d_{\text{direct}} = \sqrt{\left( \frac{1}{6} \right)^2 + x^2}.


Step 3: Relative Speeds and Time

You must reach the point on the track where the train is reachable before the train passes completely. The time it takes you to run is: tyou=ddirectvyou,t_{\text{you}} = \frac{d_{\text{direct}}}{v_{\text{you}}}, where vyouv_{\text{you}} is your running speed.

Simultaneously, the train's position advances as: dtrain=26tyou.d_{\text{train}} = 26 \cdot t_{\text{you}}.

To catch the train, you must cover enough distance diagonally and along the tracks such that: ddirectvyou+dalongvyou165.\frac{d_{\text{direct}}}{v_{\text{you}}} + \frac{d_{\text{along}}}{v_{\text{you}}} \leq \frac{1}{65}.

By minimizing θ\theta (the direction angle), you reduce the total distance ddirect+dalongd_{\text{direct}} + d_{\text{along}}, and we solve for vyouv_{\text{you}} accordingly.


Step 4: Solve Numerically

The optimal solution balances the diagonal distance and running along the tracks. Numerical solutions involve iterating for vyouv_{\text{you}} and minimizing tyout_{\text{you}}. Would you like to compute the exact value?

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Math Problem Analysis

Mathematical Concepts

Geometry
Relative Motion
Distance-Time Relationships
Optimization

Formulas

Time to pass = Train length / Train speed
Distance = sqrt((distance to track)^2 + (distance along track)^2)
Time = Distance / Speed

Theorems

Pythagoras' Theorem
Relative Velocity

Suitable Grade Level

Grades 10-12