Trains heading toward each other

[chawki]

Homework Statement

One train runs from A to B at 105 miles per hour, the other runs from B to A at 85 miles per hour.

Homework Equations

How far apart were the two trains 30 minutes prior to their crossing?

The Attempt at a Solution

if the distance between them is x, i can find the time when they cross each other.
t=x/(190)

 

[kuruman]

Homework Statement

One train runs from A to B at 105 miles per hour, the other runs from B to A at 85 miles per hour.

Homework Equations

How far apart were the two trains 30 minutes prior to their crossing?

The Attempt at a Solution

if the distance between them is x, i can find the time when they cross each other.
t=x/(190)

How far apart are the trains to begin with?

 

[chawki]

They didn't say

 

[supratim1]

30 mins * relative velocity = x. won't that be the answer?

 

[kuruman]

30 mins * relative velocity = x. won't that be the answer?

That works.

 

[supratim1]

so initial distance is not required!

 

[kuruman]

so initial distance is not required!

It is not. I misread the problem at first.

 

[chawki]

t-0.5=x/190 ?

 

[kuruman]

supratim1 has already posted the method of calculation above (#4). Be sure to have a consistent system of units.

 

[chawki]

yeah but i don't understand it
i don't know about the relative velocity

 

[cupid.callin]

here: http://en.wikipedia.org/wiki/Relative_velocity

 

[chawki]

i still don't get it...
isn't there another method more simple?

 

[kuruman]

i still don't get it...
isn't there another method more simple?

There isn't. This is a problem about relative velocity and you have to understand what that means. Here we go. Please think about and answer the following two questions.

Question 1
Suppose you are in a car traveling at 60 miles per hour going North. You look out the window and you see a road sign. Would you say that the road sign is moving? If so, how fast and in what direction?

Question 2
Suppose you are in a car traveling at 60 miles per hour going North. You look out the window and you see a car going in the opposite direction (South). The speedometer of that car reads 50 miles per hour. Would you say that that car is moving? If so, how fast and in what direction?