BREAKING: Target Test Prep releases Brand New 2026 On Demand GMAT prep course

Redeem

Relative motion confusion with Time

This topic has expert replies
Post new topic Post Reply
Previous Topic Next Topic
Master | Next Rank: 500 Posts
Posts: 209
Joined: Thu Jan 12, 2012 12:59 pm

Relative motion confusion with Time

by dddanny2006 » Mon Feb 03, 2014 2:26 am
Hey guys..
I've always wondered why we consider time to be the same in case of both bodies whether they move towards each other,or go away from each other..

Let's consider this example

Imagine that two people are 14miles apart and begin walking towards each other.Person A walks 3miles per hour,and person B walks 4miles per hour.How long will it take them to reach each other?

PersonA. Rate-3mph. Time-'t' hrs. Distance-'d'miles

PersonB. Rate-4mph. Time-'t'hrs. Distance 14-d miles


We setup a 3rd RT D. Equation by adding

R(mph) T(hrs). D(miles)
A+B. 7mph. T. 14miles

7t equals 14.
T equals 2hrs

Why is time assumed to be same always..

Let me modify it a bit by saying A and B are 8miles part,
A has a speed of 2 mph and B has a speed of 3mph.Let A cover 2miles of that distance in one hour,and let B cover 6miles of it in 2hours,and as a result they both meet.So we do see the difference in time,its not the same.So I ask again,why do we consider time to be equal always when we build a RTD table?

Thanks
Top
Source: — Problem Solving |

GMAT/MBA Expert

User avatar
GMAT Instructor
Posts: 2095
Joined: Tue Dec 04, 2012 3:22 pm
Thanked: 1443 times
Followed by:247 members

by ceilidh.erickson » Mon Feb 03, 2014 8:22 am
When a question is phrased "two people are 14 miles apart and begin walking toward each other," it is implied that they start walking at the same time, unless otherwise stated. If you're not told that one of them stops walking, we can assume that they both continue walking until they meet. Thus, at the moment when they meet, they have been walking for exactly the same amount of time.

In your second example, we would have to be told that A walks for only 1 hour, then stops. Otherwise, if they started walking at the same time, and walked without stopping, they would meet at a different point.

Unless you're told otherwise, assume that they start walking at exactly the same time, and continue without stopping until they meet each other (at the same time).
Ceilidh Erickson
EdM in Mind, Brain, and Education
Harvard Graduate School of Education
Top

GMAT/MBA Expert

User avatar
Elite Legendary Member
Posts: 10392
Joined: Sun Jun 23, 2013 6:38 pm
Location: Palo Alto, CA
Thanked: 2867 times
Followed by:511 members
GMAT Score:800

by [email protected] » Mon Feb 03, 2014 2:17 pm
Hi dddanny2006,

These types of "combined rate" questions on the GMAT are usually very specific in how they're worded. You'll be told if both people begin walking at the same time; if the two people begin walking at different times, then you'll be told that information. You'll find that a basic drawing and some calculations (using the distance formula) are what it takes to solve these problems.

GMAT assassins aren't born, they're made,
Rich
Contact Rich at [email protected]
Image
Top
GMAT Instructor
Posts: 2630
Joined: Wed Sep 12, 2012 3:32 pm
Location: East Bay all the way
Thanked: 625 times
Followed by:119 members
GMAT Score:780

by Matt@VeritasPrep » Mon Feb 03, 2014 3:48 pm
dddanny2006 wrote:Hey guys..
I've always wondered why we consider time to be the same in case of both bodies whether they move towards each other,or go away from each other..
Two ways of thinking about this:

1:: Time is the same for everyone because time travels at the same pace for everyone. (I know that physicists would quibble about this, but for our purposes it's close enough.) An hour for me is the same as an hour for you, even if our rates in space differ.

2:: If you and I are traveling toward each other and we leave at the same time, then we'll also meet at the same time: hence our time spent traveling is the same.

Our rates and distances will usually vary, of course, but our times (in these sort of problems) are generally equal, unless specified otherwise.
Top
Post new topic Post Reply
• Page 1 of 1