What would the displacement of this object be?

[mahela007]

An object is situated 10m ahead of the reference point. It starts moving in the negative direction (towards the reference point) and then continues until it reaches a point 5m behind the reference point.

What is the displacement of the object?

 

Here's my answer.

The displacement is the difference between the initial and final positions of the object. As the initial position was 10m ahead of the reference point and the final position was 5m behind the reference point the total displacement would be 15m.

However everyone else I ask (Except my teacher.. I haven't asked him yet) says that the displacement is 5 because it is only 5m behind the reference point.. Which answer is correct? 15 or 5?

I've included a (rather crude) diagram of the problem. A is the initial position. A' is the final position and O is the reference point. The object moves from A to A'.

A____________________O________A'

<-------------10m----------------><-----5m-->

[swansont]

Displacement is a vector giving you the direction and distance from a reference point. The important thing here is specifically what is being asked: are you looking for the displacement from the reference point, or the displacement from the starting point?

[mahela007]

that hasn't been given in the question. I'm asked simply to state the displacement... Would that generally mean "from the reference point" or "from the initial position"?

[swansont]

I would take it to mean from the starting point, but have to observe that it's poorly worded. It should explicitly tell you relative to whom you are making the measurement.

[mahela007]

thanks guys...