Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Alinta on 27/09/2012 05:36:26

Title: Beginner's question - velocity vs displacement?
Post by: Alinta on 27/09/2012 05:36:26
Hi all,

Unfortunately I've never had an education that involved physics (or any science beyond primary school), and I am just now finding interest in the subject in my mid-20s, so I apologise in advance for the perhaps very basic questions that I might be asking on this forum.

I've been reading an online textbook of sorts about kinematics, but I'm failing to grasp the exact difference between velocity and displacement when it comes to an object with 0 displacement or 0 velocity - since they are both described as vector quantities that have an answer of 0 if something moves but then returns to its original place.

Is the difference: displacement refers to distance in metres (distance plus direction) whereas velocity is described in a unit of time (speed plus direction)?

Thanks for any help you can give me, there may be a few more kinematics questions to come from me! Moving on to 'dynamics' next...

Title: Re: Beginner's question - velocity vs displacement?
Post by: Soul Surfer on 27/09/2012 15:41:58
To be mathematically correct, velocity, is the rate of change of displacement that is meteres oer second (in a particular direction of course).  Acceleration is of course the rate of change of velocity. meters per second per second.

So, for a body to move from a position and then return to the same position.  for example a (friction free) pendulum bob pulled to one side from its rest point and let go it swings past it rest point and then swings back to stop briefly at the position you let it go.  It accelerates up to a maximum speed at its rest point and then decelerates (negative acceleration until it stops)  its velocity is then zero it then accelerates and decelerates in the reverse direction (negative velocity) to return to its original position.
Title: Re: Beginner's question - velocity vs displacement?
Post by: JP on 27/09/2012 15:52:46
Hi Alinta,

One way to think about it is that you're taking a trip from your starting point to your ending point in a given amount of time.  Displacement just tells you how far and in which direction you'd have to go to take this trip.  Since you also want to take the trip in a certain amount of time,  average velocity tells you how fast, and in which direction you have to go if you want to go in a straight line between those points in that amount of time.  That's why it's given by the displacement vector divided by the trip time.

If your start and end points are in the same spot, then you don't have to move at all to take the trip (displacement = 0) and your speed is obviously zero since you're not moving at all (average velocity = 0). 

And yes, you're right about units: displacement is a measure of length between two points, so it's measured in units of distance.  Average velocity is a measure of speed if you traveled a straight line between those points in a given time, so it's measured in units of speed: distance/time. 

By the way, you'll probably see another type of velocity very soon in your studies: instantaneous velocity (that's why I emphasized average above).  This is given, for example, by the speedometer on your car and the direction you're driving at every instant of your trip.  If you run an errand and end up back at home, your displacement and average velocity are both zero, but your instantaneous velocity was obviously non-zero for most of your trip.
Title: Re: Beginner's question - velocity vs displacement?
Post by: Alinta on 28/09/2012 07:02:34
hmm yikes ok thanks guys!