# B Instantaneous velocity

1. Aug 11, 2016

### pravignya raj

1.one question i have just read n my book that find inst. velocity at a point ? i think it is a wrong quotation since we need atleast two points for measuring dx length so my teacher taught me that assume point p where we have to find inst. velocity thank take one point before and after p that makes dx length and solve the question but here we are not getting the accurate velocity at p. sir so dont you think that we can not find velocity at a point?
Or it is like this
2.if we go to the microscopic level and see the point then there exist a length dx from one end of point to the other end forms length dx...so from here we can find inst. Velocity at a particular point by joining one microscopic end of point to origin as vector r1 and other microscopic end of point to origin as vector r2.?
My question it is wrong to say that we can not find instantaneous velocity at a point it needs at least two point or my 2 explanation is correct ?

2. Aug 11, 2016

### andrewkirk

You are correct that to calculate velocity from observations of position you need at least two points. But in certain systems there may be other ways that only require one observation.

For instance under Maxwell's laws, when a magnet is moving near a conductor, the current in the conductor is proportional to the velocity of the magnet. So by making a single observation, through a multimeter, of the current in the wire, we could work out the velocity of the magnet.

3. Aug 11, 2016

### pravignya raj

Sir What about if see point at microscopic level then there exist length dx..

Last edited: Aug 11, 2016
4. Aug 11, 2016

### andrewkirk

You still need two observations, being the times when the object is at the beginning and end of the length dx.

5. Aug 11, 2016

### pravignya raj

Thank u sir

6. Aug 11, 2016

### Staff: Mentor

I'm guessing you haven't taken calculus yet: one of the fundamental purposes is to find the slope of a curve (such as a distance vs time) at a single point.

7. Aug 11, 2016

### pravignya raj

Just a beginner...