Lens question

Does that mean you tried to find the average speed of the image? I think their talking about "initial speed" indicates that they want instantaneous speed.

 

How do you get instantaneous speed (or more precisely, velocity) from position?

 

Yup.

 

You know the relationship between ##d_0## and ##d_i##: $$\frac 1 {d_o} + \frac 1 {d_i} = \frac 1 f.$$ Take the derivative with respect to t, of both sides of this equation, and you'll have a relationship between ##\frac {dd_o}{dt}## and ##\frac {dd_i}{dt}##.

 

With Δt = 1 s I get about a 13% difference in the final answer, using the full precision of my calculator.

I agree, if Dan shows us his working, we can tell him if he at least calculated that approximation correctly.

 

Good luck... if I want to get up for work in the morning I need to go to bed now.

 

Yes, that's what I got. Another way to get more accuracy would be to use a smaller time interval. In principle, if you make it small enough, the answer will get close enough to the exact answer to make your software happy. But you have to be very careful to avoid roundoff errors.