Optics: What is the speed of the sprinter's image?

[bcjochim07]

Homework Statement

A sports photographer has a 150 mm focal length lens on his camera. The photographer wants to photograph a sprinter running straight away from him at 5 m/s. What is the speed of the sprinter's image at the instant the sprinter is 10 m in front of the lens?

Homework Equations

The Attempt at a Solution

Here's my best guess:

(1/f)=(1/s')+(1/s) (1/.150m)= (1/s') + (1/10m) s'=.152, so the absolute value of the magnification is (.152/10m)= -.0152

and (.0152)(5m/s)= .076 m/s = 76 mm/s Is this anywhere close to correct?

 

[bcjochim07]

Any thoughts about this one?