# Cop car and opposing directions

1. Apr 2, 2009

### Jeebus

This is not a homework problem, I just want to know if this is possible.

The cop is traveling in the opposite direction of me. Let's say I'm going 60mph and am speeding. He turns around, catches up to me, and clocks me going 15mph over the speed limit.

Is this even possible? I found this link: http://hyperphysics.phy-astr.gsu.edu/hbase/sound/radar.html, but the cop is stationary. Does radars even work this way? Curious to know, thanks!

2. Apr 2, 2009

### mathman

The radar will give your speed relative to his. Presumably he knows how fast he is going and can do the arithmetic - it may even be programmed into his device.

3. Apr 3, 2009

### Lok

Not all radars are certified to be used while the cop car is moving ( usually old ones). But recent ones are and work best only in straight lines (curves give wrong results, although even that can be overcome by good programing, don't now if yet implemented). They work by adding the cop cars speed to yours. So if he is driving in the opposite direction you would have - 110mph ( your speed as seen by radar) + 50mph (cop car speed) = -60mph ( your true speed, the - sign is for opposite direction relative to the cop car)