BBC Facts

[ck1]

Quote from the BBC website

"The crossover point on a pair of scissors when they are smartly closed is moving faster than the speed of light"

Is that right, if so, how?

[gen2]

That is because as the blades become almost parallel, a very small movement of the blades closer together moves the crossover point a huge distance along its length.

[R1Geezer]

This is a misnomer, they are closing on each other faster than light speed but not actually moving at light speed.

Imagine you had 2 cars capble of 75% light speed and you put them back to back and they acclerated away from each other at 0.75C then they are parting at 1.5C, same sort of thing here.

[gen2]

I think you are misunderstanding this R1Geezer. The BBC Website does NOT say the BLADES are closing at faster than the speed of light - in fact they will not even break the sound barrier. It is the CROSSOVER POINT that moves along the blade faster than the speed of light.

[R1Geezer]

I understand, same explanation though.

[jake-the-peg]

I wonder if any of this year's A level graduates would like a Job as the BBC's Science correspondent for their web page.

They seem to have a vacancy - well they will when I've finished with them.

This is not helpful to the public understanding of Science

[New Judge]

Without wishing to get too bogged down in the question, Geezer, I just thought I�d raise a point of pedantry under your �two cars� scenario.

The two cars would not be receding at 1.5c (c = speed of light) as nothing can travel relative to another body at more than c.

Newtonian Physics dictates that the relative velocity of two bodies travelling (relative to a fixed point) at velocities u and v is equal to (u+v).

However Einstein�s physics (as explained by his theories on relativity) considers that no body may travel relative to another at at speed greater than c. The formula for calculating their relative speeds becomes:

(u+v) / (1+(uv/c2))

At low velocities there is very little difference between the two calculations. However, as velocities approaching c are considered there is considerable disparity.

Using this formula the speed of recession of the two cars is 0.96c or thereabouts.