# Light from a star travels to Earth in a straight line at a constant speed of almost 300,000 km/s. What is the acceleration of the light?

Feb 25, 2016

The answer is 0 $k \frac{m}{s} ^ 2$.

#### Explanation:

To understand what this question is asking, its important to understand what velocity and acceleration are and how the two are related.

Velocity is the rate of change of an object's position as a function of time. (i.e. how far is an object moving over a given amount of time).

This can be written mathematically as:

$v = \frac{\mathrm{dx}}{\mathrm{dt}}$

Acceleration is the rate of change of an object's velocity as a function of time. (i.e. is the object moving faster or slower as time goes on)

This is written mathematically as:

$a = \frac{\mathrm{dv}}{\mathrm{dt}}$
or,
$a = {d}^{2} \frac{x}{\mathrm{dt}} ^ 2$

If velocity is constant (as is stated in the problem), its derivative (acceleration) must be 0.

A more intuitive, less mathematical way to think about it is to say if the velocity isn't changing, then it's rate of change must be 0.