When it is stationary, the half-life of a certain subatomic particle is T0. That is, if N of these particles are present at a certain time, then a time T0 later only N/2 particles will be present, assuming the particles are at rest. A beam carrying N such particles per second is created at position x=0 in a high-energy physics laboratory. This beam travels along the x axis at speed v in the laboratory reference frame and it is found that only N/2 particles per second travel in the beam at x=2cT0, where c is the speed of light.

Answertraveling around a circular trackexplanation"traveling around a circular track" is the best answer as you could be traveling around a circular track at a constant speed. your velocity would therefore have the same magnitude but will always be constantly direction. since acceleration is the rate of change of velocity, the rate of change will only involve the direction of velocity and not the magnitude.wrong choices: "an object at rest" does not accelerate at all"increasing speed while traveling around a curve" means that the acceleration will not be constant due to only change in direction; it will be constant due to both magnitude and direction"an object in free fall" does not necessarily change direction