Earth revolves around the Sun in a circular orbit with a uniform speed. Is this motion uniform or accelerated?

Question

Earth revolves around the Sun in a circular orbit with a uniform speed. Is
this motion uniform or accelerated?

Answer 1

Kenneth is correct, except the question does ask us to assume the orbit is circular.  In that case, the only force acting on the Earth is gravity from the Sun.  Assuming the Sun is at the center of the Earth's circular orbit, that force always points toward the center of the Earth's circular orbit, and is thus always perpendicular to the Earth's direction of motion.  Thus, it cannot change the speed (magnitude) of Earth's velocity.  However, it does constantly change the direction of Earth's motion, and since vectors like velocity have magnitude and direction, changing either one constitutes a change in velocity, which is acceleration (in this case, the acceleration vector points toward the center of the circle, making it a centripetal acceleration).

 

So the motion is accelerated.  But the confusing thing about the question as stated is that this kind of motion, where the speed of the object traveling in the circle does not change, is typically called "uniform circular motion," as opposed to an object that is traveling in a circular path, but also changing speed.  But if they are asking to distinguish between accelerated and non-accelerated motion, the answer is definitely accelerated.  Another way of stating this is to say the object is not in equilibrium (since its net acceleration is not zero as it moves on the path).

Answer 2

Earth's orbit, as is the case with most planets, is elliptical, but is close to circular.  Speed does vary somewhat; consult Kepler's Laws.