How can an object moving at a constant speed have a nonzero acceleration?

Answer 1

Let's review some terms before we tackle this one. Speed is displacement per unit of time. We know 60 miles per hour is a speed. Velocity is speed with a direction vector associated with it. We know 60 miles per hour east is velocity. Acceleration is a change in velocity. That means if an object changes its speed or its direction or both, it is accelerating.

If an object has a given velocity and it slows down or speeds up, it is accelerated. But if the same object changes direction without a change in speed, it is still experiencing acceleration. A force had to act on the object to change its direction, even though its speed didn't change. Thus, an object can accelerate even though it does not change speed.

Answer 2

Consider a ball rolling on ice. After I toss the ball onto the ice, there will be no forces acting on the ball, other than gravity and the normal force from the ice. Thus, the net force will be zero. Due to Newton's second law, force = mass*acceleration, the acceleration must be zero. Note that this only applies to the center of mass.

If you are worried about the angular acceleration of the object, you still don't have reason to be worried. The speed at which the ball is rolling does not change on frictionless ice. Thus, the angular velocity is constant. Angular acceleration must then be zero. (Otherwise the angular velocity would change.)

Answer 3

If, for example, there's a cup on a table, it's not moving, but since the Earth rotates, it still has movement.

Answer 4

Because an acceleration is a CHANGE in speed or direction.