which describes motion in which the person or object is accelerating?

Question

a. a bird flies straight from a tree to the ground without changing speeds

b. a dog walks at a constant speed along a straight sidewalk

c. a girl runs along a straight path the same distance each second

d. a truck moves around a curve without changing speed 

Answer 1

Acceleration is a change in velocity.  Note that velocity is a vector which means that it has both a magnitude (which is called speed) and a direction.  A change in either magnitude or direction is due to acceleration.  Objects that are moving at constant speed in a straight line (as in choices a, b and c) are not accelerating.  An object that is changing direction by moving around a curve (as in choice d) is accelerating.  By the way, you can feel this acceleration manifested as a force (by F = ma) pushing you outwards if you are sitting in a car (or a roller coaster) that is moving around a curve.