Answer:
only one: a sky diver falling at a constant speed
Step-by-step explanation:
Let's recall that acceleration is a change in velocity, which means either a change in the speed of an object or a change in its direction.
let's now analyze each situation:
- a race car going around a circular track at 150 mph --> here the direction of motion is changing, so the object is accelerating
a sky diver falling at a constant speed --> here both the speed and the direction are constant, so the object is NOT accelerating
- a heavy box sliding across the floor, after being released --> here the speed is changing (In particular, it will decrease, as the friction will slow down the box), so the object is accelerating
- a bowling ball colliding with a pin --> here the speed is changing (when the ball colides with the pin, it slows down), so the object is accelerating
- a vibrating guitar string --> the speed of the vibrating string changes during the vibration (and the direction as well), so the object is accelerating
- a baseball flying through the air --> the speed of the baseball is changing (because there are unbalanced forces acting on the flying ball: the gravity and the air resistance), so the object is accelerating
- a child swinging on a swing --> the speed of the child is changing (and the direction as well) during the oscillation, so the object is accelerating
So, there is only 1 situation in which objects are NOT accelerating:
a sky diver falling at a constant speed