Question

choose all the examples below in which the object would have a positive acceleration. (assume the initial direction of motion is the positive direction)
a persons head is hitting an airbag in their car because they were just in a crash.
a penny was dropped off the top of the tallest building in indianapolis 2 seconds ago.
a bicycle is moving in the positive direction and increasing its velocity.
a child is riding on a carnival ride that goes around in a circle.
a person is standing on a moving floor such as those at large airports.
a car is driving down the interstate at a constant speed because the cruise control is on.

Explanation:

Step1: Analyze each example

Acceleration is the rate of change of velocity. Positive acceleration means velocity is increasing in the positive - direction or decreasing in the negative - direction.

1. When a person's head hits an airbag in a crash, the head is decelerating (negative acceleration) as it is coming to a stop quickly.
2. When a penny is dropped, it is in free - fall. In the absence of air resistance, it accelerates downwards (assuming downwards as positive in this context). The acceleration due to gravity acts in the direction of motion, so it has positive acceleration.
3. A bicycle moving in the positive direction and increasing its velocity has positive acceleration as the velocity is increasing in the positive direction.
4. For a child on a circular carnival ride, the acceleration is centripetal acceleration which changes the direction of velocity, not necessarily increasing the speed in a linear sense along a positive direction. So, it doesn't have positive acceleration in the linear positive - direction sense here.
5. A person standing on a moving floor at an airport is moving at a constant velocity (assuming the floor is moving at a constant speed), so there is no acceleration.
6. A car driving at a constant speed with cruise control on has no acceleration as the velocity is not changing.

Answer:

A penny was dropped off the top of the tallest building in Indianapolis 2 seconds ago.; A bicycle is moving in the positive direction and increasing its velocity.