Final answer:
The average acceleration of a deer running in a straight line with a constant velocity is 0 m/s² because the velocity does not change over time.
Step-by-step explanation:
If a deer is running in a straight line with a constant velocity, the deer's average acceleration is 0 m/s². This is because acceleration is defined as the rate of change of velocity over time. Since the velocity is constant and not changing, the rate of change is zero, meaning there is no acceleration.
In physics problems like the example provided regarding the 900-kg car and the 150-kg deer, conservation of momentum is typically used to find the final velocity of the system after a collision when two objects stick together. Such problems often involve calculating changes in velocity and therefore accelerations.