Final answer:
The primary factor in decreasing an object's kinetic energy is to decrease its velocity. Since kinetic energy is proportional to the square of the velocity, a reduction in velocity can significantly reduce kinetic energy, more so than a decrease in mass or other factors such as changing direction or stopping distance.
Step-by-step explanation:
The primary factor in decreasing the kinetic energy of an object is to decrease the velocity of the object. Kinetic energy (KE) is given by the equation KE = 1/2 m v^2, where m is the mass of the object and v is the object's velocity. Since kinetic energy is proportional to the square of the velocity, even a small decrease in velocity results in a significant decrease in kinetic energy. Decreasing the mass of the object also reduces kinetic energy, but not as drastically as decreasing velocity, because kinetic energy varies linearly with mass, whereas it varies with the square of the velocity. Changing the direction or decreasing the stopping distance does not directly affect kinetic energy.
Considering the provided examples, when a ball is tossed with air resistance, both the final velocity and final kinetic energy decrease as energy is transferred to the air by heat due to air resistance. In the case of a toy car going up and down a ramp, kinetic energy increases when going down due to gravity and decreases when going up as gravity opposes the motion. With friction present, such as on a carpet, kinetic energy is also decreased because friction opposes motion. Moreover, lowering the temperature in chemical reactions is a method to decrease the kinetic energy available for activation energy, thereby decreasing the reaction rate.