It is equal to the work done on it to make it acquire its velocity from rest.
Explanation
Kinetic energy is the energy possessed by an object due to its motion. Quantitatively, it is defined through the relationship between work and energy, specifically the Work-Energy Theorem, which states that the net work done on an object equals the change in its kinetic energy.
Option Analysis
- It is equal to the force required to stop it. is Incorrect: Force is an influence that can change the motion of an object. While a force is required to stop a moving body, kinetic energy is not equal to the force itself. Energy has dimensions of $[ML^2T^{-2}]$, whereas force has dimensions of $[MLT^{-2}]$.
- It is equal to the work done on it to make it acquire its velocity from rest. is Correct: The kinetic energy of a moving body is quantitatively defined as the amount of work done on the body to accelerate it from a state of rest to its current velocity. Conversely, it is also the amount of work the body can do against an opposing force before coming to rest. Mathematically, $K = \frac{1}{2}mv^2$.
- It is equal to the product of its weight and speed. is Incorrect: The product of weight (a force) and speed has the dimensions of Power ($Force \times Velocity$), not Energy.
- It is equal to the distance it travels before stopping. is Incorrect: Distance is a measure of length with dimensions $[L]$. Kinetic energy cannot be defined simply as distance, although the distance required to stop a body is proportional to its kinetic energy (assuming a constant retarding force).
Key Takeaway
Kinetic Energy is the measure of work done on an object to bring it from rest to a specific velocity. It is a scalar quantity given by the formula $K = \frac{1}{2}mv^2$.