Correct Answer:
Option C —
only iv
The three equations of motion can be applied only when acceleration is constant.
Why Constant Acceleration is the Key
These specific equations were designed based on the rule that the rate at which an object speeds up or slows down remains the same throughout the entire movement. If the acceleration changes, these equations will give an incorrect result.
Analysis of the Options:
i. In one dimensional motion: Moving in a straight line is not enough to use these equations. If a driver is constantly changing how hard they press the gas pedal, the acceleration is not steady, and the equations won't work.
ii. In circular motion: In a circle, the direction of movement is constantly shifting. Because acceleration involves both speed and direction, a change in direction means the acceleration is not constant.
iii. Simple harmonic motion: In systems like a swinging pendulum, the force and acceleration change depending on the position of the object. Since the acceleration is different at every point of the swing, these equations cannot be used.
iv. When acceleration is constant: This is the fundamental requirement. As long as the acceleration is uniform, the equations remain valid, whether the object is falling under gravity or a car is speeding up at a steady rate.
Summary
For these rules to apply, the acceleration must be a steady, unchanging value in both its strength and its direction during the time you are measuring.
Answer verified by Quintessence Classes faculty — Karan Nagar, Srinagar.
