During uniform motion of an object along a straight line, which physical quantity describing the motion remains constant with time?

Introduction / Context:
This question examines your understanding of uniform motion along a straight line, a basic idea in kinematics. Uniform motion means that the object covers equal distances in equal intervals of time in a fixed direction. Identifying which quantity remains constant in such motion helps you distinguish between speed, velocity, distance and acceleration in one dimensional motion problems.

Given Data / Assumptions:

• An object is moving in a straight line.
• The motion is uniform, meaning there is no speeding up or slowing down.
• We assume a fixed reference frame.
• We must choose which listed quantity stays constant with time.

Concept / Approach:
Uniform motion along a straight line means constant velocity. Velocity is a vector that has both magnitude (speed) and direction. In uniform straight line motion, both the speed and the direction remain unchanged, so velocity is constant. Distance from the starting point increases steadily with time and does not remain constant. Time itself always increases. Acceleration is the rate of change of velocity; if velocity is constant, acceleration is zero, but zero is a constant value. However, when the question asks which quantity remains constant describing the state of motion, velocity being constant is the key feature of uniform motion.

Step-by-Step Solution:
Step 1: Define uniform motion as motion in which equal distances are covered in equal time intervals in a given direction. Step 2: Recognise that this implies constant speed and a fixed direction of motion. Step 3: Combine constant speed and fixed direction to conclude that velocity, a vector with magnitude and direction, remains constant. Step 4: Understand that as time passes, the distance travelled from the starting point increases steadily, so distance does not remain constant. Step 5: Recall that acceleration is defined as the rate of change of velocity. For uniform motion, velocity does not change, so acceleration is zero throughout, but the important description of the motion is that velocity is constant. Step 6: Note that time is never constant; it always moves forward and increases. Step 7: Therefore, among the options, velocity is the quantity that remains constant with time in uniform straight line motion.

Verification / Alternative check:
Consider a car moving on a straight road with its speedometer fixed at 60 km/h and direction unchanged. Over each hour, it covers 60 kilometres, and its velocity vector remains the same in magnitude and direction. The distance from the starting point after 1 hour, 2 hours and 3 hours is 60, 120 and 180 kilometres respectively, which clearly changes. Time also advances. Acceleration remains zero, but when we define the motion as uniform, we focus on the constancy of velocity. This everyday example confirms the reasoning.

Why Other Options Are Wrong:
Time cannot remain constant because it continuously increases as motion proceeds. Acceleration is zero in uniform motion, but the question points to the characteristic of the motion itself, which is constant velocity; marking acceleration as constant can be misleading in this context. Distance increases steadily as the object moves, so it does not remain constant with time.

Common Pitfalls:
Some students confuse uniform motion with rest and think distance is constant. However, in uniform motion, the object is definitely moving, so distance from the start changes. Others focus on acceleration being zero and may select acceleration as the answer. While acceleration is indeed zero, the defining property of uniform straight line motion is that velocity is constant. Remembering that uniform motion means constant velocity helps you select the right option quickly.

Final Answer:
In uniform motion along a straight line, the quantity that remains constant with time is the Velocity of the object.