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

Introduction / Context:
Uniform motion is one of the simplest types of motion studied in physics. It is important to clearly understand what remains constant and what can change in such motion. This question focuses on uniform motion along a straight line and asks you to identify which physical quantity stays constant with time.

Given Data / Assumptions:

• The motion is along a straight line.
• The motion is described as uniform.
• We compare quantities such as acceleration, deceleration, force, and velocity.

Concept / Approach:
Uniform motion in a straight line means the object covers equal distances in equal intervals of time in a fixed direction. Therefore, both the speed and direction of motion remain constant, which means that the velocity remains constant. If velocity is constant, then acceleration, which is the rate of change of velocity, must be zero. There is no deceleration, and the net force along the line of motion is zero according to Newton second law.

Step-by-Step Solution:
Step 1: Recall that uniform motion implies constant speed and direction. Step 2: Therefore, the velocity, which includes both speed and direction, remains constant. Step 3: Since velocity does not change, acceleration and deceleration are both zero and do not remain non-zero constants. Step 4: According to Newton second law, if acceleration is zero, the net force must also be zero, so force is not a non-zero constant in this context. Step 5: Conclude that the correct constant quantity is velocity.

Verification / Alternative check:
Imagine a car on a straight highway moving at a steady 60 km/h without changing direction or speed. Its velocity remains constant. There is no change in speed, so acceleration is zero. There is no continuous non-zero force pushing it to increase speed; instead, forces are balanced. This simple example confirms that in uniform straight-line motion, velocity is the quantity that stays constant over time.

Why Other Options Are Wrong:

• Acceleration: In uniform motion, acceleration is zero because velocity does not change with time. It does not remain a non-zero constant.
• Deceleration: Deceleration refers to negative acceleration and is absent in uniform motion, where speed is not decreasing.
• Force: The net force is zero in uniform motion, but the question is about the motion quantity that remains constant. Velocity describes the motion directly, not the force.

Common Pitfalls:
Some students confuse the idea of uniform motion with uniformly accelerated motion. In uniformly accelerated motion, acceleration is constant but velocity changes. In uniform motion, it is velocity that is constant and acceleration that is zero. Keeping these two cases conceptually separate is important for solving kinematics problems correctly.

Final Answer:
During uniform straight-line motion, the velocity of the object remains constant with time.