Question

Which of the following is not an example of uniform motion?
A). A car covering the equal distance in equal intervals of time.
B). A bus moving with constant speed.
C). A free-fall body
D). A ball rolling on a floor without changing its velocity

Answer 1

Hint: Define uniform motion. A body in uniform motion covers equal distances in equal intervals of time. Check the given options in the question which satisfies the above condition. If they satisfy the condition then they are in uniform motion.

Complete step by step answer:
In motion, if the acceleration of the body is zero or the rate of change of velocity for time is zero, that motion is called uniform motion.
In uniform motion, the acceleration 'a' is zero
$a=0$
$\dfrac{\Delta \text{v}}{\text{ }\\!\\!\Delta\\!\\!\text{ t}}\text{=0}$
$\text{ }\\!\\!\Delta\\!\\!\text{ v=0}$
Here $\text{ }\\!\\!\Delta\\!\\!\text{ v}$ is the change in velocity.
So, we have to check all the options where acceleration is zero or velocity is constant. That will be in uniform motion.
Option(A)
A car covers ${{d}_{1}}$distance in the first t seconds and covers ${{d}_{2}}$ distance in another t second.
So,
${{v}_{1}}=\dfrac{{{d}_{1}}}{t}$ and ${{v}_{2}}=\dfrac{{{d}_{2}}}{t}$
As per given information, ${{d}_{1}}={{d}_{2}}$
Therefore, ${{v}_{1}}={{v}_{2}}$
That proves that the change in velocity is zero that means it is in uniform motion.
Option(B) Bus has the constant speed
We know that speed is the scalar quantity of velocity. Since, the speed of the bus is constant over time the acceleration will be zero because the change in velocity will be zero. That is also an example of uniform motion.
Option(C) A free-fall body
That means an object is falling under gravity. And we know the acceleration of gravity $g=9.8\dfrac{m}{{{s}^{2}}}$ so their acceleration is not zero.
So, this is not an example of uniform motion
Option(D)
In this option, velocity is also constant so it is also an example of uniform motion
The answer is (C).

Note: For a body in uniform if we plot a graph for position vs time, we will always get a straight line with a slope. If we plot a velocity vs time graph for an object in uniform motion, we will get a straight line parallel to the time axis.