04. Very Short Question Answer: Describing Motion Around Us

Q1: When is an object said to be in motion and when is it said to be at rest?

Ans: An object is said to be in motion if its position with respect to a reference point changes with time. It is said to be at rest if its position with respect to the reference point does not change with time.

Q2: What is displacement?

Ans: Displacement is the net change in the position of an object between two given instants of time. It requires specifying both a direction and its numerical value (magnitude).

Q3: What is the SI unit of distance and displacement?

Ans: The SI unit of both distance and displacement is the metre $(\text{m})$.

Q4: What is the difference between distance travelled and displacement?

Ans: Distance travelled is the total length of the path covered by an object, while displacement is the net change in position between two instants. Distance only has a numerical value, while displacement has both magnitude and direction.

Q5: Define average speed. Write its formula.

Ans: The average speed of an object is the total distance travelled divided by the time interval during which this distance is covered.

$$\text{average speed}=\frac{\text{total distance travelled}}{\text{time interval}}$$

Q6: Define average velocity. Write its formula.

Ans: The average velocity of an object in a time interval is the displacement divided by the time interval in which the displacement occurs.

$$v_{av}=\frac{s}{t}$$

where $s$ is displacement and $t$ is the time interval.

Q7: What is the SI unit of average speed and average velocity?

Ans: The SI unit of both average speed and average velocity is metre per second, represented as ${\text{m s}}^{-1}$ or $\text{m/s}$. It is also commonly measured in kilometre per hour $({\text{km h}}^{-1})$.

Q8: What is the difference between uniform motion and non-uniform motion in a straight line?

Ans: In uniform motion, an object travels equal distances in equal intervals of time, moving at a constant speed. In non-uniform motion, the object travels unequal distances in equal intervals of time, with increasing or decreasing speed.

Q9: Define average acceleration. Write its formula.

Ans: The average acceleration of an object over a time interval is the change in its velocity divided by the time interval.

$$a=\frac{v-u}{t_2-t_1}$$

where $u$ is initial velocity, $v$ is final velocity, and $(t_2-t_1)$ is the time interval.

Q10: What is the SI unit of average acceleration?

Ans: The SI unit of average acceleration is ${\text{m s}}^{-2}$ or ${\text{m/s}}^2$. Like velocity, acceleration has both magnitude and direction.

Q11: What does a straight line position-time graph indicate about the motion of an object?

Ans: A straight line position-time graph indicates that the object is moving with a constant velocity. A curved position-time graph indicates that the velocity is not constant and the object is in accelerated motion.

Q12: What does the slope of a position-time graph represent?

Ans: The slope of a position-time graph represents the magnitude of the average velocity of the object.

$$v=\frac{s_2-s_1}{t_2-t_1}=\frac{BC}{CA}$$

A steeper slope means a higher velocity.

Q13: What does a straight line parallel to the time axis on a position-time graph represent?

Ans: A straight line parallel to the time axis on a position-time graph means the position of the object is not changing with time. This represents a stationary object, i.e., the object is at rest.

Q14: What does the area enclosed by the velocity-time graph and the time axis represent?

Ans: The area enclosed by the velocity-time graph and the time axis for a given time interval is equal to the displacement of the object in that time interval. For example, for constant velocity:

$$\text{displacement}=\text{velocity}\times \text{time interval}$$

Q15: What does the slope of a velocity-time graph represent?

Ans: The slope of a velocity-time graph gives how fast the velocity is changing with time, i.e., it represents the acceleration of the object.

$$a=\frac{v-u}{t_2-t_1}=\frac{BC}{CA}$$

Q16: Write the three kinematic equations for motion in a straight line with constant acceleration.

Ans: The three kinematic equations are:

$$\begin{array}{cccc}& v=u+at& & \text{(i)}\end{array}$$

$$\begin{array}{cccc}& s=ut+\frac{1}{2}a{t}^2& & \text{(ii)}\end{array}$$

$$\begin{array}{cccc}& v^2=u^2+2as& & \text{(iii)}\end{array}$$

where $u$ is initial velocity, $v$ is final velocity, $a$ is acceleration, $t$ is time, and $s$ is displacement.

Q17: What is uniform circular motion?

Ans: When an object moves in a circular path with constant (uniform) speed, its motion is called uniform circular motion. Although the speed is constant, the direction of velocity continuously changes.

Q18: If the radius of the circular path is R and the object takes time T to complete one revolution, what is its average speed?

Ans: The average speed of the object is:

$$v_{av}=\frac{2\pi R}{T}$$

where $2\pi R$ is the circumference of the circular path and $T$ is the time taken for one complete revolution.

Q19: Why is uniform circular motion considered accelerated motion even though the speed is constant?

Ans: In uniform circular motion, the direction of velocity continuously changes even though the speed remains constant. Since acceleration results from any change in velocity (including change in direction), the motion is accelerated.

Q20: What is the difference between an instant of time and a time interval?

Ans: An instant of time is a single reading of a clock at a given point of time. A time interval is the time duration between two instants of time, i.e., between two clock readings.