CBSE Class 9 Science Chapter 04 Describing Motion Around Us MCQs Set 02

Answer: (A) Reference point

Explanation:
Position is always described relative to a chosen fixed reference point, and an object counts as 'in motion' only if its position with respect to this point keeps changing over time.

Answer: (A) How quickly one quantity changes compared to a change in another quantity, such as position changing with time

Explanation:
A rate of change is simply a ratio — here, the change in position divided by the change in time — which is exactly how average velocity is calculated.

Answer: (A) Displacement 0 m; total distance 200 m

Explanation:
Since she ends up exactly where she started, her net change in position is zero, but she has actually covered 100 m out and 100 m back, adding up to 200 m of total distance.

Answer: (A) Q, S, P, R

Explanation:
The athlete begins at O, first reaches B, continues on to A, and finally comes back to B — matching the sequence Q, S, P, R.

Answer: (A) The magnitude of displacement is always less than or equal to the total distance travelled

Explanation:
Displacement only accounts for the straight-line change between start and end points, while distance adds up every bit of the path covered, so displacement can never exceed distance — the two become equal only when the object never turns back.

Answer: (A) Simplified models reveal underlying patterns that are harder to spot in complicated, real-world motion

Explanation:
Breaking a messy, real phenomenon down into clean, idealised cases makes it possible to first understand the basic rules before applying them to trickier situations.

Answer: (A) The acceleration of the object stays constant throughout the motion

Explanation:
These equations were derived assuming acceleration doesn't change during the interval considered; the chapter explicitly notes they no longer apply once acceleration varies.

Answer: (A) The value that average velocity approaches when the time interval around an instant is shrunk to an extremely small duration

Explanation:
As the time interval used to compute average velocity is made smaller and smaller around a given moment, the result settles on a fixed value — this limiting value is called instantaneous velocity.

Answer: (A) In the same direction as the bus's velocity

Explanation:
Whenever the magnitude of velocity is increasing, the chapter states that acceleration points along the same direction as the velocity itself — unlike braking, where it opposes it.

Answer: (A) Metre

Explanation:
Both quantities describe a length, so their unit is the metre — velocity adds a 'per second' and acceleration a 'per second squared', but distance and displacement themselves are simply metres.

Answer: (A) To highlight that the idea of speed as distance divided by time has a long history rooted in ancient Indian scholarship

Explanation:
The box titled 'India's Scientific Contributions' is meant to show that this basic idea about speed was already well understood in Indian mathematical works centuries before modern physics formalised it.

Answer: (A) It lets vehicles exchange signals so drivers can be warned of possible collisions

Explanation:
This system is presented as a safety aid, tied directly to the idea of maintaining a safe distance behind another vehicle in case it brakes suddenly.

Answer: (A) 2πR / T

Explanation:
One revolution covers a distance equal to the circle's circumference, 2πR, so dividing this by the time taken, T, gives the average speed for that revolution.

Answer: (A) Object B has a higher velocity than object A

Explanation:
Slope on a position-time graph represents velocity, so a steeper line for B means it covers more distance in the same time — its velocity is greater than A's.

Answer: (A) The colour of the vehicle

Explanation:
• The chapter names velocity at braking, road surface condition, the vehicle's braking capacity and the driver's reaction time as the real factors
• A vehicle's colour plays no role in the physics of stopping distance
• So option (a) is the odd one out

Answer: (B) Positions to the right of O are positive and to the left are negative

Explanation:
The chapter fixes a simple rule for the straight-line diagram: rightward positions from the reference point O carry a plus sign, while leftward ones carry a minus sign, letting direction be shown without drawing arrows every time.

Answer: (A) An instant is a single clock reading, whereas an interval is the duration between two such readings

Explanation:
Think of glancing at a clock once versus timing how long something takes between two glances — the first is an instant, the second a time interval.

Answer: (A) 5 m s-2

Explanation:
• Use s = ut + ½at2, with u = 0 and t = 4 s
• This gives 40 = ½ × a × 16
• Solving, a = 40/8 = 5 m s-2

Answer: (C) Displacement

Explanation:
Distance and speed are scalars — a number is enough to describe them. Displacement, however, is incomplete unless you also state the direction in which the position changed, making it a vector quantity.

Answer: (B) To give a quantity that captures both how far and in what net direction an object has shifted

Explanation:
Distance alone cannot tell whether someone ended up net closer, farther, or back where they started. Displacement fills that gap by combining a numerical value with a direction from the starting position to the final one.

Answer: (B) Draw and label axes, choose a suitable scale, plot points, join the points

Explanation:
The activity first sets up the X and Y axes with the origin, then fixes a scale for each axis, then places the individual points from the table, and only at the end connects them to reveal the shape of the graph.

Answer: (A) A rectangle and a triangle

Explanation:
• The lower portion of the shaded region forms a rectangle, representing displacement at the starting constant velocity
• The upper portion forms a triangle, capturing the extra displacement caused by the increasing velocity
• Adding rectangle area and triangle area together gives the total displacement, worked out as 75 m in the chapter's example

Answer: (C) Motion in three dimensions

Explanation:
Since the road climbs and curves through space rather than staying confined to a flat plane or a straight path, this situation is used to introduce motion in three dimensions, a topic left for higher grades.

Answer: (B) Six

Explanation:
A hexagon has six straight sides meeting at six corners, so the runner must turn six times to complete one full loop — more often than on a rectangular track but far less often than on a circular one.

Answer: (B) Numbers, equations and graphs

Explanation:
The introduction promises a fuller toolkit for describing motion — going past plain description to include numerical values, mathematical equations, and graphical representations.