CBSE Class 9 Science Chapter 04 Describing Motion Around Us MCQs Set 03

Answer: (B) 1 m/s

Explanation:
He covers a total distance of 50 m (25 m there and 25 m back) in 50 s, so dividing distance by time gives 1 m/s, even though his displacement and hence average velocity is zero.

Answer: (A) When its position relative to the reference point remains unchanged over time

Explanation:
Rest and motion are both defined purely with respect to a fixed reference point. If the position of an object stays the same as time passes, it is at rest; if the position keeps changing, it is in motion.

Answer: (A) The steepness of the line, showing the rate of change of the Y-axis quantity with respect to the X-axis quantity

Explanation:
• Slope is simply a measure of how steep a line is.
• On a position-time graph, this steepness works out to velocity.
• On a velocity-time graph, the same idea gives acceleration.
• In short, slope always tells you a rate of change.

Answer: (A) Opposite to the direction of its velocity

Explanation:
Since the magnitude of velocity is shrinking, the acceleration must act against the motion to pull it down, which is why deceleration comes out with a negative sign in calculations like braking examples.

Answer: (A) Because that area is essentially velocity multiplied by the time interval, which by definition equals displacement

Explanation:
Displacement is defined as velocity times time interval, and geometrically that product is exactly what a rectangle or trapezium's area under the velocity-time line represents.

Answer: (A) The acceleration stayed the same, close to 9.8 m/s², in every interval

Explanation:
Working out the change in velocity divided by the time for each second-long interval gives 9.8 m/s² every single time, showing the fall happens under constant acceleration, later identified as acceleration due to gravity.

Answer: (A) They offer a visual way to compare motions, compute quantities, and distinguish uniform from non-uniform motion

Explanation:
The chapter even cautions that a graph is not a route map — it doesn't show the actual path taken, only how a quantity like position changes with time, which is precisely what makes it useful for comparison and calculation.

Answer: (A) To give a familiar, everyday feeling for what a change in velocity, i.e., acceleration, is like

Explanation:
That jolt is something almost everyone has experienced, so it works as an intuitive entry point before the formal definition of average acceleration is introduced.

Answer: (A) It becomes four times as large

Explanation:
Since s works out proportional to u² when v and a are fixed, doubling the initial speed multiplies the required stopping distance by four rather than just doubling it.

Answer: (A) 25 m

Explanation:
With u equal to zero, the equation simplifies to half of acceleration times time squared, which gives 0.5 × 2 × 25 = 25 m.

Answer: (A) Metre per second (m/s)

Explanation:
Average velocity and average speed share the same SI unit, metre per second, though velocity additionally carries a direction while speed does not.

Answer: (A) The track approaches a circle, and the athlete's direction of motion keeps changing continuously

Explanation:
As sides multiply and shrink, the polygon smooths into a circle, and instead of a few sharp turns, the direction of velocity now changes at every single point along the path — which is exactly how the chapter builds up to uniform circular motion.

Answer: (A) A straight line that touches the circle at exactly one point, along which the velocity acts at that instant

Explanation:
The chapter uses this geometric idea to explain that the velocity of an object in circular motion always points along the tangent at its current position, not toward or away from the centre.

Answer: (A) Linear, circular, and oscillatory

Explanation:
The chapter opens by recalling that complex real motions are first simplified into linear, circular, and oscillatory forms, which is why the rest of the chapter focuses on straight-line and uniform circular motion.

Answer: (A) Bus moves at 36 km/h -> accelerator pressed raising speed to 54 km/h -> bus travels at constant velocity -> brakes applied and bus stops

Explanation:
• The bus starts at 36 km/h.
• The driver presses the accelerator, raising the speed to 54 km/h over 10 s.
• It then cruises at this constant velocity for some time.
• Finally, seeing an obstacle, the driver brakes and the bus stops in 5 s.

Answer: (A) Changing them midway would give inconsistent or contradictory signs for displacement, velocity and acceleration in the same calculation

Explanation:
Signs attached to displacement, velocity and acceleration only make sense relative to a chosen direction; switching conventions partway through a problem would make earlier and later signs incompatible with each other.

Answer: (A) The time taken to go from 0 km/h to 100 km/h

Explanation:
This is a standard performance figure students are asked to look up and use to compute average acceleration for different cars.

Answer: (A) A vehicle overtaking another, the path of a kicked ball, and a satellite moving in a circular orbit

Explanation:
Motion in a plane is described as two-dimensional motion, and the chapter names overtaking vehicles, a kicked ball's trajectory, and a circling satellite as its examples; the mountain-road, bird, and aircraft examples are instead used later for three-dimensional motion.

Answer: (A) The magnitude of the velocity

Explanation:
A note in the chapter clarifies that the speedometer shows something close to the instantaneous speed, while the direction the tyres point gives the direction of velocity — the speedometer alone says nothing about direction.

Answer: (A) An object can be travelling very fast yet have zero acceleration, since acceleration depends on how quickly velocity changes, not on its size

Explanation:
The chapter gives the example of a bus cruising at high, unchanging velocity on a straight highway — despite its high speed, its acceleration is zero because the velocity itself is not changing.

Answer: (A) 3 m s-2

Explanation:
Average acceleration equals change in velocity divided by time taken: (25 m/s - 10 m/s) / 5 s = 15/5 = 3 m s-2.

Answer: (A) 16 m

Explanation:
Since she starts from rest, u = 0, so s = ut + (1/2)at² reduces to s = (1/2)(2)(4)² = (1/2)(2)(16) = 16 m.

Answer: (A) Because speed and velocity are themselves defined in terms of distance and displacement, so those ideas must come first

Explanation:
Average speed is defined as distance divided by time, and average velocity as displacement divided by time — both formulas rely on the earlier concepts, so building position, distance and displacement first makes the later definitions easier to follow.

Answer: (A) 15 m in the negative direction

Explanation:
Moving from 20 m to 5 m means the object ends up to the left of its starting point, so the magnitude of displacement is 15 m and, with rightward taken as positive, its direction is negative.

Answer: (A) Motion in a plane is two-dimensional, like a kicked ball's path, while motion in space is three-dimensional, like a car climbing a mountain road

Explanation:
Two-dimensional examples such as a kicked ball's trajectory or a satellite's orbit are grouped as motion in a plane, whereas examples involving height change, like a car on a winding mountain road, a flying bird or an aircraft, are placed under three-dimensional motion in space.