CBSE Class 9 Science Advanced Physics Circular Motion Very Short Answer Questions Set 01

Very Short Answer Type

Question 1. What is the direction of the linear velocity of a particle in uniform circular motion?
Answer:
The linear velocity is always along the tangent to the circle at the position of the particle, at right angles to the radius.
In simple words: The velocity points along the tangent, the way a stone flies off if the string breaks.

Teacher's Note:
1. The tangent changes direction as the particle moves, so the velocity changes even if the speed does not.
2. Tangent and radius are always perpendicular.

Question 2. What is the direction of the centripetal force?
Answer:
The centripetal force always acts towards the centre of the circle, along the radius, at right angles to the velocity.
In simple words: Centripetal force pulls the body inwards, towards the centre.

Teacher's Note:
1. It is this inward force that keeps the body on the circle.
2. Because it is perpendicular to the velocity, it changes the direction but not the speed.

Question 3. What is the angular velocity of the hour needle in a clock?
Answer:
The hour hand turns once (2π rad) in 12 h = 43200 s.
ω = 2π ÷ 43200 = π/21600 rad/s ≈ 1.45 × 10^–4 rad/s (that is π/6 rad per hour).
In simple words: The hour hand goes round once in 12 hours, so ω = 2π/12 h = π/6 rad per hour.

Teacher's Note:
1. Use ω = 2π/T with T = 12 hours.
2. Give the unit rad/s or rad/h clearly.

Question 4. Find the relation between angular velocity and linear velocity.
Answer:
The linear velocity is the angular velocity multiplied by the radius: v = rω, or ω = v/r.
In simple words: Linear speed equals radius times angular speed.

Teacher's Note:
1. Points farther from the axis move faster, though all share the same ω.
2. Check units: rad/s × m gives m/s (rad has no dimension).

Question 5. Give an example of a body moving with a uniform speed having acceleration.
Answer:
The moon moving round the earth (or a satellite round its planet). Its speed is constant, but its direction keeps changing, so its velocity changes and it has an acceleration directed towards the centre.
In simple words: A body going round a circle at constant speed is still accelerated because its direction keeps changing.

Teacher's Note:
1. Acceleration means any change of velocity, including a change of direction only.
2. A runner on a circular track at steady speed is another example.

Question 6. Give relation between linear and angular acceleration.
Answer:
The tangential (linear) acceleration is the radius times the angular acceleration: a = rα.
In simple words: Linear acceleration along the path equals radius times angular acceleration.

Teacher's Note:
1. It follows from v = rω by dividing the change by time.
2. The book calls this relation one between angular and linear velocity by mistake; it is between accelerations.

Question 7. Why circular motion is accelerated motion ?
Answer:
In circular motion the body moves on a path whose direction changes at every instant.
The velocity is a vector, so a change of direction means a change of velocity even when the speed is constant.
A change of velocity is acceleration, so circular motion is accelerated motion.
In simple words: The direction of velocity keeps changing, so the velocity changes and the motion is accelerated.

Teacher's Note:
1. Compare with a straight line at constant speed, where nothing changes.
2. Remember: velocity changes whenever speed or direction changes.

Question 8. Define Circular motion
Answer:
The motion of a body along a circular path around a fixed point (the centre) is called circular motion.
In simple words: Going round and round a fixed point on a circle is circular motion.

Teacher's Note:
1. Give an example such as a moon orbiting its planet.
2. The distance from the centre stays the same.

Question 9. State the meaning of uniform circular motion
Answer:
If a body moves along a circle and covers equal distances along the circumference in equal intervals of time, its motion is uniform circular motion.
Its speed is constant but the direction of its velocity changes continuously.
In simple words: A circular motion with constant speed is uniform circular motion.

Teacher's Note:
1. Uniform refers to the speed, not the velocity.
2. Moon around the earth is a good example.

Question 10. Define angular speed write its S.I. unit.
Answer:
Angular speed is the angle turned by the radius (position vector) of the moving body per unit time: ω = angle ÷ time = Δθ/Δt.
Its SI unit is radian per second (rad/s).
In simple words: Angular speed tells how many radians the body turns each second.

Teacher's Note:
1. For one full turn the angle is 2π rad.
2. Also written ω = 2π/T = 2πf.

Question 11. Name a physical quantity that (i) varies (ii) remains same in a circular motion.
Answer:
(i) Varies: the velocity (its direction changes continuously), and also the position and the acceleration direction.
(ii) Remains the same: the speed (in uniform circular motion), and also the angular velocity and the radius.
In simple words: In uniform circular motion the direction of velocity changes but the speed stays the same.

Teacher's Note:
1. Think of vectors that turn and scalars that do not.
2. Any one correct example for each is enough in an exam.