Page No 103: - Chapter 8 - Motion - Ncert Solutions class 9 - Science

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Question 1:

When will you say a body is in (i) uniform acceleration? (ii) non-uniform acceleration?

Answer:

(i) A body is said to have uniform acceleration if it travels in a straight path in such a way that its velocity changes at a uniform rate, i.e., the velocity of a body increases or decreases by equal amounts in an equal interval of time.

(ii) A body is said to have non-uniform acceleration if it travels in a straight path in such a way that its velocity changes at a non-uniform rate, i.e., the velocity of a body increases or decreases in unequal amounts in an equal interval of time.

Question 2:

A bus decreases its speed from 80 km h⁻¹ to 60 km h⁻¹ in 5 s. Find the acceleration of the bus.

Answer:

Initial speed of the bus, u = 80 km/h 

Final speed of the bus, v = 60 km/h 

Time take to decrease the speed, t = 5 s

Here, the negative sign of acceleration indicates that the velocity of the car is decreasing.

Question 3:

A train starting from a railway station and moving with uniform acceleration attains a speed 40 km h⁻¹ in 10 minutes. Find its acceleration.

Answer:

Initial velocity of the train, u = 0 (since the train is initially at rest)

Final velocity of the train, v = 40 km/h 

Time taken, t = 10 min = 10 × 60 = 600 s

Hence, the acceleration of the train is 0.0185 m/s².