# CBSE 9th Physics | Motion – Solved Problems

## Motion – Solved Problems

### Problem 1. A car covers the 1st half of the distance between two places at a speed of 40 km/hr and the 2nd half at 60 km/hr what is the average speed of the car ?

Solution.

Suppose the total distance covered is S. Then time taken to cover first’s distance with speed

40 km/hr.

### Problem 2. An object moving to the right has a decrease in velocity from 5.0 m/s to 1.0 m/s in 2.0 s. What is the average acceleration ? What does your result mean ?

Solution.

Given v0 = + 5.0 m/s, v = + 1.0 m/s, t = 2.0 s

Find

According to the definition of average acceleration,

The negative sign means the acceleration is opposite to velocity (deceleration). The result means that the object decreases its velocity by 2.0 m/s every s or 2.0 m/s2.

### Problem 3. The table below shows the distance in cm, travelled by the objects A, B and C during each second.

#### (iii) Which object is moving with irregular acceleration ?

Solution

The object A is moving with constant speed. The reason is that it covers equal distance=20cm. in each second,

The object C is moving with a constant acceleration. The reason is that for the object C, the distance covered increases by the same amount in each second. It can further be verified by drawing graph between S (total distance covered) and t2 (square of time taken). The graph will be a straight line.

 t2 1 4 9 16 25 s 20 80 180 320 500

The object B is moving with irregular acceleration.

### Question 4. An automobile accelerates uniformly from rest to 25 m/s while traveling 100m. What is the acceleration of the automobile ?

Solution.

Given: v0 = 0 (rest), v = 25m/s, x = 100m. Find a.

Since v2 = v02 + 2ax,

Since a is positive, it is in the direction of the velocity or motion.

Solution.

Solution.

Solution.

Solution. (i)

Solution (ii)

### Problem 9. The graph represents the velocity of a particle as a function of time.

#### (e) What is the displacement for the 8.0 s interval ?

Solution. (a)

Solution (b)

The slope of the line is zero and a = 0.

Solution (c)

Solution (d)

Solution (e)

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