Motion Quiz (Class 9 Science)

Q1. A car accelerates uniformly from a velocity of 10 m/s to 30 m/s in 10 seconds. What is its acceleration?

Hint: Use the formula for uniform acceleration:
$a = \frac{v – u}{t}$

Step-by-Step Solution:

• Initial velocity: $u = 10 \, \text{m/s}$
  

• Final velocity: $v = 30 \, \text{m/s}$
  

• Time: $t = 10 \, \text{s}$
  

$a = \frac{30 – 10}{10} = 2 \, \text{m/s}^2$

Answer: A) $2 \, \text{m/s}^2$

Q2. A body covers a distance of 100 m in 5 seconds. Its average speed is:

Hint: Average speed is given by:
$\text{Average speed} = \frac{\text{Total distance}}{\text{Total time}}$​

Step-by-Step Solution:
$\text{Average speed} = \frac{100}{5} = 20 \, \text{m/s}$

Answer: C) $20 \, \text{m/s}$

Q3. A train moving at a speed of 36 km/h comes to a stop in 10 seconds. What is the retardation?

Hint: Convert km/h to m/s first:
$36 \, \text{km/h} = \frac{36 \times 1000}{3600} = 10 \, \text{m/s}$
Then use:
$a = \frac{v – u}{t}$

Step-by-Step Solution:

• Initial velocity: $u = 10 \, \text{m/s}$

• $$Final velocity: $v = 0 \, \text{m/s}$
  

• Time: $t = 10 \, \text{s}$
  

$a = \frac{0 – 10}{10} = -1 \, \text{m/s}^2$
Magnitude of retardation: $\mathrm{\mid }a\mathrm{\mid }=1{\text{m/s}}^{}$

Answer: B) $1 \, \text{m/s}^2$

Q4. A stone is dropped from the top of a building. Which of the following statements is correct?

Hint: A freely falling body is influenced only by gravity. Its acceleration is constant.

Step-by-Step Solution:
Acceleration of freely falling stone:
$g = 9.8 \, \text{m/s}^2$

Answer: C) Its acceleration is due to gravity

Q5. The distance covered by a body under uniform acceleration is given by:

Hint: Use the equation of motion for uniformly accelerated motion:
$s = u \times t + \frac{1}{2} \times a \times t^2$

Step-by-Step Solution:

• $s$ = distance

• $u$ = initial velocity

• $a$ = acceleration

• $t$ = time

Answer: B) $s = u \times t + \frac{1}{2} \times a \times t^2$

Q6. A car starts from rest and reaches a speed of 20 m/s in 8 seconds. How far does it travel in this time?

Hint: First calculate acceleration:
$a = \frac{v – u}{t}$
Then use:
$s = u \times t + \frac{1}{2} \times a \times t^2$

Step-by-Step Solution:

• $u = 0 \, \text{m/s}$
  

• $v = 20 \, \text{m/s}$
  

• $t = 8 \, \text{s}$
  

Acceleration:
$a = \frac{20 – 0}{8} = 2.5 \, \text{m/s}^2$

Distance:

$$\begin{array}{rl}{\displaystyle s}& {\displaystyle =u\times t+\frac{1}{2}\times a\times t^2}\\ {\displaystyle }& {\displaystyle =0+\frac{1}{2}\times 2.5\times 8^2}\end{array}$$

$$\begin{array}{rl}{\displaystyle }& {\displaystyle =1.25\times 64}\end{array}$$

$$\begin{aligned} s &= u \times t + \frac{1}{2} \times a \times t^2 \\[1mm] &= 0 + \frac{1}{2} \times 2.5 \times 8^2 \\[1mm] &= 1.25 \times 64 \\[1mm] &= 80 \, \text{m} \end{aligned}$$

Answer: B) $80 \, \text{m}$