Laws of Motion — CBSE Class 9 Science

Laws of Motion Quiz – Class 9 Science

Q1. Who formulated the three laws of motion?
$\text{a) Albert Einstein} \quad \text{b) Isaac Newton} \quad \text{c) Galileo Galilei} \quad \text{d) Niels Bohr}$
Answer: $\text{b) Isaac Newton}$

Q2. A body of mass $5\,\text{kg}$ is acted upon by a force of $20\,\text{N}$. What is its acceleration?
$\text{a) } 2\,\text{m/s²} \quad \text{b) } 4\,\text{m/s²} \quad \text{c) } 5\,\text{m/s²} \quad \text{d) } 10\,\text{m/s²}$
Answer: $\text{b) 4 m/s²}$
Solution: Using Newton’s second law:
$F = m \times a \implies a = \frac{F}{m} = \frac{20}{5} = 4\,\text{m/s²}$

Q3. The first law of motion is also called the law of:
$\text{a) Force} \quad \text{b) Acceleration} \quad \text{c) Inertia} \quad \text{d) Momentum}$
Answer: $\text{c) Inertia}$

Q4. A body continues to move with uniform velocity if no external force acts on it. This statement represents:
$\text{a) Newton’s 1st Law}\text{b) Newton’s 2nd Law}$

$\text{a) Newton’s 1st Law} \quad \text{b) Newton’s 2nd Law} \quad \text{c) Newton’s 3rd Law} \quad \text{d) Law of Gravitation}$
Answer: $\text{a) Newton’s 1st Law}$

Q5. Two objects of masses $2\,\text{kg}$ and $4\,\text{kg}$ are subjected to the same force of $10\,\text{N}$. Which one will have greater acceleration?
$\text{a) 2 kg object} \quad \text{b) 4 kg object} \quad \text{c) Both same} \quad \text{d) Cannot determine}$
Answer: $\text{a) 2 kg object}$
Solution:
$a = \frac{F}{m}$
For $2\,\text{kg}$: $a = 10/2 = 5\,\text{m/s²}$
For $4\,\text{kg}$: $a = 10/4 = 2.5\,\text{m/s²}$

Q6. Newton’s third law of motion states:
$\text{“For every action, there is an equal and opposite reaction.”}$
Which of the following is an example?
$\text{a) A book resting on a table}\text{b) Recoil of a gun}$$\text{a) A book resting on a table} \quad \text{b) Recoil of a gun} \quad \text{c) Ball rolling on floor} \quad \text{d) Earth revolving around Sun}$
Answer: $\text{b) Recoil of a gun}$

Q7. A person in a bus falls forward when the bus suddenly stops. This is an example of:
$\text{a) Newton’s 1st Law} \quad \text{b) Newton’s 2nd Law} \quad \text{c) Newton’s 3rd Law} \quad \text{d) Law of Gravitation}$
Answer: $\text{a) Newton’s 1st Law}$

Q8. A force of $30\,\text{N}$ acts on a body of mass $6\,\text{kg}$. What is the acceleration produced?
$\text{a) 3 m/s²} \quad \text{b) 5 m/s²} \quad \text{c) 6 m/s²} \quad \text{d) 36 m/s²}$
Answer: $\text{a) 5 m/s²}$
Solution:
$a = \frac{F}{m} = \frac{30}{6} = 5\,\text{m/s²}$

Q9. The property of a body to resist any change in its state of motion is called:
$\text{a) Momentum} \quad \text{b) Inertia} \quad \text{c) Acceleration} \quad \text{d) Force}$
Answer: $\text{b) Inertia}$

Q10. Two ice skaters push each other. Skater A has mass $50\,\text{kg}$ and Skater B has mass $70\,\text{kg}$. If Skater A moves backward with acceleration $2\,\text{m/s²}$, what is the acceleration of Skater B?
$\text{a) 1.43 m/s²} \quad \text{b) 2 m/s²} \quad \text{c) 0.7 m/s²} \quad \text{d) 3 m/s²}$
Answer: $\text{a) 1.43 m/s²}$
Solution: By Newton’s third law, forces are equal:
$F = m_A \times a_A = 50 \times 2 = 100\,\text{N}$
$a_B = \frac{F}{m_B} = \frac{100}{70} \approx 1.43\,\text{m/s²}$.

Q10. Two ice skaters push each other. Skater A has mass $50\,\text{kg}$ and Skater B has mass $70\,\text{kg}$. If Skater A moves backward with acceleration $2\,\text{m/s²}$, what is the acceleration of Skater B?
$\text{a) 1.43 m/s²} \quad \text{b) 2 m/s²} \quad \text{c) 0.7 m/s²} \quad \text{d) 3 m/s²}$
Hint: Forces are equal and opposite (Newton’s 3rd law).
Answer: $\text{a) 1.43 m/s²}$