Class 9 How Forces Affect Motion Mock Test (CBSE)

Topics in How Forces Affect Motion

• Force
• Measuring the magnitude of a force
• Balanced and unbalanced forces
• Friction
• Newton's first law of motion
• Newton's second law of motion
• Newton's third law of motion
• Forces on a system of objects

How Forces Affect Motion practice questions with answers

1. Q1Multiple choice·1 mark
   A constant force $F$ acts on a body of mass $m_1$ for a time $t$, producing a velocity $v$. If the same force acts on another body of mass $m_2$ (where $m_2 = 2m_1$) for the same duration $t$, the ratio of the momentum imparted to the first body to that imparted to the second body is:

   1. (a)$1 : 2$
   2. (b)$2 : 1$
   3. (c)$1 : 1$
   4. (d)$4 : 1$
   Show answer and step marks

   1. Correct option: (c) $1 : 1$. Apply the relation for impulse and change in momentum: $\text{Momentum imparted } p = F \times t$. Since both force $F$ and time $t$ are identical for both bodies, the momentum imparted is equal regardless of their masses.+1

   Answer: (c)
2. Q2Multiple choice·1 mark
   A 4 kg block rests on a horizontal floor. A force of 18 N pulls it to the right, and a friction force of 6 N acts on it to the left. What is the acceleration of the block?

   1. (a)$1.5\ \text{m s}^{-2}$
   2. (b)$4.5\ \text{m s}^{-2}$
   3. (c)$5.5\ \text{m s}^{-2}$
   4. (d)$3\ \text{m s}^{-2}$
   Show answer and step marks

   1. Correct option: (d) $3\ \text{m s}^{-2}$ (net force $=18-6=12\ \text{N}$, and $a=\dfrac{12}{4}=3\ \text{m s}^{-2}$)+1

   Answer: (d)
3. Q3Multiple choice·1 mark
   A constant force of $5\text{ N}$ acts on a body of mass $2\text{ kg}$ initially at rest. What is the velocity of the body after $4\text{ s}$?

   1. (a)$5\text{ m/s}$
   2. (b)$10\text{ m/s}$
   3. (c)$20\text{ m/s}$
   4. (d)$40\text{ m/s}$
   Show answer and step marks

   1. Correct option: (b) $10\text{ m/s}$ ($a = F/m = 2.5\text{ m/s}^2$, $v = u + at = 0 + 2.5 \times 4 = 10\text{ m/s}$)+1

   Answer: (b)
4. Q4Assertion and reason·1 mark
   Assertion (A): An object moving with a constant velocity on a rough horizontal table requires a non-zero applied force to maintain that velocity. Reason (R): Balanced forces produce acceleration in a body. Options: 1. Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A). 2. Both Assertion (A) and Reason (R) are true but Reason (R) is not the correct explanation of Assertion (A). 3. Assertion (A) is true but Reason (R) is false. 4. Assertion (A) is false but Reason (R) is true.

   1. (a)Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A).
   2. (b)Both Assertion (A) and Reason (R) are true but Reason (R) is not the correct explanation of Assertion (A).
   3. (c)Assertion (A) is true but Reason (R) is false.
   4. (d)Assertion (A) is false but Reason (R) is true.
   Show answer and step marks

   1. Correct option: (c) Assertion (A) is true but Reason (R) is false (Balanced forces produce zero acceleration, not acceleration).+1

   Answer: (c)
5. Q5Assertion and reason·1 mark
   Assertion (A): A rocket can propel itself in outer space where there is no air. Reason (R): For every action, there is an equal and opposite reaction. Options: 1. Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A). 2. Both Assertion (A) and Reason (R) are true but Reason (R) is not the correct explanation of Assertion (A). 3. Assertion (A) is true but Reason (R) is false. 4. Assertion (A) is false but Reason (R) is true.

   1. (a)Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A).
   2. (b)Both Assertion (A) and Reason (R) are true but Reason (R) is not the correct explanation of Assertion (A).
   3. (c)Assertion (A) is true but Reason (R) is false.
   4. (d)Assertion (A) is false but Reason (R) is true.
   Show answer and step marks

   1. Correct option: (a) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A).+1

   Answer: (a)
6. Q6Short answer·3 marks
   A truck of mass $1000\text{ kg}$ is travelling with a velocity of $20\text{ m/s}$. The brakes are applied to stop the truck in $5\text{ s}$. (a) Calculate the acceleration of the truck. (b) Find the magnitude of the braking force acting on the truck.
   Show answer and step marks

   1. Using the relation $a = \frac{v - u}{t}$ and substituting $v = 0\text{ m/s}$, $u = 20\text{ m/s}$, and $t = 5\text{ s}$.+1
   2. Calculating the acceleration as $-4\text{ m/s}^2$ (equivalently, a retardation of $4\text{ m/s}^2$ directed opposite to the motion).+0.5
   3. Using Newton's second law $F = m \times a$ with $m = 1000\text{ kg}$ and the magnitude of acceleration $4\text{ m/s}^2$.+1
   4. Calculating the magnitude of the braking force as $4000\text{ N}$ (acting opposite to the motion).+0.5

   Answer: (a) $-4\text{ m/s}^2$ (a retardation of $4\text{ m/s}^2$ opposite to the motion). (b) $4000\text{ N}$

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