Dynamics (Newton's Laws)

Practise Dynamics (Newton's Laws) for FBISE Grade 9–10 Physics (Grade 9–10): 8 multiple-choice questions with instant checking and worked explanations. Free, no sign-up.

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1. Question 1

   A 1500 kg car traveling at $20\mathrm{m}{\mathrm{s}}^{-1}$ collides perfectly inelastically with an $800\mathrm{kg}$ truck at rest on a level road; the combined mass then slides $30\mathrm{m}$ before stopping due to kinetic friction coefficient $0.3$. What is the magnitude of the average net external force acting on the system during the sliding phase?
   • A$8.5\times 10^3\mathrm{N}$
   • B$5.0\times 10^3\mathrm{N}$
   • C$4.2\times 10^3\mathrm{N}$
   • D$6.8\times 10^3\mathrm{N}$
   Show answer and explanation

   Answer: D) $6.8\times 10^3\mathrm{N}$

   After the perfectly inelastic collision the combined mass moves with $v\approx 13.0\mathrm{m}{\mathrm{s}}^{-1}$. The only horizontal external force during the sliding phase is kinetic friction, $F_f=\mu mg$, where $m=2300\mathrm{kg}$ and $\mu =0.3$. This gives $F_f\approx 6.8\times 10^3\mathrm{N}$, which is the magnitude of the average net external force. The other options arise from common mistakes: using only the car’s mass, treating momentum change as work, or using the wrong coefficient of friction.

2. Question 2

   A $2.0\mathrm{kg}$ block rests on a horizontal surface with coefficient of static friction $0.4$. A horizontal force of $6\mathrm{N}$ is applied. What is the block’s acceleration?
   • A$3.0\mathrm{m}{\mathrm{s}}^{-2}$
   • B$0.5\mathrm{m}{\mathrm{s}}^{-2}$
   • C$0\mathrm{m}{\mathrm{s}}^{-2}$
   • D$1.5\mathrm{m}{\mathrm{s}}^{-2}$
   Show answer and explanation

   Answer: C) $0\mathrm{m}{\mathrm{s}}^{-2}$

   The block remains at rest because the maximum static friction ($7.8\mathrm{N}$) exceeds the applied $6\mathrm{N}$ force. Since there is no motion, the net horizontal force is zero and the acceleration is $0\mathrm{m}{\mathrm{s}}^{-2}$. The other options stem from mixing up static and kinetic friction or ignoring friction altogether.

3. Question 3

   A 10 N horizontal force pushes a 2 kg crate on a rough floor where the coefficient of kinetic friction is 0.2. What is the magnitude of the crate’s acceleration?
   • A$3.0\mathrm{m}{\mathrm{s}}^{-2}$
   • B$2.0\mathrm{m}{\mathrm{s}}^{-2}$
   • C$5.0\mathrm{m}{\mathrm{s}}^{-2}$
   • D$4.0\mathrm{m}{\mathrm{s}}^{-2}$
   Show answer and explanation

   Answer: A) $3.0\mathrm{m}{\mathrm{s}}^{-2}$

   The kinetic friction force opposes the motion and is $f_k={\mu }_{k}mg\approx 4\text{N}$. Subtracting this from the applied $10\text{N}$ gives a net force of about $6\text{N}$. Using $F=ma$ with $m=2\text{kg}$ yields $a\approx 3{\text{m s}}^{-2}$, which is the correct answer. The $5{\text{m s}}^{-2}$ option ignores friction, the $2{\text{m s}}^{-2}$ option confuses the coefficient with acceleration, and the $4{\text{m s}}^{-2}$ option mistakenly treats the friction force itself as the acceleration.

4. Question 4

   A book rests on a table and remains at rest. Which Newton’s law best explains this situation?
   • ASecond law
   • BFirst law
   • CLaw of universal gravitation
   • DThird law
   Show answer and explanation

   Answer: B) First law

   The book does not move because the forces on it (gravity and the normal force) cancel, giving a net force of zero. Newton's first law directly describes this condition of equilibrium, making it the appropriate law. The second law involves $F=ma$, which reduces to zero but does not capture the principle as clearly. The third law concerns action‑reaction pairs between different bodies, not the balance of forces on a single object. Universal gravitation only explains the attractive force between masses, not why the book stays still.

5. Question 5

   When a skydiver reaches constant speed during free fall, the forces acting on him are
   • Abalanced
   • Bunbalanced upward
   • Cunbalanced downward
   • Dnone of the above
   Show answer and explanation

   Answer: A) balanced

   When the skydiver reaches terminal velocity, his speed no longer changes, so acceleration is zero. By Newton's first law a zero acceleration means the sum of forces is zero; thus the gravitational force downward is exactly balanced by the upward air‑resistance force. Therefore the forces are balanced. The other options incorrectly suggest a net upward or downward force or deny the possibility of balance for a moving object.

6. Question 6

   What is the SI unit of force?
   • APascal
   • BWatt
   • CJoule
   • DNewton
   Show answer and explanation

   Answer: D) Newton

   The SI unit of force is the newton (N), defined as $1\mathrm{kg}\mathrm{m}{\mathrm{s}}^{-2}$. Therefore the correct answer is "Newton". Joule is the unit of energy/work, not force. Watt measures power, the rate of energy transfer. Pascal is the unit of pressure, which is force per unit area, not force itself.

7. Question 7

   A 0.5 kg ball is thrown vertically upward with initial speed $15\mathrm{m}{\mathrm{s}}^{-1}$. Ignoring air resistance, what is the net force on the ball at the highest point?
   • AEqual to its weight upward
   • B$4.9\mathrm{N}$ upward
   • CZero
   • D$4.9\mathrm{N}$ downward
   Show answer and explanation

   Answer: D) $4.9\mathrm{N}$ downward

   Once the ball is released, gravity is the only force acting on it throughout its trajectory (ignoring air resistance). At the highest point, although the velocity is instantaneously zero, the gravitational force continues to act downward with magnitude $F_{\text{net}}=mg=0.5\mathrm{kg}\times 9.8\mathrm{m}{\mathrm{s}}^{-2}=4.9\mathrm{N}$ downward.

8. Question 8

   According to Newton’s second law, the acceleration of an object is directly proportional to
   • Anet force
   • Bdisplacement
   • Cvelocity
   • Dmass
   Show answer and explanation

   Answer: A) net force

   Newton's second law ($F_{\text{net}}=ma$) shows that acceleration increases when the net force increases, keeping mass constant. Therefore the correct answer is "net force". Mass is inversely proportional to acceleration, not directly. Velocity is the result of acceleration, not a cause, so it is not directly proportional. Displacement measures position change and is unrelated to the instantaneous proportionality in the law.

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