Forces and laws of motion Class 9 Mcq's

Q1. Which of the following is a vector quantity?

a) Mass

b) Speed

c) Momentum(Correct)

d) Inertia

Explanation: Momentum has both magnitude and direction (same direction as velocity), making it a vector quantity. Mass, speed, and inertia are scalar attributes.

Q2. Inertia of an object is directly proportional to its:

a) Velocity

b) Mass(Correct)

c) Area

d) Volume

Explanation: Mass is the quantitative measure of inertia. A heavier object offers greater resistance to change its state of rest or uniform motion.

Q3. When a moving bus brakes suddenly, the passengers are jerked forward. This is due to:

a) Inertia of motion(Correct)

b) Inertia of rest

c) Centripetal force

d) Conservation of mass

Explanation: The lower part of the body comes to rest with the bus, but the upper part continues moving forward due to the inertia of motion.

Q4. What is the mathematical expression for Newton’s Second Law of Motion?

a) F = m / a

b) F = a / m

c) F = m × a(Correct)

d) F = m + a

Explanation: Newton’s Second Law states that force is directly proportional to the rate of change of momentum, which derives to Force = mass × acceleration (F = ma).

Q5. The SI unit of momentum is:

a) kg m/s²

b) kg m/s(Correct)

c) Newton

d) Joule

Explanation: Momentum (p) is the product of mass (kg) and velocity (m/s). Hence, its SI unit is kg m/s.

Q6. A fielder pulls his hands backwards while catching a cricket ball. This action:

a) Increases the force exerted by the ball

b) Increases the time of impact to reduce the force(Correct)

c) Decreases the time of impact

d) Completely keeps momentum zero instantly

Explanation: By pulling the hands back, the fielder increases the time over which the high velocity of the ball is reduced to zero, thereby reducing the acceleration and the impact force.

Q7. Action and reaction forces described in Newton’s Third Law:

a) Act on the same body

b) Act along different arbitrary directions

c) Act on two different bodies simultaneously(Correct)

d) Cancel each other out completely inside static objects

Explanation: Action and reaction forces are equal and opposite, but they never cancel each other because they always act on two completely distinct bodies.

Q8. Rocket propulsion works on the principle of conservation of:

a) Mass

b) Energy

c) Velocity

d) Momentum(Correct)

Explanation: The backward momentum of escaping exhaust gases gives an equal forward momentum to the rocket structure, according to the conservation of momentum.

Q9. If a constant net force acts on a body, the body will move with uniform:

a) Acceleration(Correct)

b) Velocity

c) Speed

d) Momentum

Explanation: Since F = ma, a constant net non-zero force produces a constant (uniform) acceleration in the body.

Q10. An object of mass 2 kg is sliding with a constant velocity of 4 m/s on a frictionless horizontal table. The force required to maintain this motion is:

a) 8 N

b) 0 N(Correct)

c) 2 N

d) 0.5 N

Explanation: According to Newton’s First Law, no net external force is required to keep an object moving with a uniform velocity if friction is completely absent.

Q11. Qualitative definition of force is given by:

a) Newton’s second law of motion

b) Newton’s third law of motion

c) Newton’s first law of motion(Correct)

d) Law of gravitation

Explanation: Newton’s First Law introduces the quality of force as an agent that can alter an object’s uniform state of rest or motion. (The quantitative value is defined by the Second Law).

Q12. A force of 5 N gives a mass m₁ an acceleration of 10 m/s², and a mass m₂ an acceleration of 20 m/s². If both masses are tied together, the acceleration would be:

a) 30 m/s²

b) 6.67 m/s²(Correct)

c) 15 m/s²

d) 5 m/s²

Explanation: m₁ = 5/10 = 0.5 kg. m₂ = 5/20 = 0.25 kg. Total mass M = m₁ + m₂ = 0.75 kg. Combined acceleration a = F/M = 5 / 0.75 = 6.67 m/s².

Q13. When we shake the branch of a mango tree vigorously, some mangoes detach and fall down. This happens because of:

a) Inertia of motion

b) Velocity variations

c) Newton’s third law

d) Inertia of rest(Correct)

Explanation: When the tree branch is shaken, it comes into a state of motion, while the mangoes tend to maintain their original state of rest due to the inertia of rest, causing them to detach.

Q14. Recoil of a gun is a classic demonstration of:

a) Newton’s third law of motion(Correct)

b) Newton’s first law of motion

c) Balanced forces

d) Inertia of direction

Explanation: The gun exerts a forward force on the bullet (action), and the bullet exerts an equal and opposite backward force on the gun (reaction).

Q15. If the mass of a body is doubled and its velocity is halved, its linear momentum will:

a) Become double

b) Remain unchanged(Correct)

c) Become half

d) Become four times

Explanation: Original momentum = m × v. New momentum = (2m) × (v/2) = m × v. Hence, momentum remains unchanged.

Q16. Balanced forces acting on an object can change its:

a) Shape(Correct)

b) Speed

c) Direction of motion

d) Velocity magnitude

Explanation: Balanced forces have a net result of zero, so they cannot alter speed or velocity directions. However, they can squeeze or deform an object, altering its shape (e.g., pressing a rubber ball).

Q17. A body of mass 500 g moves with an acceleration of 2 m/s². What is the net external force acting on it?

a) 1000 N

b) 1 N(Correct)

c) 10 N

d) 250 N

Explanation: First, convert mass to SI units: 500 g = 0.5 kg. Then use F = ma: F = 0.5 kg × 2 m/s² = 1 N.

Q18. Water drops stick to a wet cloth. When the cloth is shaken sharply, the water drops fly off. This is an application of:

a) Newton’s second law

b) Conservation of energy

c) Inertia(Correct)

d) Friction minimization

Explanation: When the cloth is shaken, it changes its state of motion rapidly. The water droplets tend to remain at rest due to inertia and consequently fly separate from the moving fabrics.

Q19. When an object is thrown upwards, at its highest point, what attribute safely reaches zero?

a) Inertia

b) Acceleration

c) Mass

d) Momentum(Correct)

Explanation: At the highest peak point, the velocity of the object briefly becomes zero before turning back. Because momentum equals mass times velocity ($p = mv$), momentum drops to zero too. Acceleration remains equal to gravity ($g$).

Q20. Two bodies A and B of masses 5 kg and 25 kg respectively are moving with identical uniform velocities. Compare their inertia:

a) Inertia of B is greater than inertia of A(Correct)

b) Inertia of A is greater than inertia of B

c) Both have the same inertia since velocities are equal

d) Neither body has any inertia since acceleration is zero

Explanation: Inertia depends solely on mass, not on velocity. Since body B ($25\text{ kg}$) is heavier than body A ($5\text{ kg}$), body B possesses a much larger inertia.

Quiz – Force and Laws of Motion – Class 9 CBSE

Q1. Which of the following is a vector quantity?

Q2. Inertia of an object is directly proportional to its:

Q3. When a moving bus brakes suddenly, the passengers are jerked forward. This is due to:

Q4. What is the mathematical expression for Newton’s Second Law of Motion?

Q5. The SI unit of momentum is:

Q6. A fielder pulls his hands backwards while catching a cricket ball. This action:

Q7. Action and reaction forces described in Newton’s Third Law:

Q8. Rocket propulsion works on the principle of conservation of:

Q9. If a constant net force acts on a body, the body will move with uniform:

Q10. An object of mass 2 kg is sliding with a constant velocity of 4 m/s on a frictionless horizontal table. The force required to maintain this motion is:

Q11. Qualitative definition of force is given by:

Q12. A force of 5 N gives a mass m₁ an acceleration of 10 m/s², and a mass m₂ an acceleration of 20 m/s². If both masses are tied together, the acceleration would be:

Q13. When we shake the branch of a mango tree vigorously, some mangoes detach and fall down. This happens because of:

Q14. Recoil of a gun is a classic demonstration of:

Q15. If the mass of a body is doubled and its velocity is halved, its linear momentum will:

Q16. Balanced forces acting on an object can change its:

Q17. A body of mass 500 g moves with an acceleration of 2 m/s². What is the net external force acting on it?

Q18. Water drops stick to a wet cloth. When the cloth is shaken sharply, the water drops fly off. This is an application of:

Q19. When an object is thrown upwards, at its highest point, what attribute safely reaches zero?

Q20. Two bodies A and B of masses 5 kg and 25 kg respectively are moving with identical uniform velocities. Compare their inertia: