CBSE Class 9 Physics Force and Laws of Motion MCQs Set 04

Answer: (d) acceleration

Explanation:
1. By \( F = ma \), a constant force on a body of constant mass gives a constant acceleration.
2. The velocity, speed and momentum keep changing.

Teacher's Note:
1. Force is proportional to acceleration, not to velocity.
2. Uniform acceleration means the velocity changes at a constant rate.

Answer: (d) 1000 m/s²

Explanation:
1. Mass = 1 g = 0.001 kg.
2. \( a = \dfrac{F}{m} = \dfrac{1}{0.001} = 1000 \) m/s².

Teacher's Note:
1. Always convert grams to kilograms before using F = ma.
2. A small mass gives a large acceleration.

Answer: (b) mass

Explanation:
1. Inertia is measured by mass: the greater the mass, the greater the force needed to change the state of motion.

Teacher's Note:
1. Inertia does not depend on velocity.
2. Two bodies of the same volume can have different masses and so different inertia.

Answer: (a) road on the car

Explanation:
1. The tyres push the road backward and, by the third law, the road pushes the car forward.
2. Only this external force can change the momentum of the car; the engine's force is internal.

Teacher's Note:
1. Internal forces cannot change the motion of the whole car.
2. The forward push comes from the reaction of the ground.

Answer: (b) -30 N

Explanation:
1. Taking the initial direction as positive: u = 10 m/s and v = -2 m/s, so \( a = \dfrac{-2 - 10}{4} = -3 \) m/s².
2. \( F = ma = 10 \times (-3) = -30 \) N.

Teacher's Note:
1. The negative sign shows that the force is opposite to the original motion.
2. Use signs for velocities that reverse.

Answer: (d) zero

Explanation:
1. Constant velocity means zero acceleration, so the net force is zero.
2. On a frictionless table there is no horizontal force at all (the weight is balanced by the normal reaction).

Teacher's Note:
1. No force is needed to maintain uniform motion.
2. This is Newton's first law.

Answer: (b) 5 N

Explanation:
1. \( F = \dfrac{m(v - u)}{t} = \dfrac{1 \times (30 - 20)}{2} = 5 \) N.

Teacher's Note:
1. Force = rate of change of momentum.
2. Acceleration is 5 m/s².

Answer: (a) 10 N

Explanation:
1. Bullets fired per second = 30/60 = 0.5. Momentum given to the bullets per second = \( 0.5 \times 0.05 \times 400 = 10 \) kg m/s per second.
2. The recoil force on the gun is 10 N, so an equal force is needed to hold it.

Teacher's Note:
1. Force = rate of change of momentum.
2. Do not use the mass of the gun in this calculation.

Answer: (c) -120 N s

Explanation:
1. Impulse = change in momentum = \( m(v - u) = 10(-2 - 10) = -120 \) N s.

Teacher's Note:
1. Impulse is also force x time = -30 x 4 = -120 N s.
2. The direction is opposite to the initial motion.