What is the SI unit of momentum?

The correct answer is A, the law of inertia. The first law states that a body continues in its state of rest or of uniform motion in a straight line unless an unbalanced external force acts on it, which is exactly the property called inertia.

Option B refers to the second law, which deals with the rate of change of momentum and yields the equation force equals mass times acceleration. Option C is the third law, which says that action and reaction are equal and opposite and act on different bodies. Option D, the law of gravitation, is a separate law of Newton's, stating that every body attracts every other with a force proportional to the product of their masses and inversely proportional to the square of the distance between them. The first law also defines force, by telling us that a force is what is needed to change a state of motion.

The correct answer is C, displacement. It is the shortest distance from the starting point to the finishing point together with the direction of that straight line, so it has both magnitude and direction and is therefore a vector.

Option A, distance, is the total length of the path travelled and carries no direction, so it is a scalar; option B, speed, is distance divided by time and is a scalar for the same reason, while velocity, which is displacement divided by time, is a vector. Option D, time, is a scalar. The standard illustration is a runner completing one lap of a circular track: the distance covered equals the circumference, but the displacement is zero because the runner ends where the lap began. Remember the pairs, distance with speed as scalars and displacement with velocity as vectors.

The correct answer is B, the second law. It states that the rate of change of momentum of a body is proportional to the applied force and takes place in the direction of the force. Since momentum is mass times velocity, the rate of its change for a body of constant mass is mass times acceleration, which gives the equation force equals mass times acceleration.

Option A, the first law, tells us only that a body keeps its state unless an unbalanced force acts, and so defines force without measuring it. Option C, the third law, deals with the pairing of action and reaction on two different bodies. Option D belongs to a different branch of the subject altogether. The second law also gives the definition of the newton, which is the force that gives a mass of one kilogram an acceleration of one metre per second squared.

The correct answer is B, linear momentum. Before firing, the gun and the bullet are at rest, so the total momentum of the system is zero. No external force acts along the barrel, so the total must stay zero: the bullet leaves with a large forward momentum and the gun acquires an equal momentum backwards, which is felt as the recoil. Because the gun is far heavier than the bullet, its backward velocity is small.

Option A, energy, is conserved in the wider sense but does not by itself give the direction and size of the recoil. Option C, mass, and option D, charge, are also conserved quantities but are irrelevant here. The same principle explains rocket propulsion, where hot gases are pushed out at high speed and the rocket moves the other way, and it is equally the third law of motion seen from another side.

The correct answer is B, inertia of motion. While the bus is moving, the passenger's whole body moves with it. When the brakes are applied, the feet stop along with the floor of the bus, but the upper body tends to keep moving forward at the same speed, and the passenger is thrown forward.

Option A, inertia of rest, explains the opposite case: a passenger falls backward when a stationary bus starts suddenly, because the body tends to remain at rest. Option C, inertia of direction, explains why mud flies off tangentially from a spinning wheel or why a passenger leans outward when the bus turns. Option D, centripetal force, is the force directed towards the centre that keeps a body on a circular path and has nothing to do with braking in a straight line. All three kinds of inertia are asked through everyday examples like these.