The recoil of a gun when a bullet is fired is explained by the conservation of which quantity?
- A.Energy
- B.Linear momentum
- C.Mass
- D.Electric charge
Correct answer
Explanation
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.
Read the full article: Motion, Force and Newton's Laws: Formulas and Facts
Practice Questions
View allNewton's first law of motion is also known by which name?
- A.Law of inertia
- B.Law of momentum
- C.Law of action and reaction
- D.Law of gravitation
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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.
Read the full explanation →Which of the following is a vector quantity?
- A.Distance
- B.Speed
- C.Displacement
- D.Time
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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.
Read the full explanation →The equation force equals mass multiplied by acceleration follows from which law of motion?
- A.First law
- B.Second law
- C.Third law
- D.Law of conservation of energy
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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.
Read the full explanation →What is the SI unit of momentum?
- A.Newton
- B.Joule
- C.Kilogram metre per second
- D.Newton per second
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Momentum is the product of mass and velocity, so its unit is the unit of mass multiplied by the unit of velocity, that is kilogram multiplied by metre per second.
Read the full explanation →A passenger standing in a moving bus falls forward when the bus stops suddenly. This is due to
- A.Inertia of rest
- B.Inertia of motion
- C.Inertia of direction
- D.Centripetal force
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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.
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