# 17 Conservation of Momentum Examples in Real Life

Conservation of momentum is the general law of physics, which states that the value of momentum never gets changed and remains constant in an isolated collection of objects. In other words, the law of conservation of momentum can be described as a physical quantity that remains constant before and after the collision of two or more objects in a system, i.e., in the case of a collision between two or more bodies, the value of momentum before the collision is always equal to the value of momentum after the collision. To understand the concept of the conservation of momentum, one must have good knowledge of the physical term momentum. Momentum is a physical quantity that tends to describe the quantity of motion present in a moving body. Mathematically, momentum is defined as the cross product of mass and velocity. Since velocity is a vector quantity, the quantity obtained as the result of performing a cross product of velocity and mass, i.e., momentum can also be listed under the category of vector physical quantities. This means that momentum can be described with the help of a magnitude value and direction. The direction of the momentum of a body is the same as that of the direction of its velocity. The SI unit of momentum is kilogram meters per second. Momentum is directly proportional to the mass of an object and the velocity with which the object moves in a particular direction. This means that a change in the mass or the velocity physical quantity of an object causes a proportionate change in its momentum value. An object present at rest does not possess momentum as the magnitude of the velocity of an idle body is always equal to zero. All the objects present in the universe tend to follow the law of conservation of momentum irrespective of their size, shape, location, position, and other physical or chemical parameters. Mathematically, the concept of conservation of momentum is a result of the homogeneity of space. This implies that the physical laws such as the law of conservation of momentum do not depend on the position of an object under consideration.

## Types of Conservation of Momentum

There are generally two types of the law of conservation of momentum in real life, namely the law of conservation of linear momentum and the law of conservation of angular momentum.

### 1. Law of Conservation of Linear Momentum

The law of conservation of linear momentum basically states that the momentum possessed by a body remains constant unless or until an external force comes into action. The concept of the conservation of linear momentum is based on Newton’s first law of motion, which states that an object tends to maintain its state of continuous motion or rest until it is disrupted by an external force.

### 2. Law of Conservation of Angular Momentum

The law of conservation of angular momentum states that a body tends to maintain its state of angular momentum till it encounters an external torque force. In the case of the angular momentum, instead of mass, the moment of inertia is considered; whereas, in place of velocity with which the object moves, the angular velocity with which the object turns is referred to.

## Examples of Conservation of Momentum

There are a variety of real-life applications that make use of the concept of conservation of momentum for their basic operation and working. The examples of some of such applications are listed below:

### 1. Gun and Bullet Mechanism

The bullet firing mechanism of a gun tends to form a prominent example of conservation of momentum in real life. When the trigger of the gun is pulled, the internal mechanism of the gun gets activated and a bullet gets fired in the forwarding direction. The velocity of the bullet builds up as it advances forward. This proportionally increases the momentum of the bullet. To balance this increase in momentum caused by the movement of the bullet in the system, the gun gets pulled back in the opposite direction with the same magnitude force. As a result, the momentum gets conserved within the system and the law of conservation of momentum gets verified. The gun and bullet mechanism also helps demonstrate the existence of Newton’s third law of motion in real life.

### 2. Inflated Balloon

An inflated balloon is yet another example of the objects present in our surroundings that demonstrate the law of conservation of momentum. When a balloon that is properly inflated with air is released into the environment, the air molecules present inside its structure begin to rapidly move outwards into the surroundings. The rapid movement of the air molecules means an increase in the velocity possessed by them and the increase in velocity indicates the building up of momentum value. To balance the increasing momentum value in the system, the balloon begins to move in the direction opposite to the direction of motion of the air molecules, thereby conserving the momentum and displaying the law of conservation of momentum in real life.

A Newton’s cradle is one of the best examples of real-life objects that help easily demonstrate the law of conservation of momentum as well as the law of conservation of energy in real life. A Newton’s cradle is generally considered to be a physics toy, a piece of experimental equipment, or a decorative item. It typically consists of a wooden or a steel metal frame and four to five metallic spherical balls. The spherical balls are attached to the base of the top beam of the frame at equal distances with the help of perfectly taut strings or wires. The spherical balls have the same mass values and hang a few millimetres above the base of the frame. The law of conservation of momentum and energy can be demonstrated easily with the help of Newton’s cradle by pulling ‘n’ a number of spherical balls to one side of the device and releasing them after some time from a distance. Due to the pendulum motion exhibited by the balls, the balls tend to swing towards the balls that are present at rest and undergo a collision. This leads to a transfer of energy from one ball to another and so on till the energy reaches the ball present at the extreme edge of the arrangement. As a result of the energy and momentum transfer between the balls, the same number of balls get displaced to the same distance on the other side of the toy. The same procedure can be repeated again and again by changing the number of the balls pulled and released to check if the same number of balls get displaced to the other side of the toy, thereby verifying the existence of the law of conservation of energy and momentum in real life.

### 4. Collision of Two Objects

One can easily understand the law of conservation of momentum by observing the complete process of collision of two objects moving with their respective velocities towards each other. For this purpose, suppose the two objects, say two balls having masses m1 and m2 move towards each other with velocities v1 and v2 respectively. The two objects or balls tend to collide with each other when the distance between them is nearly equal to zero. As a result of the collision, both the balls suffer the impact of equal magnitude forces in opposite directions that cause the balls to move in the opposite directions. The magnitude of the forces acting on individual balls can be calculated by evaluating the rate of change in the momentum of the balls. It can be observed theoretically as well as in practice that the sum of the momentum of both the balls before the collision is equal to the sum of the momentum of both the balls after the collision. The impact force generated due to the collision increases with an increase in the mass and velocity of the objects. This means that objects moving with a greater velocity or the objects having significantly more mass value can cause severe destruction or produce a high magnitude impact force as such objects eventually possess a high magnitude momentum.

### 5. Billiards and Snooker

Billiards and snooker games tend to form yet another example of the applications that help easily demonstrate the law of conservation of momentum in real life. Both, billiards and snooker games appear to be similar as they are played on a similar type of game set-up; however, the difference between the two games is that billiards is played on a game table that has no pockets, while the game table of a snooker game contains multiple pockets. Also, the billiards game is played with the help of three game balls, while the snooker is played with nine to fifteen balls. The player playing either of the two games hits the balls in a particular order with the help of the cue stick. As a result, the balls begin to move with a velocity value and develop momentum. The momentum remains conserved within the system throughout the process. In case the moving balls collide with each other, the magnitude and direction of the momentum possessed by each ball get altered accordingly so as to justify that the sum of total momentum in the system before the collision is always equal to the sum of momentum in the system after the collision.

### 6. Bowling

Bowling is yet another game that helps a person easily comprehend the concept of the law of conservation of momentum in real life. Bowling is basically a target sport that aims at rolling a heavy metallic ball down the playing lane in the direction of an orderly arranged stack of pins so as to knock out as many pins as possible to the ground. When the player rolls the bowling ball, it begins to move and develop velocity. As a result, a significant amount of momentum gets built up in the rolling ball. At this particular instant of time, the pins do not move at all, and hence do not possess momentum as the value of the initial velocity of the pins is still maintained equal to zero. When the bowling ball hits the pins, the rest state of the pins gets affected and the pins begin to move. This means that after and during the collision, the pins begin to gain velocity and possess momentum. In such a case, it can be observed that the momentum possessed by different objects in the bowling game system before and after the collision remains conserved throughout the process, hence the law of conservation of momentum can be verified easily bu observing a bowling game.

### 7. Rocket

A rocket is a spacecraft or an aircraft vehicle that typically makes use of thrust generated by the rocket engine for its flight. A rocket is generally launched into outer space and is used for various scientific research and development applications. The fuel used by the rockets typically includes liquid hydrogen, liquid oxygen, hydrogen peroxide, hydrazine, etc. The chemical composition of the rocket fuels tends to produce an enormous amount of energy that can help a massive body of the vehicle to shoot straight up and get launched into space. The type of fuel used by the rockets must be selected carefully on the basis of the type of application, the amount of load present within the rocket, the type of material used for the construction of the vehicle, and the physical attributes of the rocket. The law of conservation of momentum can be easily observed by looking at the flight of a rocket. This is because if you closely observe the direction of the fuel escaping the rocket and the direction in which the rocket gets launched, you will find that they are exactly opposite to each other. Also, the magnitude of the force exerted by the rocket fuel to the ground is balanced and proportionate to the magnitude of the upthrust force with which the rocket moves in the upward direction. This means that the momentum possessed by the rocket fuel and the rocket body is perfectly balanced and remains conserved in the system throughout the process, hence a rocket perfectly demonstrates the application of the law of conservation of momentum in real life.

### 8. Firecrackers

Firecrackers are basically toys that contain different harmless yet dangerous chemical substances that get triggered due to a change in the temperature or pressure value or application of a mechanical force. Most of the firecrackers lead to the formation of colourful creative light patterns in the surroundings. The colour of the light particles emitted by the firecrackers typically depends on the type of chemical contained in the internal structure of the firecrackers. It can be noted easily while lighting a firecracker that the direction in which the firecracker moves is opposite to the direction in which the energy from the chemical substances contained by the firecrackers is released. The product of the mass and the velocity of the firecracker gives the momentum of the firecracker, while the velocity with which the chemical gets released or gets consumed times the mass of the chemical substance provides the momentum of the chemical composition of the firecracker. The direction of the momentum of the firecracker is the same as that of the direction of the velocity with which it advances in the environment. Likewise, the direction of the momentum of the chemical substance is the same as that of the direction of the velocity with which it gets released or consumed. The total value of the momentum contained within the system during the process when the firecracker gets ignited and after it gets properly triggered remains constant. This indicates a prominent application of the law of conservation of momentum in real life.

### 9. Rotation of the Earth

The rotation of the earth is a classic example of the law of conservation of angular momentum as evidently, the earth is rotating along its axis since the formation of the solar system. To date, the rotation of the earth has not been affected by any external torsion force or torque, which means the angular momentum of the earth is still conserved and the same as at the beginning.

### 10. Spinning an Object

Spinning a plate or a ball on a finger or a stick is yet another example of the application of the law of conservation of angular momentum in real life. This is because in this case the object keeps spinning or maintains its state of continuous rotatory motion along its axis till it gets affected by an external force such as air resistance or drag, gravitational force, muscular force, etc.

### 11. Throwing a Ball

When you throw a ball in the air or roll it over the ground, it tends to move in the direction of the throw with a particular velocity value till an external force or a combination of two or more external forces such as the force of friction, impact force, muscular force, air resistance or drag, etc. begins to act on the surface of the ball. It means that the momentum of the ball remains conserved until an external force acts on its surface and disrupts the uniform motion of the ball.

### 12. Ice Skater

An ice skater tends to verify and demonstrate the existence of the law of conservation of angular momentum in real life when he/she begins to spin on the surface of the ice. It can be observed easily that the player is efficiently able to modulate his/her speed of spinning by moving his/her arms towards and away from his/her body. The speed with which the player spins tends to increase as the player pulls his/her arms towards the body, while the speed tends to decrease when the player moves his/her arms away from the body. This is because by pulling the arms inwards the skater tends to decrease the moment of inertia of his/her body as the mass of the body in this case is centralised. To balance this out or to maintain the angular momentum of the skater to a constant value, a proportionate increase in angular velocity gets induced. Likewise, by moving the arms outwards or away from the body, the skater increases the value of the moment of inertia of his/her body, as a result, the speed of spinning gets decreased, which in return slows down the skater’s rotation speed.

### 13. A Person Spinning in Office Chair

When a person sits in an office chair that is capable of turning 360 degrees and starts spinning, he/she can easily control the speed of the chair’s rotation by moving his/her hands towards or away from the centre of the chair. The universal law of physics that helps in this speed modulation of the chair is known as the law of conservation of angular momentum. When the user moves his/her hands toward the centre of the chair rotating along its axis, the moment of inertia of the body reduces and the rotation speed of the chair increases. Likewise, when the user moves his/her hands away from the centre of the rotating chair, the value of the moment of inertia of the body increases, while the spinning speed of the chair gets reduced. In both cases, the value of the angular momentum remains constant or is preserved. There are a variety of rides available in amusement parks and science museums that work on a similar phenomenon and help practically demonstrate the existence and application of the law of conservation of angular momentum in real life.

### 14. Fidget Spinner

A fidget spinner is basically a stress and anxiety management toy that consists of a ball bearing mechanism attached to its centre and plastic or metallic frame that covers the internal mechanical arrangement of the toy. The user is required to hold the toy from the centre in between his/her thumb and middle finger and give it a spin with the help of the push force exerted by the index finger. The spinner keeps spinning till the kinetic energy developed by it does not get completely consumed. This means that the angular momentum of the spinning toy remains conserved till it gets acted upon with an external torsion force or torque. In this particular case, air resistance, i.e., also known as drag force or mechanical force can act as an example of the external forces that may disrupt the uniform rotatory motion or spinning behaviour of the toy. In an ideal environment, where there exist no chances of air resistance or any other external force acting on the toy, the fidget spinner keeps spinning forever.

### 15. Spinning Top

A spinning top is yet another example of a daily use item that tends to demonstrate the law of conservation of momentum in real life. Here, the angular momentum of the spinning top remains conserved while it exhibits the rotatory motion and does not get affected by any form of external force.

### 16. Flip-it Fidget Toy

The flip-it fidget toy is yet another example of a stress-relieving and anxiety management toy. A flip-it fidget toy is cuboidal in shape and has curved edges and faces. If the user pushes the toy in one direction, it tends to keep rolling over and again till it encounters an external resistant force that prohibits the uniform motion of the object. The motion of the body along a straight line indicates the existence of linear momentum, the value of which can be calculated by obtaining the product of the mass of the toy and the velocity with which it tends to move. The linear momentum in a moving flip-it fidget toy remains conserved till it is externally stopped by applying mechanical force.

### 17. Straw String and Washer Project

To understand the concept of the law of conservation of angular momentum with the help of a self-made project, the user is required to bring a straw, a string, and a small heavy object such as a washer. The first step of the project is to pass the string through the straw and tie the small object to one end of the string. The second step is to allow the free end of the string to freely hang through the other side of the straw. Now hold the set-up and begin to test the execution. Spin the straw gently and make the small object attached to the string swirl around in a circular direction. Now, pull the free end of the string and observe the behaviour of the swirling object. You can easily observe that the speed with which the washer or the small object tied to the string spins varies on pulling or releasing the loose end of the string. This is because as per the law of conservation of angular momentum, the value of the moment of inertia of the rotating body or the value of the angular velocity with which it moves gets affected to maintain the value of angular momentum to a constant value.