Collision
6 Minutes
Collision and conservation of momentum for students: understand Newton's third law, forces, and carom collisions. A physics lesson in English.
- Initial and final velocities of a carom striker and coin
- Deriving momentum formulas and energy lost as heat and sound
- Real-life momentum conservation in carom, billiards, and accidents
What this lesson teaches
A collision is an event where two objects strike each other and transfer momentum, which is the product of an object's mass and its velocity. Under the law of conservation of momentum, the total momentum of the colliding bodies before impact remains equal to their total momentum after impact.
Newton's third law during impact
When two bodies collide, such as a carom striker hitting a coin, the striker exerts a force on the coin. Newton's third law states that for every action, there is an equal and opposite reaction acting between two different objects. Therefore, the force exerted by the striker is equal in magnitude and opposite in direction to the force exerted by the coin. This physical relationship is expressed mathematically as F1 equals minus F2.
Calculating force and momentum transfer
- Force equals an object's mass multiplied by its change in velocity, which is final velocity minus initial velocity, divided by time.
- For the striker, the applied force F1 equals mass M1 multiplied by final velocity V1 minus initial velocity U1, all divided by time T.
- For the coin, the reaction force F2 equals mass M2 multiplied by final velocity V2 minus initial velocity U2, all divided by time T.
- Equating F1 to minus F2 shows that the rate of momentum transferred during that specific time is conserved between both colliding bodies.
Energy changes in a carom collision
A striker pushed with two Newtons of force travels eighteen centimeters in 1.2 seconds, giving it an initial velocity of 0.15 meters per second and a momentum of 0.0012 kilogram meter per second. The resting coin begins with a velocity of zero, but after being struck, it travels ten centimeters in 1.2 seconds to reach a velocity of 0.11 meters per second and a momentum of 0.00066 kilogram meter per second. Friction between the surfaces generates heat energy, and the impact also creates sound energy. When the energy gained as heat and sound is included, the total momentum before and after the collision remains completely balanced.
Real-world examples of momentum transfer
- In billiards, striking one ball transfers momentum across the table into a second ball upon contact.
- In boxing, momentum generated by a boxer's punch transfers directly into the boxing bag.
- In vehicle accidents, police officers use momentum conservation to recreate crash scenes, determine vehicle speeds, and measure how far cars moved after collision.