Conservation of Momentum
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Every object has mass. When an object moves, it gains momentum; hence, momentum can be defined as mass in motion. The momentum of an object is dependent on two variables: its mass and its velocity. In terms of an equation, the momentum of an object is equal to the product of its mass and its velocity. It can be written as—

p = m • v

wherein p is the momentum, m is the mass, and v is the velocity of the object.

One of the most important laws in physics is the Law of Conservation of Momentum. This law corresponds with Newton’s Law of Action and Reaction, which states, “For every action, there is an equal and opposite reaction.” We usually associate the Law of Conservation of Momentum with colliding objects; hence, it can be stated, “In an isolated or a closed system (no external force in the system), the total momentum of two colliding objects before the collision is equal to the total momentum of the two objects after collision.” This tells us that the momentum in an isolated or a closed system is conserved.

As an equation, the Law of Conservation of Momentum can be expressed as—

Totalpinitial=Totalpfinal

wherein ρ initial is the initial momentum and ρ final is the final momentum of the objects in the system. Knowing the equation for momentum, the equation for Law of Conservation of Momentum can also be written as—

m1•v1(initial)+m2•v2(initial)=m1•v1(final)+m2•v2(final)

Watch the videos of real-life application of the Law of Conservation of Momentum.

In your science notebook, answer the following questions.

1. In the first video, when the two men in the cart pulled the rope, they met in the middle. Explain what would happen if the mass of the man on the left were twice the mass of the man on the right.

2. Compare the momentum and the velocity of the medicine ball with the person who threw the ball in the first part of the second video.

3. Explain what happened to the total momentum of the system after the person caught the medicine ball thrown to him in the second video.

4. An 18-wheeler and a pickup truck had a head-on collision.

A. Which of these two vehicles experienced the greatest force of impact? Provide an explanation.

B. Which of these two vehicles experienced the greatest amount of momentum change? Provide an explanation.

C. Which of these two vehicles experienced the greatest acceleration? Provide an explanation.

5. On their way back to school from a field trip, Barry and Ivan were seated in the front row of the school bus and witnessed a bug spatter onto the windshield. They started discussing the physics behind what happened. Barry argued that the momentum change of the bug is much greater than the momentum change of the bus because there was no noticeable change in the velocity of the bus compared to the obvious velocity change of the bug. Ivan disagreed and explained that both the bug and the bus experienced the same momentum change. With whom do you agree the most? Explain why you agree with one and not the other.