Lesson 9, Topic 3
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# Conservation of Linear Momentum

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The law of conservation of linear momentum states that for a closed or isolated system of colliding bodies the total momentum remains constant, provided no external forces act.
OR
If two or more bodies in a closed or isolated system collide, their total momentum before collision is equal to their total momentum after collision provided no external forces act.

A Closed / Isolated System:
A closed or isolated system means a system in which no external force acts. Or it is a system in which there is no interference by any external factor.

### Different Cases Of Collision:

If masses, M1, and M2, moving in the same direction with initial velocities U1 and U2, respectively, collide with each other and move in different directions

From conservation of linear momentum, the total momentum before collision is equal to the total momentum after collision; therefore,

M1U1 + M2U2 = M1V1 + M2V2

When two bodies moving in opposite directions collide,

M1U1 – M2U2 = -M1V1 + M2V2

When a stationary body collides with a moving body, and move apart.

M1U1 – 0 = M1V1 + M2V2

If two bodies collide with each other, and stick together, and move with common velocity.

M1U1 + M2U2 = (M1 + M2)V

V is the common velocity of the body.

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