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  1. Glossary
  2. /Physics
  3. /Conservation of Momentum

What is Conservation of Momentum?

In short

The law of conservation of momentum says that the total momentum of a system stays the same if no outside force acts on it.

In simple words

Momentum is p=mvp = mvp=mv, the mass times the velocity, in kg m/s\text{kg m/s}kg m/s. It has a direction, so a motion to the left counts as negative. In a collision or an explosion the momentum before equals the momentum after. The law follows from Newton's third law, because the forces between the objects are equal and opposite. It does not need the kinetic energy to be conserved.

Example

A 2 kg2\ \text{kg}2 kg trolley at 3 m/s3\ \text{m/s}3 m/s hits a stationary 1 kg1\ \text{kg}1 kg trolley and they stick. Total momentum before is 6 kg m/s6\ \text{kg m/s}6 kg m/s, so v=6 kg m/s3 kg=2 m/sv = \dfrac{6\ \text{kg m/s}}{3\ \text{kg}} = 2\ \text{m/s}v=3 kg6 kg m/s​=2 m/s after.

Also searched as: is conservation of momentum newton's 3rd law.

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Related terms

  • Newton's Third Law
  • Newton's Second Law (F = ma)

← All glossary terms

Practise

  1. Two identical smooth discs, each of mass 0.20 kg and radius r, lie on a smooth…Advanced
  2. In a ballistic pendulum, a bullet stops inside a hanging block, which then swings up…Medium
  3. A 1.5 kg ball moving at 5.0 m/s makes an elastic, off-centre collision with an identical…Hard
  4. An 8.0 g bullet is fired into a 1.6 kg block hanging on strings, and stays inside…Hard
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Quick questions

On a frictionless horizontal table, block A of mass 2 kg moves at 6 m/s and strikes…Hard
  • A block of mass 2.0 kg slides at 5.0 m/s on a smooth floor and hits a block of mass 3.0…Advanced
  • Ball A of mass 0.5 kg slides at 4.0 m/s on a smooth floor towards ball B of mass 1.5 kg…Advanced

    • More Physics terms

    1. Acceleration
    2. Acceleration Due to Gravity (g)
    3. Adiabatic Process
    4. Amplitude
    5. Angle of Incidence
    6. Angle of Reflection
    All Physics terms →

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