# Forces and Motion: Momentum and Collisions

## Forces and Motion: Momentum and Collisions

• Momentum is a vector quantity, depending only on an object’s mass and velocity. It is calculated by multiplying these two factors together.

• The unit of momentum is kg m/s, derived from the mass being in kilograms (kg) and velocity in metres per second (m/s).

• According to the principal of conservation of momentum, in a closed system, the total momentum before an event must be equal to the total momentum after the event, provided no external forces act on it.

• In an event such as a collision or explosion, you can use principles of momentum to determine the final velocities of objects if you know the initial velocities and masses.

• Consider Newton’s Third Law: For every action, there’s an equal and opposite reaction. This principle underpins the conservation of momentum in collisions or explosions.

• In ‘elastic collisions’, both momentum and kinetic energy are conserved. Examples include collisions between gas molecules or in ideal physics scenarios.

• For ‘inelastic collisions’, although momentum is still conserved, kinetic energy is not necessarily conserved. An example is a car crash where kinetic energy is transferred into other forms of energy, such as heat or sound.

• In ‘perfectly inelastic collisions’, the objects involved stick together and move as one mass after the collision. Even though kinetic energy is not conserved in these cases, momentum does remain conserved.

• Calculating changes in momentum can also help in understanding the force involved in a collision. The rate of change of momentum corresponds to the force applied, according to Newton’s Second Law.

• When exploring certain phenomena, such as car safety features, remember that reducing the force of an impact is about increasing the time over which the change in momentum happens. This is why features like crumple zones and airbags exist in vehicles to improve safety.

• In any calculation involving forces, motion, and momentum, always pay close attention to the direction of vectors - it can make a crucial difference to the answer.

• The usefulness of momentum calculations extends beyond collisions to include propulsion methods such as rocket launches, jet engines, and even rowing a boat. Return of a rocket to earth can also be understood through principles of momentum conservation.