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Conservation of momentum

As long as no external forces are acting on the objects involved, the total momentum stays the same in explosions and collisions. We say that momentum is conserved. You can use this idea to work out the mass, velocity or momentum of an object in an explosion or collision.
 

Example
 

A bullet with a mass of 0.03 kg leaves a gun at 1000 m/s. If the gun’s mass is 1.5 kg, what is the velocity of the recoil on the gun?
 

momentum of bullet = mass x velocity

= 0.03 kg x 1000 m/s

= 30 kg m/s
 

Notice that we have unequal numbers of each type of atom on the left-hand side compared with the right-hand side. To make things equal, we need to adjust the number of units of some of the substances until we get equal numbers of each type of atom on both sides of the arrow.
Here is the balanced symbol equation:
 

2Cu + O2

2CuO

Reactions and compounds

New substances are formed by chemical reactions. When elements react together to form compounds their atoms join to other atoms using chemical bonds. For example, iron and sulfur (often spelt 'sulphur') react together to form a compound called iron sulfide (often spelt 'sulphide'), and sodium and oxygen react together to form sodium oxide.
 

• electrons are transferred from one atom to another, so that one atom gives electrons and the other takes electrons, or
• electrons are shared between two atoms.

momentum (kg m/s) = mass (kg) x velocity (m/s)
 




Notice that momentum has:
 



• magnitude (an amount because it depends on the object’s mass); and
• direction (because it depends on the velocity of the object).