Momentum and Collision
Car and Truck in Head-on
Collision
Collisions between objects are governed by laws of momentum and
energy. When a collision occurs in an isolated system, the total
momentum of the system of objects is conserved. Provided that there
are no net external forces acting upon the objects, the momentum of
all objects before the collision equals the momentum of all objects
after the collision. If there are only two objects involved in the
collision, then the momentum change of the individual objects are
equal in magnitude and opposite in direction.
Certain collisions are referred to as elastic collisions. Elastic
collisions are collisions in which both momentum and kinetic energy
are conserved. The total system kinetic energy before the collision
equals the total system kinetic energy after the collision. If total
kinetic energy is not conserved, then the collision is referred to as
an inelastic collision.
The animation below portrays the elastic
collision between a 1000-kg car and a 3000-kg truck. The
before- and after-collision velocities and momentum are shown in the
data tables.
In the collision between the truck and the car, total system
momentum is conserved. Before the collision, the momentum of the car
is +20 000 kg*m/s and the momentum of the truck is -60 000 kg*m/s;
the total system momentum is -40 000 kg*m/s. After the collision, the
momentum of the car is -40 000 kg*m/s and the momentum of the truck
is 0 kg*m/s; the total system momentum is -40 000 kg*m/s. The total
system momentum is conserved. The momentum change of the car (-40 000
kg*m/s) is equal in magnitude and opposite in direction to the
momentum change of the truck (40 000 kg*m/s) .
An analysis of the kinetic energy of the two objects reveals that
the total system kinetic energy before the collision is 800 000
Joules (200 000 J for the car plus 600 000 J for the truck). After
the collision, the total system kinetic energy is 800 000 Joules (800
000 J for the car and 0 J for the truck). The total kinetic energy
before the collision is equal to the total kinetic energy after the
collision. A collision such as this in which total system kinetic
energy is conserved is known as an elastic collision.
For more information on physical descriptions of motion, visit
The Physics Classroom Tutorial.
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