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Momentum
A moving object has a property that is called momentum (moh-MEHN-tuhm).
Momentum is a measure of mass in motion; the momentum of an object is the
product of its mass and its velocity. Momentum is similar to inertia.
To calculate an object’s momentum, you can use the following formula:
momentum = mass · velocity
p = mv
In this formula, p stands for momentum, m for mass, and v for velocity. Momentum
is usually measured in units of kilogram meters per second, or kg m/s. Momentum,
like acceleration and force, is a vector. The direction of momentum is the same as
the direction of the velocity.
During a collision between two objects, each object exerts a force on the other.
The colliding objects make up a system—a collection of objects that affect one
another. As the two objects collide, the velocity and momentum of each object
change. However, as no other forces are acting on the objects, the total momentum
of the system is unchanged. The principle of conservation of momentum states that
the total momentum of a system before a collision is the same as the total momentum
after a collision, as long as no outside forces are acting on the system.
total momentum
momentum 1
momentum 2
1
forces in collision
2
Before the collision The momentum of the first car is greater than
the momentum of the second car.
Their combined momentum is the
total momentum of the system.
APPENDIX
total momentum
momentum 1
momentum 2
3
During the collision The forces
After the collision The momentum
on the two cars are equal and
opposite, as described by Newton’s
third law. Momentum is transferred
from one car to the other during
the collision.
lost by one car was gained by the
other car. The total momentum of
the system remains the same as it
was before the collision.
Collisions are not the only events in which momentum is conserved. For
example, if you throw a ball while you are standing on in-line skates, the forward momentum of the ball will equal the backward change in your momentum.
In fact, momentum is conserved whenever the only forces acting on objects are
action/reaction force pairs. Conservation of momentum is really just another way
of looking at Newton’s third law.
R56 McDougal Littell Science
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