in collisions ,total ____ remains unchanged provided that no external force acts.
Answers
In a collision, strong mutual forces act between a few particles for a short time. These internal forces are significantly larger than any external forces during the time of the collision. The laws of conservation of linear momentum and energy, applied to the "before" and "after" situations, often allows us to predict the outcome of a collision. A great deal can be learned about the interactions between the colliding particles from the observed collision products.
Note:
external forces are small (and are ignored) during the collision
particles before and after the collision can be different (for example: nuclear reactions)
the collision force does not need to be a contact force
A very important parameter that is often used in the description of collisions is the linear momentum of the objects involved. The linear momentum p of an object with mass m and velocity v is deined as
p = m v
The linear momentum is a vector. This means that it is characterized by both a magnitude and a direction. The direction of the linear momentum of an obejct is equal to the direction of the velocity of that object.
Under certain circumstances the linear momentum of a system is conserved. The linear momentum of a particle is related to the net force acting on that object. The rate of change of linear momentum of a particle is equal to the net force acting on the object, and is pointed in the direction of the force. If the net force acting on an object is zero, its linear momentum is constant (conservation of linear momentum).
Suppose a force F acts during a collision. The result of the collision force will be a change in the momentum of the particles involved. The amount of change depends not only on the average value of the force, but also on the time period during which it acts. The change in momentum dp is related to the collision force F as
The total change in the of momentum during the collision is given by
The total linear momentum p of a system of particles is defined as the vector sum of the individual linear momenta. If the net external force acting on a system of particles is zero, the linear momentum of the system is conserved.
Consider the collision shown in Figure 1. If there are no external forces acting on this system (consisting of the two masses) the total momentum of the system is conserved. The first class of collisions we will discuss are the elastic collisions. Collisions are called elastic collisions if the total kinetic energy of the system is conserved. This means that none of the energy associated with the motion of the objects involved in the collision is transferred into internal energy (like for example the energy associated with the deformation of the colliding objects).