Definitions

Interactions - when one object influences another we say that the two objects interact. To analyze an interaction we must detect changes in the system of objects we are studying. Physicists divide all interactions between objects into four fundamental groups:

· Gravity
· Electromagnetism
· Weak nuclear interaction
· Strong nuclear interaction

System - the complete set of objects that we are studying. When we chose a system so that all the interactions we are concerned with occur between objects in the system, we have chosen a closed system.

Example of an interaction
Suppose you are driving your car to work and you accidentally hit the rear of the car in front of you at an intersection. Your car comes to a stop but the car you hit is propelled into a small tree on the sidewalk. A policeman comes to investigate the accident.

What objects should the policeman list as the system?

What changes occurred to the system to indicate there was an interaction?

Conservation laws - although we analyze interactions by detecting changes in the system, some things stay the same during an interaction. A system's physical quantities of momentum and total energy stay the same during an interaction.

Momentum - a vector quantity that includes an object's mass and velocity

Momentum = mass ´ velocity

Symbolically p = mv

Momentum is conserved in all interactions; this property is useful for determining unknown quantities such as mass or velocity of objects in the interaction.

We use an equation such as:

Where the subscripts i and f denote initial and final velocities for the two objects. If there are more than two objects in the system we would include their initial and final momentums as well.

Example of momentum
A ball of mass 0.5 kg traveling at 15 m/s to the right strikes another ball of mass 1 kg that is initially at rest. The smaller ball bounces off the larger ball and now has a velocity of 6 m/s to the left. Calculate the final velocity of the 1 kg ball using conservation of momentum.

Suppose rather than bouncing off, the smaller ball sticks to the lager ball. Calculate the speed and the direction of the two balls after the collision.

Charge - the electric charge of a system is also conserved during an interaction. If you count the charges of objects in the system before an interaction, you should have the same net charge afterwards.

Example of charge conservation
Tritium with an electrical charge of +1 changes into an electron with a charge of -1 and two other objects. What are the charges on the other objects?