Action and Reaction Notes Newton’s Third Law of Motion Action and Reaction Newton’s Third Law of Motion: when one object exerts a force on a second object, the second one exerts a force on the first that is equal in size and opposite in direction. OR… for every action force there is an equal and opposite reaction force. These forces always act in pairs. These forces always act on DIFFERENT objects. Newton’s Third Law Examples: - jumping on a trampoline (you place a force on the trampoline which places an equal and opposite force back on you – causing you to go “up”) - jumping from a boat (you place a force on the boat which sends the boat backward, but the boat places an equal and opposite force back on you causing you to move forward) - a rocket (gases are pushed out by the engine which place an equal and opposite force back on the rocket – causing it to fly) Action Force/ Reaction Force Diagram Reaction Force Action Force Momentum Momentum: a property a moving object has because of its mass and velocity Momentum = mass x velocity p=mxv Momentum is measured in kg x m/s Example: - two trucks may have the same velocity, but the one that has the greater mass will have greater momentum. EXAMPLES of Momentum Ifthe velocity of a bus & a car traveling along the highway is equal, the bus will have more momentum because it has more mass. MORE LESS If the mass of two bicycles and their riders are equal, the one that is moving at a greater velocity has more momentum because it is moving faster. Law of Conservation of Momentum Law of Conservation of Momentum: the total amount of momentum of a group of objects does not change unless outside forces act on the objects. Example: Pool table - no balls moving = zero momentum - cue ball moving toward the group of balls has momentum - cue ball hits the group of balls and scatters the balls; spreads the total momentum out amongst all of the moving pool balls Law of Conservation of Energy applied to momentum Energy cannot be created or destroyed, but can change forms. Momentum is a result of an object’s motion. Energy can be transferred from one moving object to another in a collision. All energy within the system can be accounted for because any energy left after the transfer of energy from one object to another in a collision changes to different forms of energy. EXAMPLES In a game of billiards or pool: Potential energy for the motion of balls on a pool table is stored in the person holding the stick as he/she prepares to hit the ball. Most of this energy is changed to kinetic energy (of the moving cue ball). When the cue ball collides with the group of balls causing them to scatter and spread out, kinetic energy of the cue ball is shared among all moving balls. Remaining energy is changed to sound, heat from friction, etc. In conclusion, all potential energy originally stored in the person holding the pool stick is used. Therefore, no energy is created or destroyed.