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Chapter 4 & 5 - Newton’s 3rd Law of Motion and Momentum. Action - Reaction. Newton’s Third Law. To every action there is always an opposed equal reaction. We call one force the action force and the other the reaction force. Simple Rule to Identify Action Reaction.
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Chapter 4 & 5 - Newton’s 3rd Law of Motion and Momentum Action - Reaction
To every action there is always an opposed equal reaction We call one force the action force and the other the reaction force.
Simple Rule to Identify Action Reaction • Action: Object A exerts a force on object B • Reaction: Object B exerts a force on object A • Example: Man pushes on wall, wall pushes back at man
Action-Reaction Examples • Cannon is fired - Cannon exerts force on ball (accelerates ball - ball exerts force on cannon (it recoils) • Rocket pushes on ground ground - ground pushes back
Balloon exhaust (air) pushes on outside air molecules - air molecules push back • Horse pushes on ground - ground pushes back
Action-Reaction act on Different objects • Action-Reaction forces do not cancel out because they act on different objects • Two forces acting on the same object can cancel out each other producing a net force of zero • ∑F=0
Tug of War Example • Each team pulls and produces enough rope tension to keep the rope from accelerating (Equal and opposite forces) • The winning team? • The team that produces more force against the ground, not the team that exerts the greatest force on the rope, wins the Tug O’ War
Helicopter Example • The whirling blades are shaped to push air particles down (action) • The air forces the blades up (reaction) • This upward force is called lift
Momentum and Collisions • Momentum = mass x velocity • P = m x v • Impulse = change in momentum • Or, change in m x v in a time interval
Increasing Momentum • 1) Apply the greatest force you can • 2) Extend the time of application of the force • Example: Long barreled cannons • Long barrels produce higher velocities than short ones as long as the force remains relatively large! • Why? The force of the exploding gun powder acts on the cannonball for a longer time
Decreasing Momentum • Long time means less force, if force is able to decrease • Example: you are in a fast-moving car and the brakes fail. Would you want to stop by hitting a brick wall or hit a haystack and move until you eventually come to rest? • By hitting the haystack you extend the contact time (time in which your momentum is brought to zero by opposing forces) • Force is reduced and you survive • If force is constant or is able to increase over time than - Long time less force is thrown out the window.
Decreasing Momentum • Short time means more force (inversely related) • Example: When boxing, if you move into the punch you are in trouble. Move away and the force is decreased, force is given chance to decrease. • Example: Breaking a stack of bricks. Would you want a short quick blow or a long extended blow? Remember: force increases with more momentum, quicker blow, more momentum, thus more force
Law of Conservation of Momentum • In the absence of an external force, the momentum of a system remains unchanged
Collisions • Elastic - When objects collide without being permanently deformed and without generating heat • Inelastic - Whenever colliding objects become tangled or coupled together • In a perfect inelastic collision both objects stick together.
