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Learn about Newton’s Third Law, momentum, and collisions, including action-reaction examples, tug of war, helicopter lift, increasing/decreasing momentum, conservation of momentum, and types of collisions. Master the key principles of motion and force!
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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.
