Transcript Momentum
Momentum Ch. 6 Momentum • Inertia in motion! • We know that a heavy truck is harder to stop than a small car moving at the same speed • Basically that fact is saying the truck has more momentum Momentum Formula • Momentum = mass x velocity • when we don’t care about direction: Momentum = mass x speed What can we determine from the formula about the heavy truck? What can we determine from the formula about velocity? • Bullet vs. large ship • Bullet may not have a lot of mass, but large velocity • Large ship might be moving slowly, but lots of momentum Impulse • If the momentum of an object changes, then either the mass or the velocity or both change • If the mass remains unchanged, then the velocity changes and acceleration occurs Impulse • What produces acceleration? • Force! • The greater the force acting on an object, the greater its change in velocity – hence, the greater its change in momentum Time • How long a time the force acts is important! • If you apply a brief force to a stalled car, you produce a change in its momentum • Apply the same force over an extended period of time, and you produce a greater change in the car’s momentum Change in Momentum and Time A force sustained for a long time produces more change in momentum than does the same force applied briefly So both the force and time interval are important in changing momentum Impulse =Ft The quantity of force x time interval = impulse Impulse Changes Momentum • The greater the impulse (Ft) exerted on something, the greater the change in momentum • Impulse = change in momentum Delta Symbol • Delta symbol represents a “change in” • So to represent the impulse-momentum relationship (greater impulse – greater momentum) Ft= (mv) Impulse = Change in Momentum Impulse and Change in Momentum ALWAYS Linked On Your Own • Pg. 86 – For each case (1-3) give me the main idea and an example. – Must have labeled pictures describing your example! • Review Questions pg. 96 – #1-11 • Plug and Chug pg. 96 – -#1-6 Bouncing • Impulses are greater when an object bounces. • The impulse required to bring an object to a stop and then to “throw it back again” is greater than the impulse required to merely bring the object to a stop Bouncing • If a flowerpot falls from a shelf onto your head, sad day. If it bounces from your head, really sad day. • Suppose that you catch a falling pot with your hands. You provide an impulse to reduce its momentum to zero – If you throw the pot upward again, you have provided additional impulse – This increased amount of impulse is the same that your head supplies if the flowerpot bounces from it Bouncing So basically, force is greater when bouncing occurs Real Use! • The fact that impulses are greater when bouncing occurs was used with success during the California Gold Rush • A curved paddle that caused the incoming water to bounce upon impact increases the impulse on the wheel Think Back to Newton’s nd 2 Law • From Newton’s 2nd law you know that to accelerate an object, a net force must be applied to it • If you wish to change the momentum of an object, exert an impulse on it • Only an impulse external to a system will change the momentum of the system – If no external impulse then no change in momentum A Cannon • A cannon is being fired • The force on the cannonball inside the barrel is equal and opposite to the force causing the cannon to recoil • Since these forces act for the same time, the impulses are also equal and opposite • Newton’s 3rd law applies to impulses too! Conservation of Momentum • These impulses are internal to the system comprising the cannon and cannonball, so they don’t change the momentum of the cannon-cannonball system • Before firing, the system is at rest and the momentum is zero • After firing, the net momentum is still zero • Net momentum is neither gained nor lost Conservation of Momentum • Momentum, like the quantities velocity and force, has both direction and magnitude – Vector quantity • Like velocity and force, momentum can be cancelled – So although the cannonball gains momentum when fired and the recoiling cannon gains momentum in the opposite direction, there is no gain in the cannon-cannonball system Conservation of Momentum In the absence of an external force, the momentum of a system remain unchanged Bouncing and Conservation of Momentum Practice • Pg. 96 – Review questions #12-17 – Plug and chug #7-8 – Ranking #1-3 More Practice! • Pg. 98 – Exercises # 1- 11 odd, 12-16 all Collisions • Momentum is conserved in collisions – The net momentum of a system of colliding objects is unchanged before, during, and after the collision • This is because the forces that act during the collision are internal forces – Forces acting and reacting within the system itself Collisions Net momentum before collisions = net momentum after collision Elastic Collisions • When a moving billiard ball makes a head-on collision with another ball at rest, the moving ball comes to rest and the other ball moves with the speed of the colliding ball. • This is an elastic collision http://www.physicsclassroom.com/mmedia/momentum/cthoe.gif Inelastic Collisions • Deformation, or generation of heat, or both • In a perfectly inelastic collision, both objects stick together. Ch. 6 Notes • Notes for each section of ch. 6 • You will have a quiz over these notes (momentum) tomorrow!