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)

Impulse and Change in Momentum ALWAYS Linked 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

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 2nd 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

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!