CHAPTER # 8 Momentum

So What’s Momentum ? Momentum = ______ x _________ This can be abbreviated to : . ______________________ Or, if direction is not an important factor : . . momentum = mass x _______ So, A really slow moving truck and an extremely fast roller skate can have the same momentum.

1 kg 10 m/sec 1000 kg .01 m/sec Question : Under what circumstances would the roller skate and the truck have the same momentum ? __________________________________________________________________________________________________________________ A 1000 kg truck moving at 0.01 m/sec has the same momentum as a 1 kg skate moving at 10 m/sec. Both have a momentum of 10 kg m/sec. ( 1000 x .01 = 1 x 10 = 10 )

Impulse and Momentum If momentum changes, it’s because _____ or __________ change. Most often _________________ so velocity changes and that is ___________. And ________ x ______________ = force Applying a force over a time interval to an object changes the momentum Force x time interval = ______________ Impulse = F t or Ft = mv Ft = mv

MOMENTUM An object at rest has no momentum, why? ____________________________________________________________________________ To INCREASE MOMENTUM, ____________________________________________________________________________ Examples : pulling a sling shot drawing an arrow in a bow all the way back a long cannon for maximum range hitting a golf ball or a baseball . (follow through is important for these !) FORCE TIME

MOMENTUM SOME VOCABULARY : _______ : impact force X time (newton.sec) . Ft = impulse ________ : the force acting on an object (N) . usually when it hits something. ____________ : average force of impact

MOMENTUM Decreasing Momentum Ft Which would it be more safe to hit in a car ? Knowing the physics helps us understand why hitting a soft object is better than hitting a hard one. Ft mv mv Ft

MOMENTUM In each case, the momentum is decreased by the same amount Hitting the haystack ______________ the impact time (the time in which the momentum is brought to zero). The longer impact time __________________________ and decreases the deceleration. Whenever it is desired to decrease the force of impact, ____________________________________ !

DECREASING MOMENTUM If the time of impact is increased by 100 times (say from .01 sec to 1 sec), then the force of impact is reduced by 100 times (say to something survivable). EXAMPLES : Padded dashboards on cars Airbags in cars or safety nets in circuses Moving your hand backward as you catch a fast-moving ball with your bare hand or a boxer moving with a punch. Flexing your knees when jumping from a higher place to the ground. or elastic cords for bungee jumping Using wrestling mats instead of hardwood floors. Dropping a glass dish onto a carpet instead of a sidewalk.

EXAMPLES OF DECREASING MOMENTUM Bruiser Bruno on boxing … Increased impact time reduces force of impact Barney Jervais on bungee Jumping … Ft = change in momentum Ft = change in momentum POOF ! CRUNCH ! Ft = Δmv applies here. mv = the momentum gained before the cord begins to stretch that we wish to change. Ft = the impulse the cord supplies to reduce the momentum to zero. Because the rubber cord stretches for a long time the average force on the jumper is small.

Questions : When a dish falls, will the impulse be less if it lands on a carpet than if it lands on a hard ceramic tile floor ? _______________________________________________________________________________________________________________________________________________________________________. If a boxer is able to increase the impact time by 5 times by “riding” with a punch, by how much will the force of impact be reduced? ____________________________________________________________________________________

BOUNCING IMPULSES ARE ______ WHEN AN OBJECT BOUNCES The impulse required to bring an object to a stop and then to throw it back upward again is greater than the impulse required to merely bring the object to a stop. When a martial artist breaks boards, does their hand bounce? Is impulse or momentum greater ? Example : The Pelton Wheel.

CONSERVATION OF MOMENTUM To accelerate an object, a force must be applied. The force or impulse on the object must come from _________________. EXAMPLES : The air in a basketball, sitting in a car and pushing on the dashboard or sitting in a boat and blowing on the sail don’t create movement. Internal forces like these are balanced and cancel each other. If no outside force is present, ______________ in momentum is possible.

The Law of Conservation of Momentum In the absence of an external force, the momentum of a system remains ______________________________. This means that, when all of the forces are internal (for EXAMPLE: the nucleus of an atom undergoing . radioactive decay, . cars colliding, or . stars exploding the net momentum of the system before and after the event is the same.

QUESTIONS 1. Newton’s second law states that if no net force is exerted on a system, no acceleration occurs. Does it follow that no change in momentum occurs? __________________________________________________________________________________________ 2. Newton’s 3rd law states that the forces exerted on a cannon and cannonball are equal and opposite. Does it follow that the impulse exerted on the cannon and cannonball are also equal and opposite? _______________________________________________________________________________________________________________________________________

COLLISIONS ELASTIC COLLISIONS INELASTIC COLLISIONS Momentum transfer from one Object to another . Is a Newton’s cradle like the one Pictured here, an example of an elastic or inelastic collision?

Problem Solving #1 A 6 kg fish swimming at 1 m/sec swallows a 2 kg fish that is at rest. Find the velocity of the fish immediately after “lunch”.

Problem Solving #2 Now the 6 kg fish swimming at 1 m/sec swallows a 2 kg fish that is swimming towards it at 2 m/sec. Find the velocity of the fish immediately after “lunch”.

Problem Solving #3 & #4 Now the 6 kg fish swimming at 1 m/sec swallows a 2 kg fish that is swimming towards it at 3 m/sec. Now the 6 kg fish swimming at 1 m/sec swallows a 2 kg fish that is swimming towards it at 4 m/sec.

MOMENTUM VECTORS Momentum can be analyzed by using vectors The momentum of a car accident is equal to the vector sum of the momentum of each car A & B before the collision. A B

MOMENTUM VECTORS (Continued) When a firecracker bursts, the vector sum of the momenta of its fragments add up to the momentum of the firecracker just before it exploded. The same goes for subatomic elementary particles. The tracks they leave help to determine their relative mass and type.

CHAPTER #8 - MOMENTUM Finish