Momentum “The Big Mo” “Momentum’s on our side!” “Feel the Momentum shift!” (insert any other sports cliché here)

How is velocity affected by force?  Newton’s 1st law says if no net force acts on an object its velocity will not change  How much the velocity changes depends on two things: The magnitude of the force, and how long the force acts on it

So then what?  Newton’s 2nd law can help us with the math: F = (m)(a) Can be rewritten as: F = (m)(∆v/∆t) Which can be rearranged to show: (F)(∆t) = (m)(∆v)

 The left side of the equation is called impulse (the product of the magnitude of the force and the time it is applied)  The right side of the equation is called momentum (the product of an object’s mass and velocity)

Taking it one step further…  The equation: (F)(∆t) = (m)(∆v)  Can be rewritten again: (F)(∆t) = (m 2 v 2 ) - (m 1 v 1 )  Momentum is symbolized by “p”, so the equation is written as: (F)(∆t) = p 2 - p 1 This equation is called the impulse-momentum theorem (impulse causes a change in momentum)

Example #1 An 2200 kg SUV traveling at 60 mph (26 m/s) stops in.22 s when it hits a concrete wall. What is the force applied by the wall to stop the SUV? (F)(∆t) = p 2 - p 1 F(.22s) = 2200(0) (26) F = N (the answer makes sense: the force is negative to show it is acting opposite the motion, and it is pretty big)

Oh yeah, one more thing:  Momentum is a conserved quantity  What does is mean to be conserved? The momentum of a system is neither lost nor gained Meaning, the total momentum of a system remains the same  In math terms, it is written as: p before = p after

To Use the Conservation of Momentum  Keep in mind that since momentum is dependent on velocity, it is considered a vector quantity  This means direction is important  Use your signs correctly in a situation; one direction is “+”, the opposite direction is “-”

Example #2 A.105 kg hockey puck, moving at 24 m/s, is caught by a 75 kg goalie at rest. What is the speed of the goalie after the catch? p before = p after (m)(v) puck + (m)(v) goalie = (m)(v) puck+goalie (.105)(24) + (75)(0) = (75.105)(x) X =.0336 m/s (notice the answer makes sense; it is really small, the sign is positive, meaning the goalie will move in the same direction the puck was traveling)

Example #3: A 4.00 kg model rocket is launched, shooting 50.0 g of fuel from its exhaust at 625 m/s. What is the velocity of the rocket after the fuel is burned? p before = p after (m)(v) fuel + rocket = (m)(v) rocket +(m)(v) fuel ( )(0) = (.050)(625) + (4.00)(x) X = m/s (the answer makes sense; the velocity is negative, meaning the rocket moves in the opposite direction of the fuel)

Homework  Chapter 9  Pages  #15, 24, 25, 31, 36, 40, 42