AP Physics B Ch 7 Review PPT

8/9/2019 AP Physics B Ch 7 Review PPT

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Chapter 6 energy

transformations involved

only one object

Chapter 7 momentum is

transferred when 2 or more

objects interact in a collision

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Impulse changes momentum

Bat applies an average

force over a short time

interval to accelerate the

ball

Must stop the ball, change

its direction and send it in

the opposite direction

v F ma mt

F t m v

(! !

(

( ! (g g

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Impulse = Favgy(time

Impulse from the bat changes the

momentum of the ball

Actual force builds to a very highmaximum value and then drops off

We will most often study forces that

have a constant average value

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variable force

constant force

Impulse = area under force time graph

Impulse = vector quantity in

same direction as force vector

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A ball hits a wall and bounces off

at the same speed.

What is the direction of the

impulse acting on the ball?

f iImpulse = p = p - p(

pf

pi

6

(p


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Linear Momentum

Product of an objects mass and velocity

vector quantity with same direction as velocity

vector

Units:

units for momentum are the same for impulse

p mv!ur r

2

m mkg kg s N s

s s

y ! ! y

g g

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Impulse momentum theorem

Impulse = change in momentum

Equivalent or identical?

equivalence means that if you calculate the

impulse you know what the change in

momentum is equal to equivalence means that if you know the mass

and velocity change of the object then you know

the value of the impulse that changed its velocity

0 0f f F t m v mv mv p p py ( ! ( ! ! ! (r r r r r r r

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Airbag-watermelon drop-balloon toss2 identical 1 kg watermelons are dropped from rest from a 3 meter diving board

concrete vf= 0

3 m

conservation of

energy to solve for

velocity just before

collision

211 9.8 3 12

7.7 /

PE KE

v

v m s

!

!

!

g g g g

3 m

water vf= 0

(p for both cases is the same = 0 7.7 kgm/s = - 7.7 kgm/s

Impulses for both water and concrete are both 7.7 kgm/s

Collision time for concrete (t = .001 s Force = - 7700 N (up)

Collision time for water(t = 1 s Force = - 7.7 N

TIME IS THE DETERMINING FACTOR, NOT THE FORCE

+

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Application ofImpulse Momentum Theorem

Airbag principle water balloon toss

2 extremes in changing momentum from large value to

zero

Very high force acting over a very short time

Lower force acting over a longer time

Time is what determines the force NOT force which

determines the time

1000 0.01 10

10 1 10

F t p

N s N s

N s N s

7 y ( ! (

!

!

r r

g g

g g

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Force plate ball drop example


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Conservation ofLinear Momentum

During collision each ball exerts an equal force on the other in the

opposite direction according to Newtons3rd L

aw of MotionTherefore since the time interval is the same for both forces it can be

said that the impulses of A on B and B on A are equal and opposite in

direction

Impulse from A changes Bs momentum; Impulse from B changes As

momentum 12


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Total Linear Momentum stays constant

A B

B A

Bf Bi Af Ai Ai Af

Bf Af Ai Bi

B Bf A Af B Bi A Ai

F t F t

p p

p p p p p p

p p p p

m v m v m v m v

( ! (

( ! (

! !

!

!

r rgr r Impulses are equal and opposite

change in momentums are equal and opposite

Total momentum after the collision equals the total momentum beforethe collision.

Assumes that system is closed and isolated

Isolated = no external forces

Closed = no mass or particles being added or removed from system 13


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Conservation of Momentum Examples

When two particles stick together after the collision thereis only one mass (total of m1 + m2) and only one final

velocity vf

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Momentum is a vector

Total momentum before

collision = 0

Must have + and signs on the

final velocities for total

momentum after the collision =

push to be equal to 0

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7-4 Conservation of Energy and Momentum

in Collisions

Momentum is conserved

in all collisions.

Collisions in which

kinetic energy is

conserved as well are

called elastic collisions,and those in which it is

not are called inelastic.


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Elastic, Inelastic Collisions

Momentum is always conserved for alltypes of collisions, in a closed, isolated

system

However energy has the ability to do work

in the form of deformation of one of the

objects in the collision or to generate heat

Collisions are classified by amount of

kinetic energy that is used up doing workduring collision

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Completely Elastic total KE before = total KE after

Inelastic total KE before > total KE after

some KE used up to compressand expand the ball during

collision Completely Inelastic

objects stick together aftercollision

may or may not be moving

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7-5 Elastic Collisions in One Dimension

Here we have two objects

colliding elastically. We

know the masses and the

initial speeds.

Since both momentumand kinetic energy are

conserved, we can write

two equations. This

allows us to solve for thetwo unknown final

speeds.


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Ballistic Pendulum

They may be a bullet shot

into a block which rises or a

ball which swings down and

strikes a block which then

moves to the right.

Use energy conservation to

solve for speed height

change relationship

Use momentumconservation to solve for

velocities before/after

collision

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Ballistic Pendulum

Completely inelastic collision between

bullet and block

KE of bullet > KE of bullet+block some of bullets KE used to deform block

cannot equate final PE of pendulum with

initial KE of bullet

cannot equate initial and final KEs

momentum is conserved for the collision

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2 dimensional collisions

total initial momentum in x = total final momentum in x

total initial momentum in y = total final momentum in y


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A two dimensional collision

U

J

i f!

r r

xi xf!

1v

1'v

2'v

1 1 1 1 2 2' cos ' cosm v m v m vU!

yi yfP P!

1 1 2 20 ' sin ' sinm v m vU J! 24

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AP Physics B Ch 7 Review PPT