Linear Momentum
description
Transcript of Linear Momentum
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Linear Momentum
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What we will cover.
Momentum and Its Relation to Force
Collisions and Impulse
Conservation of Momentum
Conservation of Energy and Momentum in Collisions
Elastic & Inelastic Collisions in One Dimension
Mr. Baldwin PHYSICS
IMPULSE & MOMENTUM 31-Oct-13
Aim: Why is a follow through so important?
Do Now: If a toddler and a sumo wrestler are both running
toward you, which one would be easier to stop? Why?
Homework: ESSAY is Due tomorrow
Let’s start with everyday language.
What do you say when a sports team is on a roll?
They may not have the lead, but they sure may have ___________
MOMENTUM
A team that has momentum is hard to stop.
An object with a lot of momentum is also hard to stop
Momentum is defined as the product between an object’s mass and its velocity. It is a vector & symbolized by the symbol p.
CHECK: Can you derive the units for momentum?
The unit is: kg∙m/s
The rate of change of momentum is equal to the net force:
This is nothing but Newton’s second law.5
Momentum and Its Relation to Force
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Let’s practice
A 1200 kg car drives West at 25 m/s for 3 hours. What is the car’s momentum?•Identify the variables:
– m = 1200 kg – v = 25m/s, West– t = 3 hrs
p = mv = 1200 kg x 25 m/s= 30,000 kg∙m/s, West
How hard is it to stop a moving object?To stop an object, we have to apply a force over a period of time.
This is called ImpulseThe definition of impulse J: the change in momentum of an object OR force times time of contact.
CHECK: What is the unit of Impulse?The unit is: N∙s
tpF
ptF JtFp
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Collisions and Impulse
During a collision, objects are deformed due to the large forces involved.
Why does an egg break or not break?
• An egg dropped on a tile floor breaks, but an egg dropped on a pillow does not. Why?
In both cases, m and Δv are the same, so Δp is the same. If Δt goes up, what happens to F, the force?
Right! Force goes down. When dropped on a pillow, the egg starts to slow down as soon as it touches it. A pillow increases the time the egg takes to stops.
CHECK: Why are dashboards padded?
ptF
Practice ProblemA 57 gram tennis ball falls on a tile floor. The ball changes velocity from
-1.2 m/s to +1.2 m/s in 0.02 s. What is the average force on the ball?
Identify the variables:
m = 57 g = 0.057 kg
Δv = +1.2m/s – (-1.2m/s) = 2.4 m/s
t = 0.02 s
using FΔt= mΔv F x (0.02 s) = (0.057 kg)(2.4 m/s)
F= 6.8 N
Sample Regents Question
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Mr. Baldwin PHYSICS
IMPULSE & MOMENTUM 1-Nov-13
Aim: What happens when objects collide?
Do Now: What would happen if a 2 tonne truck rear ends a 1
tonne pick up? How would the wreckage move afterwards?
Homework: DO the first five problems on Momentum and
Impulse worksheets.
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Conservation of MomentumDuring a collision, measurements show that the total momentum does not change. The total momentum in any collision is conserved.
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Conservation of Momentum
More formally, the law of conservation of momentum states:
The total momentum of an isolated system of objects remains constant.
Mr. Baldwin PHYSICS
IMPULSE & MOMENTUM 4-Nov-13
Aim: How does a rocket (space shuttle) take off?
Do Now: QUIZ
Suppose you are an astronaut that became stranded in space when your tether line breaks. You begin to float away from your space station with only a wrench in hand. Explain how would you save your life? How would you return to the space station? (Use the terminology you have learned.)
Homework: Design an Egg Drop Experiment
PHY 205 BALDWIN 17
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Conservation of Momentum
Momentum conservation works for a rocket. WHY?
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Conservation of Momentum
Momentum conservation works for a rocket. WHY?
What about Newton’s 3rd law? Does it work here?
What are the equations associated with momentum and impulse?
PHY 205 BALDWIN 20
t
pF
JtFpvmp
A 0.2 kg baseball is travelling at 40 m/s. After being hit by a bat, the ball's velocity is 50 m/s in the opposite direction. Find a) the change in momentumb) the impulse , Jc) the average force, F, exerted by the bat if the ball and bat are in contact for 0.002 s.d) In which direction is the force acting?
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18 /
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0.2 40 / 50 /
0.2 50 / 0.2 40 /
10 / 8 /
18 /0.002
0.000 0
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i f
f i
m kg v m s v m s
J p mv mv kg m s kg
kg m s
m s
J kg m s kg m s
p kg m st s F
tN
s
A 1000 kg car travelling 22 m/s (about 50 mi/hr) hits a concrete bridge support and comes to a stop in .5 s. a) Find the average force acting on the car b) if the bridge support had been cushioned so that the stopping time was increased to 3 s, what would have been the average force.c) In which direction is the force acting and why?
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1000 22 / 0 / 0.5
1000 0 / 1000 22 /
22000 /
22000 /
440000.5
i f
f i
m kg v m s v m s t s
p mv mv kg m s kg m s
p
p kg m sF
t s
kg m s
N
CHECK…YOUR TURN!!!
Mr. Baldwin PHYSICS
Conservation of Momentum 7-Nov-13
Aim: How is momentum conserved in elastic and inelastic
collisions?
Do Now: QUIZ (10 minutes)
A 2000 kg car is travelling at 20 m/s. The driver hits the gas and increases his speed to 50 m/s. Find a) the change in momentum of the carb) the impulse, J, imparted to the carc) the average force, F, exerted by the engine if his foot was on the gas for 5.0 sd) In which direction is the force acting? Can you explain?
Homework: Design an Egg Drop Experiment
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What can you tell about your impulse when you jump off from a high place? Why do you bend your
knees?The impulse tells us that we can achieve the SAME change in momentum with a large force acting for a short time OR a small force acting for a longer time.
This is why you should bend your knees when you land after jumping; it increases time; it reduces force.
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Why do airbags work?
Why does landing on a pillow hurts less than landing on concrete.
What can you tell about your impulse when you impact your
airbags?
How is this similar to when you bend your knees after landing from
a jump?
IT’S ALL ABOUT THE FORCE!
Impulse/Impact
Decreasing momentum in a short time
Car hitting concrete wall-- great damage by quick change
Karate master-- great momentum of hand causes breaking of bricks
Impulse
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Can you tell me what a collision is?
Collisions are the interactions of two or more objects.
Momentum is conserved in ALL collisions.
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Elastic Collisions
Collisions in which kinetic energy is conserved.
Momentum is perfectly transferred from one object to the next.
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Inelastic Collisions
With inelastic collisions, some of the initial kinetic energy is lost to thermal, sound or potential energy.
A completely inelastic collision is one where the objects stick together afterwards, so there is only one final velocity.
CHECK: CAN YOU GIVE ME SOME EXAMPLES OF ELASTIC & INELASTIC COLLISIONS?
OK…How would you classify these types?• Two lumps of clay colliding• Rubber ball on hard surface• Two billiard balls colliding• Two-Car collision• Two bumper cars collision• Wad of gum tossed onto a rotating cd• Meteor impacting the Earth
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You are to design and build a device that will allow a chicken egg to survive the impact of dropping from various heights. In the process you will make a hypothesis about what you think will happen to the egg by explaining its inertia, motion, gravity, momentum and impulse.
BE CREATIVE!!!32
Design an Experiment & Build a Device
http://weirdsciencekids.com/EggDropExperiment.html