Lesson 11 Momentum

7/29/2019 Lesson 11 Momentum

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Do now!

Can youcontinue thesheet youstarted

yesterday on

stoppingdistances?


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Last lesson

Stopping distances


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Stopping distances

The distance a car takes to stop iscalled the stopping distance.


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Two parts

The stopping distance can bethought of in two parts


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Stopping distances

Thinking distance is the distance traveled whilstthe driver is thinking (related to the driversreaction time).


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Thinking distance

This isaffected by themental stateof the driver(and thespeed of the

car)


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Braking distance

This is the distance traveled by thecar once the brakes have beenapplied.


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Braking distance

This affected by the speed andmass of the car


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Braking distance

It is also affected by the roadconditions


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Braking distance

And by the condition of the carstyres.


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Todays lesson

Momentum

YouTube -Spectacular100mph Train

Crash Test
http://www.youtube.com/watch?v=zJflu7z4QyI&feature=PlayList&p=A93E7141D869A85C&index=4http://www.youtube.com/watch?v=zJflu7z4QyI&feature=PlayList&p=A93E7141D869A85C&index=4http://www.youtube.com/watch?v=zJflu7z4QyI&feature=PlayList&p=A93E7141D869A85C&index=4http://www.youtube.com/watch?v=zJflu7z4QyI&feature=PlayList&p=A93E7141D869A85C&index=4http://www.youtube.com/watch?v=zJflu7z4QyI&feature=PlayList&p=A93E7141D869A85C&index=4http://www.youtube.com/watch?v=zJflu7z4QyI&feature=PlayList&p=A93E7141D869A85C&index=4http://www.youtube.com/watch?v=zJflu7z4QyI&feature=PlayList&p=A93E7141D869A85C&index=4http://www.youtube.com/watch?v=zJflu7z4QyI&feature=PlayList&p=A93E7141D869A85C&index=4http://www.youtube.com/watch?v=zJflu7z4QyI&feature=PlayList&p=A93E7141D869A85C&index=4http://www.youtube.com/watch?v=zJflu7z4QyI&feature=PlayList&p=A93E7141D869A85C&index=4


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Momentum

What makes an object hard to stop?

Is it harder to stop a bullet, or a trucktravelling along the highway?

Are they both as difficult to stop as

each other?


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Momentum

The bullet is hard to stop because itis travelling very fast, whereas thetruck is hard to stop because it has a

very large mass.


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Momentum

It makes sense to assume that a bullettravelling twice as fast would be twiceas hard to stop, and a truck twice the

mass would also be twice as hard tostop.


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Momentum

Momentum is a useful quantity toconsider when thinking about"unstoppability". It is also useful

when considering collisions andexplosions. It is defined as

Momentum (kg.m.s-1) = Mass (kg) x Velocity (m.s-1)

p = mv


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An easy example

A lorry has a mass of 10 000 kg and avelocity of 3 m.s-1. What is itsmomentum?

Momentum = Mass x velocity

= 10 000 x 3

= 30 000 kg.m.s

-1

.


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Conservation of momentum

In a collision between two objects,momentum is conserved (totalmomentum stays the same). i.e.

Total momentum before the collision = Total momentum after

Momentum is not energy!


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Law of conservation of momentum

The law of conservation oflinear momentum says that

in an isolated system, momentumremains constant.

We can use this to calculate what happens after a collision (andin fact during an explosion).


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A harder example!

A car of mass 1000 kg travelling at5 m.s-1 hits a stationary truck ofmass 2000 kg. After the collisionthey stick together. What is their

joint velocity after the collision?


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A harder example!

5 m.s-11000kg

2000kgBefore

AfterV m.s-1

Combined mass = 3000 kg

Momentum before = 1000x5 + 2000x0 = 5000 kg.m.s-1

Momentum after = 3000v


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A harder example

The law of conservation of momentum tellsus that momentum before equalsmomentum after, so

Momentum before = momentum after

5000 = 3000v

V = 5000/3000 = 1.67 m.s-1


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Momentum is a vector

Momentum is a vector, so ifvelocities are in opposite directionswe must take this into account inour calculations


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An even harder example!

Snoopy (mass 10kg) runningat 4.5 m.s-1 jumps onto askateboard of mass 4 kg

travelling in the oppositedirection at 7 m.s-1. What isthe velocity of Snoopy andskateboard after Snoopy has

jumped on?

I lovephysics


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10kg

4kg-4.5 m.s-17 m.s-1

Because they are in opposite directions, wemake one velocity negative

14kg

v m.s-1

Momentum before = 10 x -4.5 + 4 x 7 = -45 + 28 = -17

Momentum after = 14v


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Momentum before = Momentum after

-17 = 14v

V = -17/14 = -1.21 m.s-1

The negative sign tells us that the

velocity is from left to right (we choosethis as our negative direction)


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Thats it!

Now lets try some questions!

Lesson 11 Momentum

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