do now! can you continue the sheet you started yesterday on stopping distances?
TRANSCRIPT
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Do now!
Can you continue the
sheet you started
yesterday on stopping
distances?
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Last lesson
Stopping distances
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Stopping distances
The distance a car takes to stop is called the stopping distance.
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Two parts
The stopping distance can be thought of in two parts
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Stopping distancesThinking distance is the distance traveled whilst the driver is thinking (related to the driver’s reaction time).
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Thinking distance
This is affected by the mental state of the driver (and the speed of the car)
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Braking distance
This is the distance traveled by the car once the brakes have been applied.
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Braking distance
This affected by the speed and mass of the car
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Braking distance
It is also affected by the road conditions
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Braking distance
And by the condition of the car’s tyres.
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Today’s lesson
Momentum
YouTube - Spectacular 100mph Train Crash Test
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Momentum
What makes an object hard to stop? Is it harder to stop a bullet, or a truck
travelling 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 it is travelling very fast, whereas the truck is hard to stop because it has a very large mass.
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Momentum
It makes sense to assume that a bullet travelling twice as fast would be twice as hard to stop, and a truck twice the mass would also be twice as hard to stop.
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Momentum
Momentum is a useful quantity to consider when thinking about "unstoppability". It is also useful when considering collisions and explosions. 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 a velocity of 3 m.s-1. What is its momentum?
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 (total momentum 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 of linear momentum says that
“in an isolated system, momentum remains constant”.
We can use this to calculate what happens after a collision (and in fact during an “explosion”).
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A harder example!
A car of mass 1000 kg travelling at 5 m.s-1 hits a stationary truck of mass 2000 kg. After the collision they stick together. What is their joint velocity after the collision?
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A harder example!
5 m.s-1
1000kg
2000kgBefore
After
V 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 tells us that momentum before equals momentum after, so
Momentum before = momentum after5000 = 3000v
V = 5000/3000 = 1.67 m.s-1
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Momentum is a vector
Momentum is a vector, so if velocities are in opposite directions we must take this into account in our calculations
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An even harder example!
Snoopy (mass 10kg) running at 4.5 m.s-1 jumps onto a skateboard of mass 4 kg travelling in the opposite direction at 7 m.s-1. What is the velocity of Snoopy and skateboard after Snoopy has jumped on?
I love physics
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An even harder example!
10kg
4kg-4.5 m.s-1
7 m.s-1
Because they are in opposite directions, we make 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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An even harder example!
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 choose this as our “negative direction”)
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That’s it!
Now let’s try some questions!