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Do now!

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Title: Momentum Author: ocuser14 Last modified by: sporter Created Date: 10/12/2006 8:11:49 AM Document presentation format: On-screen Show (4:3) Company – PowerPoint PPT presentation

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Title: Do now!


1
Do now!
Can you talk with your partner about what we
learned last lesson?
2
Last lesson
  • Stopping distances

3
Stopping distances
  • The distance a car takes to stop is called the
    stopping distance.

4
Two parts
  • The stopping distance can be thought of in two
    parts

5
Stopping distances
  • Thinking distance is the distance traveled
    whilst the driver is thinking (related to the
    drivers reaction time).

6
Thinking distance
  • This is affected by the mental state of the
    driver (and the speed of the car)

7
Braking distance
  • This is the distance traveled by the car once
    the brakes have been applied.

8
Braking distance
  • This affected by the speed and mass of the car

9
Braking distance
  • It is also affected by the road conditions

10
Braking distance
  • And by the condition of the cars tyres.

11
Todays lesson
  • Momentum

YouTube - Spectacular 100mph Train Crash Test
12
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?

13
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.

14
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.

15
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 (kgm/s) Mass (kg) x Velocity (m/s)
  • p mv

16
An easy example
  • A lorry has a mass of 10 000 kg and a velocity
    of 3 m/s. What is its momentum?
  • Momentum Mass x velocity
  • 10 000 x 3
  • 30 000 kgm/s

17
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).

18
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!
19
A harder example!
  • A car of mass 1000 kg travelling at 5 m/s hits a
    stationary truck of mass 2000 kg. After the
    collision they stick together. What is their
    joint velocity after the collision?

20
A harder example!
Before
2000kg

1000kg
5 m/s
Momentum before 1000x5 2000x0 5000 kgm/s
Combined mass 3000 kg
After
V m/s
Momentum after 3000v
21
A harder example
  • The law of conservation of momentum tells us that
    momentum before equals momentum after, so
  • Momentum before momentum after
  • 5000 3000v
  • V 5000/3000 1.67 m/s

22
Momentum is a vector
  • Momentum is a vector, so if velocities are in
    opposite directions we must take this into
    account in our calculations

23
An even harder example!
  • Snoopy (mass 10kg) running at 4.5 m/s jumps onto
    a skateboard of mass 4 kg travelling in the
    opposite direction at 7 m/s. What is the velocity
    of Snoopy and skateboard after Snoopy has jumped
    on?

I love physics
24
An even harder example!
Because they are in opposite directions, we make
one velocity negative
10kg

4kg
-4.5 m/s
7 m/s
Momentum before 10 x -4.5 4 x 7 -45 28
-17
14kg
v m/s
Momentum after 14v
25
An even harder example!
  • Momentum before Momentum after
  • -17 14v
  • V -17/14 -1.21 m/s

The negative sign tells us that the velocity is
from left to right (we choose this as our
negative direction)
26
Thats it!
  • Now lets try some questions!
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