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Look at the ball and the pins
above. Why is it when I bowl, sometimes I strike (all pins down), but
other times only 2/3 pins were knocked down?
When a car ramped into a tree, the
tree collapsed. But, when a lorry which is traveling at the same speed as
the car, knocked the tree, the tree does not collapsed.
Why?
This is because of Momentum.
What is that?
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Momentum is a mass in motion.
All moving objects have momentum.
The faster it moves, the more
momentum it has. The bigger its mass, the more its momentum.
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Momentum = mass x
velocity
=
mv |
For example, comparing momentum of a car and a lorry
traveling at same velocity.
| Vehicle |
mass, m (kg) |
velocity, v (ms-1
) |
momentum (kgms-1) |
| Car |
1000 |
10 |
10000 |
| Lorry |
5000 |
10 |
5000 |
Or comparing momentum of a car speeding from 10 ms-1
to 20 ms-1.
| Car |
mass, m (kg) |
velocity, v (ms-1
) |
momentum (kgms-1) |
| before |
1000 |
10 |
10000 |
| after |
1000 |
20 |
20000 |
Is momentum a scale
quantity or vector
quantity?
Of course, it is a vector quantity
because it has direction. Its direction is the same as the direction
of its velocity.
What is the unit of momentum?
Since momentum = mv, therefore
(m = mass, v = velocity)
unit of momentum = (unit of mass) x
(unit of velocity)
= (kg) x (ms-1 )
= kgms-1
What is the momentum of
trolley P and Q?
| P |
Q |
| mass, m (kg) |
velocity, v (ms-1
) |
momentum (kgms-1) |
mass, m (kg) |
velocity, v (ms-1
) |
momentum (kgms-1) |
| 2 |
-3 |
-6 |
3 |
2 |
6 |
A ticker timer is used to
study the motion of an object for a short period of time in the
Physics laboratory. When the ticker timer is connected to the
alternating current (a.c.), the vibrating steel strip will vibrate
50 times every second and make 50 dots a second on a ticker tape
being pulled by a trolley. The time interval between 2 consecutive
dots (time of 1 tick) is 1/50 or 0.02 s.
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The
diagram on the right shows the ticker tape pulled by a trolley to
the right. So the first tick on the right (initial motion) and the
last tick is on the left (final motion).
 |
| The ticker tape can be analysed
to determine the time, displacement, average velocity, acceleration
and type of motion of the trolley. |
| To determine
the displacement. If 2 ticker tape has the same
number of ticks, the tape that is longer shows that the trolley that
pulls it was moving faster than the trolley that pulls the tape
which is shorter.
If the length of two ticker tape are the same, the one that has
more ticks on it shows a slower motion. When the distance between 2
dots increases, the speed increases.
- If the ticks are near to one another = the trolley is moving
slowly.
- If the ticks are far apart = the trolley is moving fast.
 |
The length of a ticker tape also
represent the speed the trolley that pulls it. 
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Observation on the ticker tape |
Type of motion |
|
the ticks are of uniform distance from one
another |
constant velocity |
|
the ticks are getting further apart |
accelerating |
|
the ticks are getting closer |
decelerating |
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A ticker tape
is also used to determine the speed of
the trolley: |
A ticker tape is can also
be used to determine
the acceleration of the trolley.
Take note that the time taken to change from the initial velocity
to the final velocity is 7 ticks and not 8 ticks. |
| Friction-compensated runway means the
frictional force of the runway is compensated by the gravitational
force of the trolley. A friction-compensated runway allows the
trolley to move with uniform velocity. |
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Let's study
momentum in collision and explosion.
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Explosion