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Based on the
calculations carried out above, the theoretical net force acting on the plate
is 3.18N. The interpolate the maximum net force acting on the plate by
experiment, a trend line has been added the graph (see figure 2) and the
maximum force is when the trend line intercepts the y-axis which is at – 2.9N.
Hence the proposed result is quite close to the theoretical result with an
error of 9%. This is acceptable as experimental inaccuracies are always going
to occur. Possible explanations are that the apparatus such as the manometer
was not calibrated properly resulting in systematic error, the spring gauge
having a worn spring resulting in random
error. This apparatus error with the possibility of the plate not being
completely perpendicular to the jet contribute to the error. 

Initially,
the graph shows that the net force acting on a plate is roughly constant from
the height of 0.7 m to 0.1 m. This implies that the relatively larger
differences in length between the nozzle and the plate do not make a difference
to the net force acting on the plate. It also means that based on Newton’s 3rd
Law which states that every force has an equal and opposite reaction that all
the upwards acting force is a result of the air jet coming from the nozzle. The
air during this has lower velocity but as the pressure of the jet is very high,
all the surrounding air is sucked in which in turn increases the mass of the airflow which increases the momentum which
increases the force.

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From 0.1 m and below
the net force decreases sharply to the point where 0.05 m below, the net force
is negative which appears to be acting downwards. The reason for the results
and occurring phenomenon is that the air jet is hitting the plate at such high
speeds and escaping rapidly, it creates an area of low pressure below the plate,
almost a vacuum. The atmospheric air above the plate become an area of
relatively high pressure. The change from high pressure to low pressure causes
the plate to be sucked downwards rather than being pushed upwards by the air
jet.For figure
3, note that the pressure is given as manometer water level and not in Pascals.
As the relationship between manometer water level and pressure is positively
linear, converting it to Pascals would result in the same shape of the graph
and hence increase the chance of error. Figure 3 shows that as the height of
the pitot tube decreases, the pressure increases rapidly. The reason for this
is that the jet boundary increases with height. This means that the area is
increased and as the force is applied over a larger area, the pressure
subsequently decreases as . Radially, as
the distance from the centre increases, the pressure greatly decreases. This is
due to the pressure at the centre is the highest and as the radial distance
increases, the pitot tube experiences a force closer to the boundary layer
which results in a lower pressure due to the increased area. At a height of 50
cm, the pressure values are quite similar for all the values from -8 cm to 8
cm. This is because the jet is spread out with evenness at this height. At a
height of 10 cm, the pressure difference from -1 cm to 1 cm is very high
reaching zero pressure at radial distances of -3 cm and 3 cm. This backs up the
theory from experiment 1 where it was stated that as the distance between the
plate and the nozzle decreased, the pressure increased. Figure 4 shows the jet
boundary layer approximation where a trend line has been added to show that as
the height increases, the boundary also increases. The boundary layer is
essentially where the pressure drops to zero. This can be explained using
Newton’s second law as when the height increases, the velocity of the jet
essentially reduces. This means that the momentum of the air jet decreases, the
force also decreases as the force is proportional to the rate of change of
momentum. This is backed up by the decreased readings in pressure as the height
increases as pressure is directly in proportional to force. 

Conclusion

The two experiments helped
achieve a better understanding of the flow of an air jet and the properties it
has. Experiment 1 concluded that when a flat surface gets to a very close
distance to a nozzle, a phenomenon occurs where the flat surface gets sucked
towards the nozzle rather than away as expected. It was concluded from
experiment 2 that the fluid velocity decreases as the distance between the
nozzle and the respective object increases, something that was not obvious
based solely on the results and discussion of experiment 1. Experiment 2 also
helped approximate the boundary layer of an air jet. These properties were
related to the fundamental theory that is Newton’s second law.

The experiment was
carried out in a well-fashioned manner
with readings being taken multiple times and averaged for experiment 1 to rule
out anomalies. However, as most readings were taken on analogue scales, the
chance of parallax error is higher but as mentioned in the results and
discussion, 9 % error is quite acceptable for the experiment. The experiment
put the practical skills to the test and demanded the use of theoretical subject knowledge. The experiment was carried
out safely with effective communication and coordination between team members
to obtain the results.  

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