4.Discussions

4. Discussion

4.1 Key findings & Analysis of results


From our results, we can tell that as the length of the ruler piece increase, the force exerted on the scale decreases, and this applies to all motors regardless of their gear ratio. The highest reading we got for the 100:1 HP gear ratio motor is an average of 1.78 N when the distance from the centre of the motor shaft to the wooden wedge is 12.0 cm, and 12.0 cm is the shortest distance we used. The lowest reading we got for the 100:1 HP gear ratio motor is an average of 0.86 N when the distance between the motor shaft to the wooden wedge increases to 24.0 cm, and 24.0 cm is the longest length we wanted to measure. This was the same as our first hypothesis, whereby the longer the distance from the centre of the motor shaft to the red line on the ruler, the force exerted on the scale decreases.

It is because using our key findings, we see that the force applied on the scale has the biggest magnitude when the shortest 12.0 cm ruler is used. As we increased the distances using multiples of 2, the force exerted on the scale drops by a factor of approximately 1.1, which explains why our graph show the curves going downwards. For example, a 12.0 cm ruler piece attached to a 100:1 HP gear ratio motor has an average reading of 1.78 N exerted on the scale whereas a 14.0 cm ruler attached to the same motor has a reading of 1.53 N exerted on the scale. On top of that, this applies to all the motors, regardless of the gear ratio. This can be proven by using a data set from another motor, the 75:1 HP motor. When the 12.0 cm ruler piece is attached to it, an average reading of 1.31 N is exerted on the scale. But, if we change the distance between the centre of the motor shaft to the wooden wedge to 14.0 cm, the average reading exerted on the scale would drop to 1.11 N. Thus, from this we can conclude that the shorter the distance, the greater the force exerted onto the scale.

Also, the higher the gear ratio of the motor, the more torque they have, which in turn means they would exert more force on the scale compared to a motor with a smaller gear ratio when it has the same ruler piece length as the higher gear ratio motor. This matches with our second hypothesis, whereby the larger the gear ratio, the higher the torque.

It is because using our key findings if we were to use one 100:1 HP gear ratio motor and attach a 12.0 cm ruler piece onto it, we would get an average reading of 1.78 N exerted on the scale. However, if we were to use a 75:1 HP gear ratio motor and attach the same ruler piece, we would get an average reading of 1.31 N exerted onto the scale. Because voltage given for every motor is 6 V, and the distance is the same for each motor. It is because in this case, we are comparing both motors using the same distance of 12.0 cm, then it means that the torque is different for both motors, by using the equation to find torque, which is the magnitude of the force multiplied by the length of the beam relative to the pivot.


The special observations we have is that sometimes when we close the switch, our readings may be 10 g - 20 g off from our desired reading. This would mean that we are off by 0.10 N - 0.20 N if the mass is converted into force, which we can find by multiplying the reading shown on the scale with the gravitational acceleration of 9.8 m/s2. This would thus give us a torque value that is not accurate compared to the torque value stated in the online shop where we bought our motors from. Because of this, we have to keep testing in order to get a good reading.





4.2 Explanation of key findings

According to the data recorded on how force and distance are related, we can tell that shorter distances are indeed better at producing more force, as it is because more force was exerted on the scale as the distance decreases. This can be illustrated by the equation of finding torque, which is:

(Magnitude of the force) X (perpendicular length of beam relative to the pivot) = Torque

Because we are using the same voltage throughout, this means that a motor should produce the same torque. Because the distance changes, the force should decrease accordingly. This is backed up by a textbook, Physics by Cutnell, J.D., Johnson, K.W. and Fisher, K.D, whereby they stated that  torque is dependent on the magnitude of the force, the point where the force is applied about the axis of rotation and the direction of the force (Cutnell, Johnson, and Fisher, 2009). Because of this, the force exerted on the scale decreases as the distance from the centre of the motor shaft to the wooden wedge increases.
Regarding the gear ratio, we see that the higher the gear ratio, the higher the torque. It is because the gear ratio multiplies the torque by that amount. There are two pieces of research evidence to support. First, the gears act as torque multipliers (How A Car Works, 2017). If the first gear has a ratio of 3:1, it multiplies the engine's torque output by three when passing it on to the final drive (How A Car Works, 2017). Secondly, if the speed is reduced by the ratio, the torque is increased by the same ratio. (NORD Drivesystems, 2015). This means that if the gear ratio is higher, the torque is increased by a higher amount.


The reason behind our special observation is because is because the batteries do not provide a stable source of current, and this affect the torque of the motor. We used alkaline batteries in our experiment, and according to a book called Physics: A Conceptual World View, alkaline batteries are primarily used in applications where fairly large, continuous currents are needed  (Kirkpatrick and Francis, 2009). Also, for DC motors, the output torque is proportional to the current going into the motor (DC motors, n.d).





4.3 Evaluation of Hypothesis

Our first hypothesis which is, as the distance from the centre of the motor shaft to the wooden wedge, the force exerted on the scale decreases, was proven to be correct. It is because of our key findings, as stated above, has shown that as the distance from the motor shaft to the wooden wedge increases, the force exerted on the scale decreases. This applies to all motors of the different gear ratio. As mentioned above, it is backed up by a textbook whereby they stated that torque is dependent on the magnitude of the force, the point where the force is applied about the axis of rotation and the direction of the force (Cutnell, Johnson, and Fisher, 2009).

Our second hypothesis, which is as the higher the gear ratio, the higher the torque produced by the motor, was proven to be correct. It is because our key findings have shown that motors with higher gear ratio produce higher torque. This in turn means that more force is exerted onto the scale compared to a motor with a smaller gear ratio, when the same distance from the centre of the motor shaft to the wooden wedge is used for both motors. This is backed up by our research evidence, whereby if the speed is reduced by the ratio, the torque is increased by the same ratio. (NORD Drivesystems, 2015).

In addition, according to the online shop we bought our motors from, it shows that the torque for motors with higher gear ratios were higher compared to the torque of a motor with a smaller gear ratio. For example, the 100:1 gear ratio motor has a torque of 0.21 Nm corrected to 2 decimal places. The 75:1 gear ratio motor has a torque of 0.16 Nm corrected to 2 decimal places. Lastly, the 50:1 gear ratio motor has a torque of 0.11 Nm corrected to 2 decimal places.

From the two sources mentioned for the special observations, we can conclude that the unstable current caused the special observations. It is because of the alkaline batteries which failed to provide a constant current to the motor, it thus caused some of the readings to be inaccurate as alkaline batteries are primarily used in applications where fairly large, continuous currents are needed (Kirkpatrick and Francis, 2009).

4.4 Limitations and Areas for improvement

1. We could have used a variable power supply instead of a set of batteries because batteries do not provide a stable voltage and current, which would affect the torque of the motor. However, the variable power supply works in a way that you can set a fixed current and voltage you need for an appliance.

2. We could remove the batteries first before carrying the holder. It is because when we are keeping the equipment, sometimes we would just grab the holder with their wires and forgot that there are batteries in the holder. As a result, when we gripped the holder such that it connected the wires together, it closes the circuit, and would hurt someone's hand due to the heating of the wire when the current is passing through the wire.

3. We could also remember to open the switch after we are done. It is because when the motor is left stalled, it starts to heat up, and because it is on top of the wooden structure, the wooden structure may catch on fire due to the heat and wood is flammable. On top of that, even though this does not happen, someone's hands may be injured once touching the motor body and the motor would also spoil. This happened once when one of our teammates forgot to open the switch at home. Although he suffered a minor burn, the motor was spoilt and a new one must be bought.

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