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ENGINEERING, PPT 2024 HSC 3 MC

A simplified image of a bicycle chain drive is shown.
 

If a cyclist is pedalling at 70 revolutions per minute (RPM), what is the RPM of the driven wheel?

  1. 3.2
  2. 21.7
  3. 226.2
  4. 546.0
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\(C\)

Show Worked Solution

\(\text{Velocity Ratio}\ (VR)\ = \dfrac{\text{output teeth}}{\text{input teeth}} = \dfrac{13}{42}=0.3095\)

\(\text{Output speed (RPM)}\ = \dfrac{\text{input speed}}{VR} = \dfrac{70}{0.3095}=226.2\)

\(\Rightarrow C\)

ENGINEERING, PPT 2023 HSC 12 MC

A rail switch lever is shown.
 


 

What is the calculated mechanical advantage when operating this rail switch lever?

  1. 3.7
  2. 6.3
  3. 14.7
  4. 16.0
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\(  D \)

Show Worked Solution
\(MA\) \(=\dfrac{d_{input}}{d_{output}} \)  
  \(= \dfrac{1100+100}{75}\)  
  \(=16.0\)  

 
\(\Rightarrow  D \)

♦ Mean mark 51%.

ENGINEERING, PPT 2017 HSC 21c

The diagram shows a gear train.
 

The mechanical advantage of the gear train is 1.6 : 1 with an efficiency of 80%.

Calculate the number of teeth on the driven gear.   (3 marks)

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`18\ text{teeth}`

Show Worked Solution
`n` `=(MA)/(VR)`  
`VR` `=(MA)/n`  
  `=1.6/0.8`  
  `=2`  

 

`VR` `=text{Driven}/text{Driver}`  
`:.\ text{Driven}` `=VR xx text{Driver}`  
  `=18\ text{teeth}`  

♦ Mean mark 48%.

ENGINEERING, PPT 2017 HSC 21a

Using examples, explain why gears are used in the design of vehicles.   (3 marks)

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→ Speed, rotation direction and torque can all be changed in engineering mechanisms using gears.

→ Different gear ratios with different types of gears can be combined to change direction or increase torque or speed.

→ For example, a truck cannot travel fast but it can carry tonnes of goods, whereas a performance motorcycle is unable to carry any meaningful load but can travel at high speeds.

Show Worked Solution

→ Speed, rotation direction and torque can all be changed in engineering mechanisms using gears.

→ Different gear ratios with different types of gears can be combined to change direction or increase torque or speed.

→ For example, a truck cannot travel fast but it can carry tonnes of goods, whereas a performance motorcycle is unable to carry any meaningful load but can travel at high speeds.

ENGINEERING, PPT 2019 HSC 13 MC

The diagram shows a seesaw being used by two children, one of mass 25 kg and the other of mass 20 kg. It has a corroded pivot mechanism that reduces its efficiency by 25%.
 

What is the mechanical advantage of the seesaw?

  1. 0.25
  2. 0.75
  3. 0.80
  4. 1.00
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`B`

Show Worked Solution

→ If the arms are the same length, then the MA and VR should be 1. 

→ However, the MA is reduced by 25% due to the decrease in efficiency, as losses in efficiency only change the MA, not the VR.

`=>B`


♦ Mean mark 50%.

ENGINEERING, PPT 2022 HSC 21c

To change a tyre on a bus, a system of pulleys, as shown, is used to lift the wheel and tyre to re-fit them back on the bus. The wheel and tyre have a combined mass of 95 kg.
 

What effort is required to lift the wheel and tyre if the efficiency of the pulley system is 80%?   (3 marks)

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`296.875\ text{N}`

Show Worked Solution
`W` `= 95 xx 10 = 950\ text{N}`  
`VR` `= 4\text{ (number of ropes to load)}`  
`eta` `= (MA)/(VR)`  
`MA` `= VRxxeta`  
  `= 4xx0.8`  
  `= 3.2`  
`MA` `= L/E`  
`E` `= L/(MA)`  
  `= 950/3.2`  
  `=296.875\ text{N}`  
Mean mark 56%.