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How do levers and linkages multiply force and change motion?

The three classes of lever, the principle of moments, mechanical advantage, and linkages (reverse-motion, bell-crank, parallel).

A CCEA A-Level Technology and Design answer on the three classes of lever, the principle of moments, calculating mechanical advantage and velocity ratio, and common linkages such as reverse-motion, bell-crank and parallel linkages.

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  1. What this dot point is asking
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What this dot point is asking

CCEA expects you to know the three classes of lever, apply the principle of moments, calculate mechanical advantage and velocity ratio, and recognise common linkages (reverse-motion, bell-crank, parallel). Moments and mechanical-advantage calculations are a regular feature.

The answer

The three classes of lever

The principle of moments

Mechanical advantage and velocity ratio

Worked example: a wheelbarrow as a second-class lever

Examples in context

Example 1. Scissors and bolt cutters. Both are first-class levers; long handles and short blades give a large mechanical advantage, which is why bolt cutters have such long arms, more effort distance for more cutting force.

Example 2. Digger arm. Hydraulic rams act through bell-crank and parallel linkages so the bucket moves and stays oriented correctly, a real machine built from the linkage types listed above.

Try this

Q1. State the principle of moments. [2 marks]

  • Cue. For equilibrium, the sum of clockwise moments about a pivot equals the sum of anticlockwise moments (effort times effort arm = load times load arm).

Q2. A lever lifts a 500 N load with an effort of 125 N. Find the mechanical advantage. [2 marks]

  • Cue. MA=500/125=4\text{MA} = 500/125 = 4.

Q3. Which class of lever is a wheelbarrow, and which element is in the middle? [2 marks]

  • Cue. Second class; the load is in the middle (between the fulcrum at the wheel and the effort at the handles).

Exam-style practice questions

Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

CCEA 20186 marksA crowbar is used as a lever to lift a 600 N load. The load is 0.10 m from the pivot and the effort is applied 0.60 m from the pivot. Using the principle of moments, calculate the effort needed and the mechanical advantage.
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The principle of moments states that for equilibrium the clockwise moment equals the anticlockwise moment about the pivot:

effort×effort arm=load×load arm.\text{effort} \times \text{effort arm} = \text{load} \times \text{load arm}.

So:

E×0.60=600×0.10=60 N m.E \times 0.60 = 600 \times 0.10 = 60\ \text{N m}.

E=600.60=100 N.E = \frac{60}{0.60} = 100\ \text{N}.

The mechanical advantage (MA) is the load divided by the effort:

MA=loadeffort=600100=6.\text{MA} = \frac{\text{load}}{\text{effort}} = \frac{600}{100} = 6.

So an effort of 100 N lifts the 600 N load, a mechanical advantage of 6 (the lever multiplies the force six times).

Markers reward the moments equation, the correct effort of 100 N, and MA = load/effort = 6.

CCEA 20214 marksState the order of the effort, load and fulcrum (pivot) for a first-class and a second-class lever, and give an example of each.
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The class of lever depends on what lies in the middle:

  • First-class lever: the fulcrum (pivot) is in the middle, between the effort and the load. Example: a see-saw, scissors, or a crowbar/pliers.
  • Second-class lever: the load is in the middle, between the fulcrum and the effort. Example: a wheelbarrow, a nutcracker, or a bottle opener.
  • (Third-class, for completeness: the effort is in the middle; example tweezers or a fishing rod, with MA less than 1.)

Markers want the correct middle element for each class (fulcrum in the middle = first; load in the middle = second) and a valid example for each.

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