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How do mechanical devices change the type, direction and magnitude of movement and force?

The functions of mechanical devices including the four types of motion, the three classes of lever with mechanical advantage and velocity ratio calculations, linkages, cams, followers and cranks and sliders.

A focused answer to Edexcel GCSE Design and Technology 1.5 on mechanical devices, covering the four types of motion, the three classes of lever with mechanical advantage and velocity ratio calculations, linkages, cams, followers and cranks and sliders.

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  1. What this dot point is asking
  2. The four types of motion
  3. Levers and mechanical advantage
  4. Linkages, cams, followers and cranks

What this dot point is asking

This is Edexcel key idea 1.5, mechanical devices. Edexcel wants the performance, principles, applications and design influence of devices that produce movement and change the magnitude and direction of forces. This page covers the types of motion (1.5.1), levers and mechanical advantage (1.5.2), linkages (1.5.3), cams (1.5.4), followers (1.5.5) and cranks and sliders (1.5.7). Gears, pulleys and belts (1.5.6, 1.5.8) are on a paired page because they share the ratio calculations. Expect short open-response questions, motion-naming, and calculation questions, which Edexcel guarantees in Section A.

The four types of motion

Mechanisms exist to change one type of motion into another, or to change the size or direction of a force. Knowing the four motions lets you describe what a mechanism does.

Levers and mechanical advantage

A lever can magnify force. Mechanical advantage (MA) measures how much:

MA=loadeffort=effort distance from pivotload distance from pivotMA = \frac{\text{load}}{\text{effort}} = \frac{\text{effort distance from pivot}}{\text{load distance from pivot}}

A crowbar with the effort applied far from the pivot and the load close to it has a large MA, so a small effort lifts a large load. The trade is distance: the effort end moves further than the load end. The related measure is the velocity ratio (VR), the ratio of how far the effort moves to how far the load moves.

Linkages, cams, followers and cranks

  • Linkages (1.5.3): rigid bars joined by pivots that change the direction or type of motion. A reverse motion linkage makes the output move opposite to the input; a bell crank changes the direction of motion through (typically) 90 degrees.
  • Cams (1.5.4): a shaped disc on a rotating shaft. Edexcel names the pear-shaped, eccentric (circular) and drop (snail) cams; the profile sets the pattern of rise, dwell and fall.
  • Followers (1.5.5): rest on the cam and move up and down. Edexcel names roller, knife and flat followers, chosen for the cam profile and wear.
  • Cams and followers convert rotary motion into reciprocating motion.
  • Cranks and sliders (1.5.7): convert rotary motion into reciprocating motion (or the reverse), as in an engine piston or a pillar drill quill.

Exam-style practice questions

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

Edexcel 20224 marksA class 1 lever is used as a crowbar. The effort is applied 60 cm from the pivot and the load is 15 cm from the pivot. Calculate the mechanical advantage and state the effort needed to lift a load of 600 N. (4 marks)
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This is a calculation question (Section A carries 10 marks of calculations). Markers award method and the correct value with units.

Mechanical advantage uses the distances from the pivot: MA=effort distanceload distance=6015=4MA = \dfrac{\text{effort distance}}{\text{load distance}} = \dfrac{60}{15} = 4 (2 marks: formula and value).

The effort is the load divided by the mechanical advantage: effort=loadMA=6004=150 N\text{effort} = \dfrac{\text{load}}{MA} = \dfrac{600}{4} = 150\ \text{N} (2 marks: method and value with units).

Markers reward the correct use of distances for MA and the rearrangement for effort. A common error is multiplying instead of dividing, or forgetting the unit (N).

Edexcel 20213 marksExplain how a cam and follower converts the rotary motion of a motor into a different type of motion, naming the motion produced. (3 marks)
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A 3-mark Explain rewards the mechanism described and the correct motion named.

A cam is a specially shaped disc fixed to a rotating shaft. As the motor turns the cam (rotary motion), the changing radius (rise and fall) of the cam pushes a follower resting against its edge up and down (1). This converts the rotary input into reciprocating (straight-line, back-and-forth) output motion (1). The shape of the cam (pear, eccentric or snail) sets the pattern of the rise, dwell and fall of the follower (1).

Markers reward (1) the cam turning and pushing the follower, (2) naming the output as reciprocating motion, (3) linking the cam profile to the movement pattern. Naming the wrong output motion (for example oscillating) loses the central mark.

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