Machines — Part 2: Pulleys: visual lesson, ICSE Class 10 Physics (Science Paper 1)

Lesson

Wheels with a groove

Part 2 — a pulley is a grooved wheel turning on an axle, with a string or rope round it. Arranged in blocks, pulleys lift engines and building materials.

In this lesson you will

State MA, VR and the use of a single fixed pulley
Find MA, VR and efficiency of a single movable pulley
Calculate MA, VR and efficiency of a block and tackle, including the weight of the movable block
Explain why a single fixed pulley is used though its MA is 1

Table of three pulley systems and where each is used
Pulley system	Where it is used
Single fixed pulley	Drawing water from a well, raising a flag
Single movable pulley	Lifting loads on a building site
Block and tackle	Cranes, garage engine hoists, ships

Diagram

Single fixed and single movable pulleys

Single fixed pulley: the axle is fixed; effort and load are on the two ends of one string. Ideal MA = 1 and VR = 1 (effort moves as far as the load). In practice MA < 1 because of friction. Use: it changes the direction of the effort — pulling down is easier and safer than lifting up, and you can use your own weight. Single movable pulley: the pulley moves with the load, hanging in a loop of string fixed at one end. Two strands support the load, so VR = 2 and ideal MA = 2.

Worked example

Worked example: a movable pulley with a heavy block

Problem. A single movable pulley of weight 20 N is used to lift a load of 180 N. Ignoring friction, find the effort, MA, VR and efficiency.

Effort: two strands support load + pulley: $2E = 180 + 20$ , so $E = 100\ \text{N}$ .

MA $= 180 \div 100 = 1.8$ ; VR = 2.

Efficiency $= 1.8 \div 2 = 0.9$ , that is 90%: the pulley’s own weight wastes some of the input.

Table of results for the movable pulley example
Quantity	Value
Effort	100 N
MA	1.8
VR	2
Efficiency	90%

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