What it shows
Several rope sections supporting a load share its weight. The mechanical advantage equals the number of supporting rope sections, so you pull a longer length with less force.
The science
In a block and tackle, the same rope passes round several pulleys and each supporting section carries part of the load. With four sections, the pull needed is about a quarter of the weight, but you must pull four times the length. The work done is the same in the ideal case, so a machine only trades force for distance. Friction means the real effort is a little larger.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Four empty thread spools or round discs
- Strong string, about 4 metres
- Two ice-cream stick frames or cardboard holders
- A hook and a half-brick or a bag of sand
- A spring balance or a rubber band
- A measuring tape
Safety
For this project: Keep feet clear of the load. An adult must supervise lifting heavier loads and cut the frame pieces.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Make two blocks, each with two spools on a skewer fixed in a frame.
Hang one block from a support and attach the load to the other.
Tie the string to the top block, then thread it over the spools in turn.
Pull on the free end and raise the load.
Measure the force needed with the rubber band or balance.
Mark how far the load rises and how far you pulled the rope.
Calculate the velocity ratio and the mechanical advantage.
Repeat with fewer pulleys and compare the efficiency.
What you should see
The pull needed is about a third to a quarter of the load, and the rope pulled is four times the height lifted.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is mechanical advantage?
The load divided by the effort.
2What do you give up to gain force?
Distance moved.
3Why is real effort a little greater than ideal?
Friction in the pulleys.
4Where are block and tackles used?
In cranes and ship rigging.
Take it further
Compute efficiency as work out divided by work in for each setup.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
Science projects
Physics Working Models Explained
Twenty classic models, how each works and how to make it
Science projects
Mini and Major Project Guide
For diploma and BTech students: choose, plan, build, document and present