6.9Chapter 6: Forces, Motion and Pressure

Tension and upthrust — Alhussan Physics - Stage 8 Revision Notes & Topical Questions

5 min read 7 parts Lesson 9 of 19 1 quiz

📚 Alhussan Physics - Stage 8 Hub / Topical Revision

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Lesson summary

Physics 6.9 Tension and upthrust — Elastic Extension and Floating — Alhussan International Academy — Physics (Stages 7–9) notes: Describe stretching of springs, elastic limit, and explain floating using upthrust.Stretching — Ropes, cords and springs all stretch when you pull them…

Bungee cord
Hooke's law
Newtonmeter
Apparent weight
Archimedes' principle
Tension = the force in a rope or spring that is being stretched.It balances the climber's weight

Part 1 of 5Tension

Stretching

Ropes, cords and springs all stretch when you pull them.
Climbing ropes are designed to stretch a certain amount in case the climber falls, so the climber is not injured.

Tension

Tension the force in a rope or spring that is being stretched.

It balances the climber's weight.
As you pull, the particles inside move apart and the force of attraction pulls them back together.

Part 2 of 5Elastic and plastic materials

Elastic

An elastic material returns to its original length when the force is removed.

Plastic

A material that does not go back to its original length after stretching.

Extension

Extension new length − original length.

Bungee cord

It is made of many elastic cords bound together so that it stretches a very long way.

Part 3 of 5Stretching a spring

Uses

Springs are used in spring balances, trampolines and car suspension, so the right spring must be chosen for each job.

Hooke's law

Within a limit the extension of a spring is proportional to the force applied.
A bigger force produces a bigger extension.
Doubling the force doubles the extension.

Newtonmeter

It contains a spring and can measure forces because the extension is proportional to the force.

Elastic limit

Beyond this load the spring does not return to its original length when the force is removed.
The spring is then permanently extended.
Elastic materials break if the force is too big.

Part 4 of 5Upthrust and floating

Upthrust

Upthrust the upward force a fluid exerts on an object in it.

Cause

Water particles collide with the bottom of the boat and push it up.
A large area produces a big force that can balance a big weight.

Floating

An object floats when the upthrust equals its weight, so the forces are balanced.

Sinking

If the weight is greater than the upthrust the object sinks.

Part 5 of 5Apparent weight and displaced water

Apparent weight

An object weighed underwater seems to weigh less because of upthrust.
It is easier to pull yourself out of a swimming pool than to pull yourself up when not in water.

Archimedes' principle

The upthrust equals the weight of the water displaced.
A weight of 10 N with an upthrust of 4 N has an apparent weight of 6 N.

Situation	Weight compared with upthrust	Result
Floating	Equal	Stays at the surface
Sinking	Weight greater	Goes down
Rising	Upthrust greater	Goes up

Key terms

Term	Meaning
Tension	The force in a rope or spring that is being stretched.
Extension	The increase in length of a stretched object.
Elastic limit	The point beyond which an object does not return to its original length.
Hooke's law	Extension is proportional to the force, up to the elastic limit.
Upthrust	The upward force of a fluid on an object in it.
Archimedes' principle	Upthrust equals the weight of the fluid displaced.