GRADE 8 SCIENCE · MAKE THE MECHANISM VISIBLE

Efficiency, friction & energy audits

Compare useful output with total input and account for the difference.

Class notes

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Define useful before calculating

Efficiency = useful output energy ÷ input energy × 100%. A motor that receives 500 J and delivers 350 J of useful mechanical work is 70% efficient. Its remaining 150 J is transferred to other outputs or stores.

Useful is defined by the job: heating is useful in a heater but often unwanted in a lifting motor.
Compare like units and use the same operating interval.

Friction changes the account

Friction opposes relative sliding and can transfer mechanical energy into thermal energy. Lubrication and bearings can reduce some losses, but real devices also have other constraints. Friction can be essential: a bicycle needs tyre grip and controlled braking.

Do not describe energy as destroyed.
A design that removes all friction would not automatically be safe or useful.

Compare measurements with predictions

An ideal geometric ratio describes a lossless prediction. Measurements of force and work describe the real operation. For a steady machine, actual MA divided by SR gives efficiency as a decimal.

A value above 100% for a simple passive machine signals an incomplete energy account or measurement/calculation error.
A practical investigation needs uncertainty, repeated trials and a clear boundary.

Improve the whole design

A highly efficient device may still be slow, costly or unsuitable for the user. Compare efficiency with power, reliability, safety and maintenance. Explain a trade-off using evidence instead of choosing the biggest number automatically.

PhET · Energy Skate Park

Watch friction change the energy stores

Choose a screen with friction and energy displays. Start the skater from a fixed height. Compare friction off and on using the energy bars or pie chart. Reset to the same height for a fair comparison.

The skater is not a literal model of every machine. Use it to track conversion to thermal energy, then apply that account to a mechanism.

MEASURE · CALCULATE · EXPLAIN

Audit the input and useful output

Change one measurement, predict the result, then compare the calculated account. These are editable teaching measurements, not a physical simulation.

Input force (N)Output force (N)Input travel (m)Output travel (m)

WATCH THE PHENOMENON

Efficiency and simple machines

The Science Classroom · Mechanism in action

Watch with a prediction

Record the input energy and useful output for one example. Account for the difference before calculating a percentage.

Open on YouTube →

WORK THE REASONING

Audit a lifting machine

Input force = 50 N and travel = 4 m, giving 200 J of input work.
The 120 N load rises 1 m, giving 120 J of useful work.
Efficiency = 120 ÷ 200 × 100 = 60%.
The other 80 J is accounted for by other energy transfers; the machine has not lost energy from existence.

TRY IT

Predict, then explain

A device receives 240 J and supplies 180 J of useful work. Find efficiency and the remaining energy.

Your reasoning

Compare your reasoning

Efficiency = 180 ÷ 240 × 100 = 75%. The remaining 60 J is transferred into other outputs or stored forms.

TAKE IT FURTHER

Test the idea

Design a fair comparison of two mechanisms. State what you would measure and which conditions must be held fixed.

Your planApply it to the design challenge →

CHECK YOUR UNDERSTANDING

Put the mechanism to work

Read further: OpenStax · Simple Machines →