lesson
A muscle pulls when it contracts
1 · Learn
The skeleton supports the body, protects organs and provides a framework for movement. Muscles exert forces when they contract; they pull rather than actively pushing bones back. At a joint, opposing muscles can produce movement in different directions.
In a simplified elbow model, the biceps contracts to bend the elbow while the triceps relaxes; the triceps contracts to straighten it. Tendons connect muscles to bones. Real movement involves coordinated muscles and control, not isolated strings alone.
2 · Worked example
Pulling one string in a hinged-card model bends it. A second opposing string returns it, illustrating why one pulling structure cannot push in the reverse direction.
3 · Your turn
Why does straightening the elbow need an opposing muscle action rather than the biceps pushing?
Check your answer
A contracting muscle pulls; the triceps provides the opposing pull needed to straighten the elbow in the simplified model.
4 · Apply your learning
Annotate a safe paper joint model and state one difference between it and a real limb.
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Check a calculation · Year 5 onward
Use this only when your teacher or adult allows a calculator for the task. Show your thinking first. It does not replace mental or written arithmetic and does not mark your lesson answer.
Powers, roots and trigonometry · Year 7 onward
Use only when the task allows a calculator. Choose one operation at a time and record your working. Trigonometry here always uses degrees (DEG), not radians. Inverse sine, cosine and tangent return an angle, not a reciprocal. Keep exact fractions and π in answers when requested.
Approximate numerical results use up to 12 significant digits. Inverse sine returns −90° to 90°, inverse cosine 0° to 180°, and inverse tangent −90° to 90°; these are principal values, not every possible solution of a trigonometric equation.
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