B.1 · 身体产生运动

What would explain this observation?

• Landing with bent knees increases the time over which momentum changes. The movement can reduce average impact force for the same momentum change.
• Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.

Build the model

• Joint movement results from muscle forces acting through lever systems. The centre of mass and base of support 支撑面 affect stability. Impulse connects force-time evidence to momentum change.
• impulse: Force integrated over time, equal to momentum change; base of support: The area supporting a body.

Choose evidence that can test it

• A biomechanical analysis must define the segment, direction and phase of movement. Net force and a single muscle force are not interchangeable. A longer lever arm can change torque without changing applied force.
• Use teacher-approved video of a safe movement with consent. Calibrate distance and time, identify frames consistently, and avoid making medical claims from a two-dimensional recording. Camera perspective can bias apparent joint angles.

Work from known quantities

• State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
• Known: momentum changes by 150 kg metres per second. Average force magnitude = momentum change/stopping time. For 0.30 s, force = 150/0.30 = 500 N. For 0.60 s, force = 150/0.60 = 250 N. This idealized average excludes the detail of a changing force curve.

Example:

Momentum change is 120 kg metres per second over 0.40 s. Find average force magnitude. Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.

Check the conclusion and its limits

• The largest force is not the same as average force. A school movement study should not deliberately create high-impact landings to test a prediction.
• Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.

Warn:

A video from one angle proves the cause of a sports injury. This claim is false: The largest force is not the same as average force. A school movement study should not deliberately create high-impact landings to test a prediction.

Key:

Biomechanics: movement and impulse: A biomechanical analysis must define the segment, direction and phase of movement. Net force and a single muscle force are not interchangeable. A longer lever arm can change torque without changing applied force.