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Pulleys - Pull Distance

Name of Corresponding Unit Plan: Pulley - Pull Weight

Grade Level: 2-8

VT Grade Expectations
VT S:21 Students demonstrate their understanding of Force by investigating variables that change an object's speed, direction, or both, and identifying and describing the forces that cause the change in motion.

NY Standards
NY Standard 1: Scientific Inquiry
Key Idea 1: Clarify ideas through reasoning, research, and discussion

Content Areas: Science

Recommended Length/Duration: 30-50 minutes

Learning Goals: Students will measure the length of rope needed to lift a resistance using a variety of pulley systems.

Description/Sequence:

  1. The teacher should review that a machine makes a task easier by increasing power or increasing speed of motion. Mechanical advantage allows a person to exert a stronger force with less effort.
  2. The teacher should remind students that whenever you decrease one thing (effort) you increase another (effort distance)
  3. Review the worksheet diagramming the different pulley systems to be tested and the data that should be recorded.
  4. In groups or as a learning center, have students rig each pulley system and measure the length of rope needed to pass through the system to raise the resistance one meter (adjust this distance to suit your work environment). Be sure they measure from a consistent reference point (floor for resistance, top pulley for the effort distance)
  5. Have students calculate mechanical advantage by dividing the Effort Distance by the Resistance Distance.
  6. Have students answer the questions on the worksheet consolidating their findings and understanding.
  7. When everyone has had a chance to complete the activity, discuss their findings. Guiding questions might include:
    • Which system had the highest/lowest mechanical advantage?
    • Did any of the systems have about the same mechanical advantage?
    • Did it make any difference whether you pull up or pull down?
    • Is there a way to determine mechanical advantage without actually measuring the rope?
  8. A way to determine mechanical advantage unique to pulley systems is to count the number of rope strands that support the weight of the resistance. The number of strands equals the mechanical advantage. Note that this is affected by whether you are pulling up or pulling down, as a downward rope generally does not support the resistance weight.
  9. Students data may reflect differences between the pull distance and pull weight for the same pulley system. This reflects the difference between the Ideal (theoretical) Mechanical Advantage and the Actual Mechanical Advantage. Friction is the difference. A more efficient machine reduces the effect of friction and the IMA and AMA are nearly the same value.

Assessments: Assessment can be informal based on participation and general understanding, or more formal in terms of accuracy of measurements and calculations.

Materials/Resources:

Pulley - Pull Distance Worksheet (pdf)
Ring stands, clothes hooks, ceiling supports or some other overhead anchor to support pulleys, pulleys of various types, line appropriate for the size of the pulleys, clips or extra line to rig pulleys, resistance weight, meter stick or tape measure

Special Considerations: Review safety precautions – students should be careful not to drop weights to hurt themselves or damage the equipment or room. Larger pulleys are heavier and more expensive, but much more durable and generally easier to rig.