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Pulley - Pull Weight

Name of Corresponding Unit Plan: Pulley - Pull Distance

Grade Level: 2-8

VT Grade Expectations
VT S:5 Students demonstrate their ability to REPRESENT DATA by displaying and labeling data for separate trials/observations.
VT S:6 Students demonstrate their ability to ANALYZE DATA by interpreting patterns or trends in data.
VT S:7 Students demonstrate their ability to EXPLAIN DATA by providing a reasonable explanation that accurately reflects data.
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: Mathematical Analysis
Key Idea 1: M1.1 Use algebraic and geometric representations to describe and compare data.
Key Idea 2: M2.1 Use deductive reasoning to construct and evaluate conjectures and arguments.
NY Standard 1: Scientific Inquiry
Key Idea 1: Clarify ideas through reasoning, research, and discussion
Key Idea 3: S3.1 Use various means of representing and organizing observations

Content Areas: Science

Recommended Length/Duration: 30-50 minutes

Learning Goals: Students will measure the pull weight 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. Review the worksheet diagramming the different pulley systems to be tested and the data that should be recorded.
  3. In groups or as a learning center, have students rig each pulley system and measure the pull weight needed to lift the resistance.
  4. Have students calculate mechanical advantage by dividing the Resistance weight by the Effort weight.
  5. Have students answer the questions on the worksheet consolidating their findings and understanding.
  6. 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 pull weight?
  7. 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.
  8. 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:

Pulleys - Pull Weight Worksheet (pdf)
Ring stands, clothes hooks, door frame, 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.