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Pulleys: Lifting Objects
Mechanical Advantage and Work Trade-offs
Related Labs
Pulleys are simple machines that help lift heavy objects by changing the direction of a force or reducing the amount of force needed. They are used in cranes, elevators, flagpoles, theater rigging, and many gym machines. Studying pulleys helps students connect forces, motion, and energy in a practical way. They are a clear example of how physics can make work easier without creating extra energy.
A fixed pulley changes the direction of the pull, while a movable pulley can reduce the input force by sharing the load across multiple rope segments. In an ideal pulley system, the mechanical advantage equals the number of rope segments supporting the load. The tradeoff is that you must pull more rope when the required force is reduced. Real pulleys also involve friction, so actual systems need slightly more force than ideal calculations predict.
Key Facts
- Mechanical advantage: MA = load force / input force
- Ideal mechanical advantage for many pulley systems equals the number of supporting rope segments
- Work relationship in an ideal machine: F_in d_in = F_out d_out
- A fixed pulley has MA = 1 and mainly changes the direction of the force
- A single movable pulley has ideal MA = 2, so F_in = W/2 for load weight W
- Efficiency = useful work out / work in x 100%
Vocabulary
- Pulley
- A pulley is a wheel with a groove that guides a rope or cable to help lift or move a load.
- Load
- The load is the object or weight being lifted by the pulley system.
- Tension
- Tension is the pulling force carried through a rope, string, or cable.
- Mechanical advantage
- Mechanical advantage tells how much a machine multiplies the input force.
- Fixed pulley
- A fixed pulley is attached in place and changes the direction of the applied force without ideally reducing its size.
Common Mistakes to Avoid
- Counting all rope segments instead of only the segments supporting the load, which gives the wrong mechanical advantage. Only the rope sections directly holding up the moving load should be counted.
- Assuming a fixed pulley reduces the needed force, which is wrong in the ideal case. A fixed pulley mainly changes the direction of the pull and has mechanical advantage 1.
- Forgetting that pulling less force means pulling more rope, which breaks the work relationship. In an ideal system, reduced force is balanced by greater input distance.
- Ignoring friction in real pulleys, which leads to underestimating the input force. Actual systems are less efficient than ideal ones, so more force is needed than simple ideal formulas predict.
Practice Questions
- 1 A 240 N crate is lifted with a single movable pulley that has ideal mechanical advantage 2. What input force is needed to lift the crate if friction is ignored?
- 2 A block and tackle has 4 rope segments supporting a 600 N load. Assuming an ideal system, find the mechanical advantage and the input force needed to lift the load.
- 3 Explain why a pulley system with greater mechanical advantage does not reduce the total work needed in an ideal case, even though it reduces the input force.