Grade 6

Elementary

8 min

Lesson 10 of 12

Pulleys and Ropes

Not Started

Using wheels and cables to lift loads and change force direction.

How could a single person lift a heavy piano or a massive boat engine without using a motor? The secret lies in a simple wheel and a piece of rope that can effectively 'multiply' your strength.

Learning Objectives

1.By the end of this lesson, you will be able to distinguish between fixed, movable, and block-and-tackle pulleys.
2.You will understand how adding more pulleys reduces the effort needed to lift a load.
3.You will be able to calculate the mechanical advantage based on the number of rope segments.

The Fixed Pulley: Changing Direction

A pulley is a simple machine made of a wheel with a groove for a rope or cable. The simplest version is the fixed pulley. It is attached to a permanent structure, like the top of a flagpole. When you pull down on one end of the rope, the load goes up. While it feels like you are doing the same amount of work, the fixed pulley changes the direction of the force. This is helpful because you can use your own body weight to pull down, which is often easier than lifting up!

Imagine you need to lift a flag weighing

10\text{ Newtons}

to the top of a pole. 1. You loop the rope over a fixed pulley at the top. 2. To lift the flag, you must pull down with a force of

10\text{ Newtons}

. 3. The Mechanical Advantage (MA) is

1

because the force you put in equals the force that comes out:

MA = \frac{\text{Load}}{\text{Effort}} = \frac{10}{10} = 1

Quick Check

Does a single fixed pulley reduce the amount of force (effort) needed to lift an object?

Think about the Mechanical Advantage of 1.

Answer

No, it only changes the direction of the force.

The Movable Pulley: Cutting the Weight

A movable pulley is different because the pulley itself is attached to the load and moves with it. One end of the rope is tied to a fixed point, and you pull up on the other end. Because the rope is essentially 'doubled up,' the weight is shared between the fixed anchor and your hand. This means you only have to provide half the force to lift the object! However, there is a trade-off: you have to pull the rope twice as far as the object actually moves.

You are lifting a bucket of sand weighing

40\text{ Newtons}

using one movable pulley. 1. The load is

40\text{ N}

. 2. Since there are

2

rope segments supporting the load, the effort is divided by

2

. 3. Your effort:

\text{Effort} = \frac{40\text{ N}}{2} = 20\text{ N}

4. To lift the bucket

1\text{ meter}

, you must pull

2\text{ meters}

of rope.

Quick Check

If a movable pulley has a Mechanical Advantage of 2, how much effort is needed to lift a 100 N box?

Divide the load by the Mechanical Advantage.

Answer

50 Newtons.

Block and Tackle: The Ultimate Multiplier

When we combine fixed and movable pulleys into one system, we call it a block and tackle. This system allows us to reach a very high Mechanical Advantage (MA). The rule for calculating MA is simple: count the number of rope segments that are directly supporting the movable pulley(s). The more segments you have, the less effort you need. If you have 4 segments supporting a load, you only need $1/4$ of the force to lift it!

A mechanic uses a block and tackle system with

4

rope segments to lift a

2000\text{ Newton}

engine. 1. Identify the number of supporting segments:

n = 4

. 2. Calculate the required effort:

\text{Effort} = \frac{\text{Load}}{n} = \frac{2000\text{ N}}{4} = 500\text{ N}

3. The mechanic only feels like they are lifting

500\text{ Newtons}

, making the job much safer and easier!

Key Takeaways

1Fixed pulleys change the direction of force but have a Mechanical Advantage of 1.
2Movable pulleys reduce the effort needed by sharing the load weight across rope segments.
3Mechanical Advantage (MA) is equal to the number of rope segments supporting the load.
4The 'Trade-off Rule': As effort decreases, the distance you must pull the rope increases.

Quick Check

Multiple Choice

Which type of pulley is attached to the object being lifted?

Multiple Choice

If a pulley system has a Mechanical Advantage of $3$ , how much rope must you pull to lift a load $2$ meters?

True/False

A block and tackle system combines both fixed and movable pulleys.

What's Next?

Review Tomorrow

In 24 hours, try to draw a diagram of a movable pulley and explain to someone why it makes lifting easier using the word 'segments'.

Practice Activity

Find a set of blinds in your house. Observe the cords and pulleys at the top. Are they fixed or movable? How many rope segments can you count?

Browse

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Pulleys and Ropes

Learning Objectives

The Fixed Pulley: Changing Direction

The Movable Pulley: Cutting the Weight

Block and Tackle: The Ultimate Multiplier

Key Takeaways

Quick Check

What's Next?

Pulleys and Ropes

Learning Objectives

The Fixed Pulley: Changing Direction

The Movable Pulley: Cutting the Weight

Block and Tackle: The Ultimate Multiplier

Key Takeaways

Quick Check

What's Next?