Lesson 3.1: Wheels, Pulleys, and Blocks

The invention of the wheel is considered usually to be the biggest accomplishment of early people.  The wheel is the basic part of a pulley. 

Read the introduction to this lesson Wheels, Pulleys, and Blocks on page 140 of your textbook. Then, answer the following question.

Question 1. What is a pulley?

 

Then, return here to continue this lesson.

 

 

Pulleys and combinations of pulleys are very common in many machines.  You have likely seen pulleys in clotheslines.  One of the pictures on page 141 is a picture of a pulley used in a clothesline.  The investigation can be done if you have access to three pulleys and a spring scale.

Read the investigation Pulleys Make Work Easier - Sometimes on pages 140 and 141 to ensure that you understand what you are expected to do.  Perform the investigation and record your data in the table below.    If you do not have access to the equipment, use the data that was obtained by a student who performed the investigation.

Data:

Step 1
Force Required to Suspend the Block

Step 5
Reading on the Scale

Step 6
How far the String was Pulled

Part 1

 

 

  

Step 1
Force Required to Suspend the Block

Step 6
Reading on the Scale

Step 7
How far the String was Pulled

Part 2

 

 

  

Part 3

 

 

  

Student Data:

Step 1
Force Required to Suspend the Block

Step 5
Reading on the Scale

Step 6
How far the String was Pulled

Part 1

4 N

4 N
10 cm

Step 1
Force Required to Suspend the Block

Step 6
Reading on the Scale

Step 7
How far the String was Pulled

Part 2

4 N

2.1 N
20 cm

Part 3

4 N

1.4 N
30 cm

Analysis:

Question 2. Answer the Analyze questions on page 141 in your textbook.

Question 3. Calculate the work done, with and without the pulley, in each pulley set-up.  You must convert your distances from cm to m by dividing by 100 cm/m.  Then, use the formula  W = F·d (from Lesson 1.4) to calculate the work.  Use the following table to show your work.

Work Required to Lift the Block Without the Pulley

Work Required to Lift the Block With the Pulley

Part 1

Part 2

Part 3

Question 4. Based on your calculations in question 3, which part required the most work using the pulley system?  Because a machine is supposed to make work easier, explain why more work is needed using a pulley system.

Question 5. Predict how far the string would be pulled if a force of 1 N was required to raise the block 10 cm. 

 

Then, return here to continue this lesson.

 

 

 

 

 

 

 

 

 

 

 

Answers to Questions:

Question 1. What is a pulley?

A pulley is a simple machine is made up of a grooved wheel around which a rope turns.  A pulley can also be just the grooved wheel.

 

Click to return to where you left off in this lesson.

 

 

 

 

 

 

 

 

 

 

 

Answers to Questions:

Question 2. Answer the Analyze questions on page 141 in your textbook.

1.  The single pulley required the most effort to lift the block.  A single pulley changes only the direction of the force. The force required to lift the block is the same (4 N) whether the pulley is used or not.  If you just lift the block with the spring scale, the force of 4 N is applied upwards.  If you use the single pulley, you apply a force of 4 N downwards.

2.  The three pulley set-up used the least force to lift the block (1.4 N).  The pulley set-up is a simple machine that makes work easier by requiring less force to raise the block.  Notice that the string had to be pulled three times as far to lift the block the same 10 cm distance.

3.  The one pulley set-up required the string to be pulled a distance of 10 cm.  The two pulley set-up required the string to be pulled a distance of 20 cm.  The three pulley set-up required the string to be pulled a distance of 30 cm.  Notice the 1:2:3 ratio in distances compared to the 1:2:3 ratio in number of pulleys.

Question 3. Calculate the work done, with and without the pulley, in each pulley set-up.  You must convert your distances from cm to m by dividing by 100 cm/m.  Then, use the formula  W = F·d (from Lesson 1.4) to calculate the work.

The following are the calculations based on the student sample data.  If you did the experiment yourself, you might have different numbers but the calculations should be similar.

Work Required to Lift the Block Without the Pulley

Work Required to Lift the Block With the Pulley

Part 1

W = (4 N)(0.1 m) = 0.4 J W = (4 N)(0.1 m) = 0.4 J

Part 2

W = (4 N)(0.1 m) = 0.4 J W = (2.1 N)(0.2 m) = 0.42 J

Part 3

W = (4 N)(0.1 m) = 0.4 J W = (1.4 N)(0.3 m) = 0.42 J

Question 4. Based on your calculations in question 3, which part required the most work using the pulley system?  Because a machine is supposed to make work easier, explain why it would take more work using a pulley system.

According to the calculations for the student data, parts 2 and 3 required more work than part 1.  The reason for this is that each pulley has some friction, and the rope passing over the pulley has some friction.  It takes more effort to overcome the friction; therefore, more work has to be done.  The machine makes work easier in that it takes less effort, but that effort must be exerted over a longer distance.

Here is an analogy.  Suppose you have to carry a large box of books up a flight of stairs.  You could carry the entire box up the stairs, but it is very heavy for you.  A much stronger person could carry the box up the stairs.  So, you take a manageable load of books out of the box, carry them up the stairs, and repeat the process until the whole box of books is upstairs.  You did a lot more work because you had to move a much greater distance than if you had carried the whole box at one time.

Question 5. Predict how far the string would be pulled if a force of 1 N was required to raise the block 10 cm. 

The force that is needed to raise the block without a pulley is 4 N.  The force required with the pulley is 1 N or 1/4 the force.  Therefore, the force must be exerted over a distance 4 times as far as the block is raised or 4 times 10 cm, which is 40 cm.

                            


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