Lab #4-3: Simple Pulley Lab

Introduction:

A pulley is a simple machine consisting of a wheel turning on an axle. Pulleys are often used singly and in combinations to do work. We’ll be doing a series of problems in this chapter and the future that involve the use of pulleys. So, today we’re going to:

  1. Look at some simple examples of how pulleys work.
  2. Draw some free-body diagrams for each of the systems.
  3. Use the springs scales to measure forces.
  4. Calculate and compare these to the theoretical forces that are involved.

One thing that we’ll have to ignore is the small bit of friction that exists when using a pulley. This is going to hamper our results a bit, but overall we should be able to get some good results. Each of the following set ups needs to be created using the provided materials.

Materials:

  • 2 Pulleys (1 stationary, 1 free)
  • 2 m of string
  • Scissors
  • 1 kg weight
  • 1 ring stand (if needed)
  • 1 spring scales (Newtons)

Pre-Lab:

In this lab, we’ll be using a mass of 1 kg for all trials. If we have a 1-kg mass, what should it’s weight (Fg) in newtons be?

We’re going to compare what it should be theoretically to the reading on the scale. The spring scale may not be calibrated perfectly, so if its off by a small amount, that’s OK. As long as it measures consistently, we should get out of this what we want. Lift the 1-kg mass up using the spring scale.

What does the scale read? ______Newtons

Procedure:

You’ll need to cut the string to various lengths. The length doesn’t matter, but remember you have a limited amount of string. So don’t use it all at once.

Set Up 1:

  1. Using the stationary pulley attached to the wood on the lab bench, feed the string through the pulley and suspend the 1-kg mass from 1 end while pulling the other end with the spring as shown in Figure 1.
  2. Hold the mass stationary.

Figure 1

  1. Draw the free-body diagram for both the 1-kg mass AND for the spring scale on which you’re pulling.
  1. Record the reading on the spring scale: ______Newtons.
  2. There is a tension force (FT) pulling on both the spring scale AND on the 1-kg mass. How do you think these compare? Why?

______

Set Up 2:

  1. Take on loop end of the string and loop it over the bolt on top of the stationary pulley as shown in Figure 2. We’re taking this pulley out of the equation this time, we just need a contact point.
  2. Feed the string through the free pulley.
  1. Hook the free pulley to the 1-kg mass.
  1. Attach the spring scale to the end of the string and lift the mass directly up as you’ll see in Figure 3.
  1. Hold the mass stationary.

Figure 2

  1. Draw the free-body diagram for both the 1-kg mass w/ pulleyAND for the spring scale on which you’re pulling.

Figure 3

  1. Record the reading on the spring scale: ______Newtons.
  2. Why is the reading on the scale lower this time?

______

Set Up 4:

  1. Create the final set up as shown in Figure 4
  1. Pull the lift the 1-kg mass until it is stationary in the air.
  1. Record the reading on the spring scale: ______Newtons.
  1. Draw the free-body diagram for both the 1-kg mass w/ pulley AND for the spring scale on which you’re pulling.

Figure 4

  1. Does the reading on the scale make sense? Why or why not?

______