Summary
Overview
Can your students lift a book off the floor with just one finger? Find out and learn about simple machines in this fun lesson plan about levers.Learning Objectives
- Understand the working principles of a lever
- Apply these principles to lift a heavy object with one finger
- Identify which variables can be changed when using a lever, and the effect of changing them
NGSS Alignment
This lesson helps students prepare for these Next Generation Science Standards Performance Expectations:- 3-5-ETS1-3. Plan and carry out fair tests in which variables are controlled and failure points are considered to identify aspects of a model or prototype that can be improved.
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Science & Engineering Practices
Planning and Carrying out Investigations. Plan and conduct an investigation collaboratively to produce data to serve as the basis for evidence, using fair tests in which variables are controlled and the number of trials considered.
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Disciplinary Core Ideas
ETS1.C: Optimizing the Design Solution. Different solutions need to be tested in order to determine which of them best solves the problem, given the criteria and constraints.
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Crosscutting Concepts
Systems and System Models. A system is a group of related parts that make up a whole and can carry out functions its individual parts cannot.
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Materials
For the entire class:
- Book or other heavy object
- Stiff meter stick/yard stick (make sure it is not too flexible)
- Thick marker
For each student (make small groups if there are not enough supplies):
- Pencil or thin marker or crayon
- Stiff ruler with inch markings (do not use flexible rulers)
- An item to use as a weight, like a box of crayons
Background Information for Teachers
This section contains a quick review for teachers of the science and concepts covered in this lesson.A force is a push or a pull on an object. We exert forces every day, whether we are pulling open a door, pushing a child on a swing, or picking up a book. Sometimes we use machines to help us exert bigger forces. This is called mechanical advantage.
A lever is a type of simple machine that can be used to increase a force. A lever consists of a beam attached to a pivot (called the fulcrum). A lever allows you to take an input force (the effort) and amplify the output force (the load). A seesaw at the playground is a simple example of a lever. Imagine that you have three children who are all exactly the same weight. One of the children can lift the other two children on the seesaw by sitting twice as far away from the fulcrum (Figure 1).
A seesaw is balanced on a fulcrum so that 2 meters of the board hang off each side. Three people of equal weight are able to balance on the seesaw if a person on one side of the seesaw stands at the furthest end and the other 2 people stand together 1 meter away from the end on the opposite side. Two people weigh twice as much as the single person but they are twice as close to the fulcrum so the forces exerted on both sides of the seesaw are the same.
Figure 1. A seesaw is an example of a lever.
You can make a miniature version of a seesaw using a pencil and a ruler. Place a pencil flat on a desk and put a ruler on top of it at the halfway point (the 6 inch mark). Place an object like a box of crayons on one end of the ruler, as shown in Figure 2. Then press down on the on the ruler just on the other side of the pencil (the 5 inch mark). How hard is it to lift the box? Try pressing down all the way at the end of the ruler instead (the 0 inch mark). It should be easier to lift the box. Now try moving the pencil closer to the box of crayons, and press down on the other end of the ruler again. It should be even easier to lift the box, because the effort (your finger pressing down on the ruler) is relatively much farther away from the fulcrum (the pencil) than the load (the box of crayons). In this lesson plan, your students will experiment with a similar setup to see if they can lift a heavy object off the floor with just one finger.
Figure 2. A simple lever made from a ruler and pencil. The ruler is the beam and the pencil is the fulcrum. By pushing down on the far end of the ruler, you can lift the box of crayons.
There are other examples of levers all around us that might not be as obvious. Grab a pair of scissors— they are also a type of lever! Is it easier to cut through a piece of cardboard using the tip, or the part of the blade closer to the handle? If you cut with the part of the blade closer to the handle, the mechanical advantage is bigger (the force of you squeezing the handles is amplified more), so it is easier to cut. Now go try to open (or close) a door, first by pushing near the doorknob, and next by pushing near the hinges. You should notice that it is much harder to open (or close) the door by pushing close to the hinges.
It might seem like levers give you "something for nothing" because they can take a small input force and turn it into a larger output force. However, while a lever allows you to decrease the effort force required to lift an object, the distance over which you must exert this force increases. For example, in Figure 1, in order to lift the children on the right up a given distance, the child on the left must move twice as far. This means that energy is conserved; what you gain in force, you lose in distance. This is a very important concept in physics—you never get anything for free!
Mathematically, the equation for balancing a lever can be expressed as:
Equation 1:
