Teacher Guide: Triple Beam Balance
Learning Objectives
Students will…
• Identify the parts of a triple beam balance.
• Understand that a triple beam balance is an example of a lever.
• Find the mass of an object using a triple beam balance.
Vocabulary
fulcrum, lever, mass, rider, triple beam balance
Lesson Overview
A triple beam balance is commonly used to measure
mass in a laboratory. The Triple Beam Balance Gizmo™
allows students to practice finding the mass of objects
using this device.
The Student Exploration sheet contains one activity. In
this activity, students learn to use a triple beam balance.
Suggested Lesson Sequence
1. Pre-Gizmo activity
( 10 – 20 minutes)
Students can make a simple singlebeam balance using a ruler, a pencil,
and a few pennies. Set the ruler on the
pencil so that it is perfectly balanced.
Place a stack of 3-5 pennies on the
ruler, one inch from the fulcrum (pencil).
On the opposite side, place a single
penny one inch from the fulcrum. Slide
the single penny along the ruler until the stack of pennies is balanced. With this model,
students will learn the two principles that allow triple beam balances to work:
•
As a mass moves farther from the fulcrum, the force it exerts on the beam
increases.
•
The force a mass exerts on the beam is proportional to the distance from the
mass to the fulcrum. In other words, a stack of three pennies located one inch
from the fulcrum is balanced by one penny placed three inches from the fulcrum.
2. Prior to using the Gizmo
( 10 – 15 minutes)
Before students are at the computers, pass out the Student Exploration sheets and ask
students to complete the Prior Knowledge Questions. Discuss student answers as a
class, but do not provide correct answers at this point. Afterwards, if possible, use a
projector to introduce the Gizmo and demonstrate its basic operations. Demonstrate how
to take a screenshot and paste the image into a blank document.
3. Gizmo activities
( 15 – 20 minutes per activity)
Assign students to computers. Students can work individually or in small groups. Ask
students to work through the activities in the Student Exploration using the Gizmo.
Alternatively, you can use a projector and do the Exploration as a teacher-led activity.
4. Discussion questions
( 15 – 30 minutes)
As students are working or just after they are done, discuss the following questions:
•
How can you tell that a triple beam balance is an example of a lever?
•
What is the largest mass that could be accurately measured on the triple beam
balance shown in the Gizmo?
•
What would you predict is the mass of the light bulb and the paper clips?
•
Why does moving the riders along the beams help you to balance the mass on
the measurement tray?
5. Follow-up activity
( 30 – 60 minutes)
Collect a large number of rocks or other objects, label them, and find their mass using a
triple beam balance. Then have your students practice finding the masses of the rocks.
Use your answer key to assess student results, and assist students that need extra help.
Most balances, if calibrated properly, are accurate to within 0.5 grams or better.
Students should zero the balance with the adjustment knob before each measurement.
When using a triple beam balance, the most common mistake is to leave the 10-gram or
100-gram rider between notches. Be sure that students understand that you must move
the rider over until the pointer is below the zero line, and then move the rider back to the
nearest notch.
Another common error is to misread the value of the 1-gram rider. For example, students
may misread 5.6 g as 0.56 g, and get a mass value of 130.56 g rather than the correct
135.6 g.
Scientific Background
A triple beam balance is an example of a lever. A lever consists of a rod or beam that pivots
over a fulcrum. On a lever, a large force exerted close to the fulcrum can be balanced by a
smaller force that is exerted farther from the fulcrum. On a triple beam balance, an unknown
mass on the measurement tray is balanced by sliding the riders away from the fulcrum.
On a lever, the weight to be lifted is the load and the force used to lift the weight is the effort.
The distance between each force and the fulcrum is called the arm. The law of the lever states
that in a balanced lever, the following is true:
load × load arm = effort × effort arm
For example, a 10-gram object that is 2 centimeters from the fulcrum can be balanced by a 4gram object that is 5 centimeters from the fulcrum.
A triple beam balance usually has 100-gram, 10-gram, and 1-gram riders. When each rider is in
the “0” position, their collective weight exactly balances the weight of the measurement tray. An
adjustment knob below the measurement tray is used to zero the balance. This accounts for
minor variations in atmospheric pressure. When a rider is moved to the right, the torque
(twisting force) that it exerts increases proportionally. For example, the 10-gram rider in the “60”
position exerts 6 times as much torque as the 10-gram rider in the “10” position. Once the object
on the tray is perfectly balanced by the riders on the beams, its mass can be read. In the
example shown below, the object on the measurement tray has a mass of 245.6 grams.
Measurement Connection: Mass and weight
Students are often confused by the terms “mass” and “weight.” Mass refers to the amount of
matter in an object, while weight is the force of gravity on an object. In the metric system, units
of mass include grams (g) and kilograms (kg). Weight is measured in newtons (N). An object’s
weight is calculated using the universal law of gravitation:
FG =
Gm1m 2
r2
On Earth’s surface, where m1 (Earth’s mass), r2 (Earth’s radius squared) and G (gravitational
constant) are constant, the equation can be rewritten w = 9.8m, where m is measured in kg.
It is common for science textbooks and teachers to assert that balances measure mass, while
spring scales measure weight. This is not exactly true. A balance or a spring scale can be
calibrated in units of weight or mass. A balance compares the unknown weight of an object to
the weight of a set of known masses. A balance that is balanced on Earth will also be balanced
on any other planet. A spring scale, on the other hand, will show a different result on another
planet because the gravity pulling objects down will be either stronger or less strong than on
Earth. Therefore, spring scales are useful for showing how weight changes on different planets,
while balances are useful for demonstrating that the masses of objects do not change if the
objects were moved to another planet.
Selected Web Resources
Reading triple beam balances: http://www.wisc-online.com/objects/index_tj.asp?objID=GCH202
Triple beam balances: http://www.newton.dep.anl.gov/askasci/gen99/gen99154.htm
Mass vs. weight: http://www.nyu.edu/pages/mathmol/textbook/weightvmass.html
Ruler levers: http://www.teaching-point.net/samples/science/physicalscience/Studentsample.pdf
Related Gizmos:
Levers: http://www.explorelearning.com/gizmo/id?646
Weight and Mass: http://www.explorelearning.com/gizmo/id?653