Does Weight Matter?

Engineers must understand how friction affects many situations, from the bottom of skis in which friction is a disadvantage to hiking boots where friction provides traction. Scientists must think like engineers when designing experiments, as students do in this activity.

After this activity, students should be able to:

• Explain that, for friction due to surface roughness, the frictional force is proportional to the weight of the object being moved across a surface.
• Calculate the coefficient of friction for an object whose weight and the measured frictional force between the object and a surface are given.

Students should have a basic understanding of friction and how to measure it using coffee mugs and spring scales, as was done in the previous lesson, Discovering Friction, and its activity, Sliding and Stuttering.

Since students have already formulated hypotheses concerning the effects of weight on friction (as part of the associated lesson), little further introduction to the activity is needed.

Remind the students, however, about the importance of controlling variables in scientific experiments. In this case, weight is the only variable that differs each time they drag their coffee mugs across a surface and measure the resulting frictional force. Other variables, such as the type of surface used, must be controlled, that is, not varied during the experiment.

Also, point out that since weight is the subject of the experiment, it is the total weight of the object being dragged that is important. If students start with an empty coffee mug, its weight is not zero, but rather the weight of the mug itself. (This is easily determined by hanging the mug from the spring scale.) Then, each time they add weight to the mug, the new total weight is the weight of the mug plus whatever additional weight was placed in the mug. Show students the weight sets that are available for them to use, and then let them proceed with planning and executing their experiments.

Part 1: Designing the Experiment

Although students should already be familiar with the basic method for measuring friction using spring scales and coffee mugs, they will still need to make several decisions in order to design an experiment to test for the effects of weight on friction. Write the following list of questions on the board (or provide them in a handout):

• How much additional weight will you use? Will you test just one additional weight, or several different weights?
• What surface will you use for dragging your mug over?
• How many trials will you do?
• Will you measure and record both types of friction (static and kinetic)?
• How will you record your data?

Then, have students meet in their groups to discuss these questions. Once all members of a group agree on the answers, and you have verified that the answers are reasonable, the group is ready to conduct its experiment.

Part 2: Conducting the Experiment

Provide student groups with the materials, and direct them conduct their experiments.

Part 3: Analyzing the Data

Remind students that scientists typically report the results of their experiments in the form of graphs. Have students use their data to prepare graphs similar to the one shown in the Lesson Background and Concepts for Teachers section of the associated lesson. Then ask them to write a paragraph describing what their data show about how weight affects friction.

Use the first three Investigating Questions to check for understanding of the process of scientific inquiry involved in this activity. Also, check to see that students are reading the spring scales with reasonable accuracy and are accurately recording their data.

As students prepare their graphs, check to see that they put the dependent variable (friction force) on the y-axis and the independent variable (weight) on the x-axis. Make sure that their graphs also include the origin (0,0), axes labels (including units), and a legend if both the static and kinetic friction are shown in one graph. Have students analyze their graphs and determine the relationship between the friction force and the weight. Specifically discuss in terms of how the dependent variable changes when the independent variable changes.

As necessary, remind students that spring scales have a limit to how much force they can measure (scales with a 500 g limit are recommended for this activity). If they put too much additional weight in their mugs, the frictional forces may exceed the spring scale capacity and could even damage them. Expect them to be able to obtain reliable results for their weight experiments using added weight between 50 and 250 grams.