1. Contributed by: Engineering K-Ph.D. Program, Pratt School of Engineering, Duke University Hands-on Activity: Does Weight Matter?

2. Keywords  friction, static friction, kinetic friction frictionstatic frictionkinetic friction

3. Learning Objectives  Students will 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.

4.  Students will be able to calculate the coefficient of friction for an object whose weight and the measured frictional force between the object and a surface are given.

5. Materials List  spring scales, preferably having a 20 N capacity 0.5 N accuracy, one per team of 2 to 4 students.  ceramic coffee mugs, one per team of 2-4 students  scissors (one per team)

6.  tape (masking or wide transparent), one roll per team or one roll shared between two teams  string, about 30 cm per team  200 g, 500 g, 1 kg weights

7. Introduction  Write the hypothesis you wrote about weight and friction.   If____________________________________ _then________________________________ ______.

8.  Besides weight and friction what are the other variables that need to be kept constant when performing an experiment?  _____________________________________ _____________________________________ ______

9.  In this case, weight is the only thing that should differ each time they drag their coffee mugs across a surface and measure the resulting frictional force.  What is the weight of the empty coffee mug? _____________________

10.  This must be added to the 200g, 500g, and 1kg when recording data.

11. Procedure Part 1: Designing the Experiment  How much additional weight will you use?  Will you test just one additional weight, or several different weights?

12.  What surface will you use for dragging your mug over?  How many trials will you do?

13.  Will you measure and record both types of friction (static and kinetic)?  How will you record your data?

14. Part 2: Conducting the Experiment  Use the list of materials above and record all your data in a table. This data will be used to make a graph like the one that you saw in the lesson.

15. Part 3: Analyzing the Data  Use the graph below to create a graph of the data obtained for the three weights used. There should be three dots for static friction and three dots for kinetic friction.

17.  Label Y-axis Force (Newton), Label X- axis Weight (gram).  Number the axis as needed.  Use two different colors for static and friction or use squares and triangles instead of dots to tell the difference.  Connect the dots with a line. If you did a good job the line will be straight.