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BOSTON UNIVERSITY

LESSON 4: Bobsled Challange

Subject Area

Grade

Estimated Length

Prerequisite knowledge/skills

1. The ability to draw free-body diagrams (FBD)
2. Friction
3. Engineering Design

Description of new content

Goals

Gain an understanding of the concept of friction and explore the effects on velocity and acceleration using Newton’s three laws of motion, and become familiar with the engineering design process.

1) Investigate the effects of materials on the friction between a "bobsled" and the track.
2) Investigate the effects of weight on the motion of the bobsled.
3) Design an appropriate bobsled for either fast or slow movement.

Materials Needed

The following is a list of parts that can be used for the "Bobsled Challange". These are only suggested parts. Be creative with the actual matials you let the students use. You will also need to make a smooth inclined track for the bobsleds to race down.

Procedure

Initial Preparation:

1) The rules of the Bobsled challange are that a maximum of 4 batteries can be used as weights in the bobsleds. The students should try to make their bobsled either go slowest or fastest down the track.
2) Draw a free body of the bobsled on an incline. Include all relevant forces on the sled. Hint: Think about what force is causing the sled to slide down the ramp, what force is opposing the motion, and don’t forget the normal force that is always perpendicular to two surfaces in contact.
3) Determine what materials are available to construct your bobsled.
4) Given the materials available describe what aspects of the design can be controlled?

Friction Investigation:
1) Select a specific base type for the bobsled.
2) Select three specific different materials to attach to the bottom of the bobsled base.
3) In the same track, determine the time it takes the bobsled to reach the bottom of the track using each type of material one at a time, including a trial with no material on the sled. Record the time data and a basic description of the material used in the provided table.

Weight investigation:
1) Using the base alone. Perform a series of trials with differing numbers of batteries (serving as weight) in the bobsled. Be sure to use the same track as the friction investigation. Record the weight description and time data in the provided table.

Free design:
1) Based on the two previous investigations, design your own bobsled to try to maximize or minimize (your choice) the time it takes the sled to reach the bottom of the track.
2) Describe qualitatively which objective you are trying to achieve (slow or fast) and what rationale you are using to design your sled as it relates friction and weight. Note any interesting decisions you make about the configuration of how the sled is put together as well.

Evaluation

1) Describe what makes your final design work the best in terms of the physical quantities changed in the space provided below.

2) Did you observe any significant differences between the four available tracks? Describe and explain your answer, including a hypothesis of what might cause these differences.

3) If you had more time what further changes could you make to your best design to further optimize it? Describe and explain.

4) Describe what variations (errors) could be present in the system. Hint: Think in terms of how the bobsled is built, what materials were used (cloth and batteries), and what tracks were used.

Extensions

This exercise introduces students to the concepts of static and kinetic friction. The lesson also introduces students to engineering design as an iterative process. Problems solving and thinking skills are reinforced and creativity encouraged through the open-ended final sled design. Students who are familiar with friction and Newton’s Laws will have an easier time determining what types of changes to make to their design, while students with a low familiarity with these concepts have a trial and error approach available that will most likely still result in overall success.

The lesson is designed to take approximately 45 minutes to complete, however, more advanced students may finish in less time and should be encouraged to try additional iterations of design with the extra time available.

To add excitement to the lesson, students could be encouraged to compete with each other in the two categories of faster or slower design.

As a follow up in the classroom, basic calculations of force and/or coefficient of friction could be calculated if the track angle, and final bobsled weight are measured prior to leaving the museum. Additionally, average velocity of the sled could be calculated if the track length is measured.

Massachusetts state frameworks covered in this lesson:
Primary: Massachusetts State Framework 1.9.
Secondary: Massachusetts State Framework 1.1, 1.2, 1.6, 1.7, 1.8, 1.10