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# Professional Development Modules

## The Downhill Racer - Forces and Motion

Author: Sandy Van Natta
View Student Lesson Plan

#### Abstract

Module Description

Motion of a car traveling along an inclined plane is investigated. Participants will observe the pattern of water drops left by the moving car. The changing distances between the successive drops indicate that the car was not traveling at a constant speed. The pattern of drops can be used for data collection and graphs representing the motion of the car drawn. Analysis of the graphs leads participants to the conclusion that the car was increasing in velocity, or accelerating, down the ramp.

#### Objectives

• Participants will identify the unbalanced force causing the motion of a toy car on an inclined plane.
• Participants will collect data and construct graphs representing the motion of a toy car on an inclined plane.
• Participants will interpret the curved line on a distance vs. time graph as representing a change in velocity (acceleration).
• Participants will be able to compute the acceleration of the car by finding the slope of the best fit line on a velocity vs. time graph.
• Participants will develop and simple lesson implementation plan for their own classrooms based upon changes in motion caused by forces.

#### Materials

• Plastic pipette, thin stem - one per group
• Scissors
• Metric ruler
• Pliers - for teacher use only
• 4-oz plastic cup - one per group
• Pencil compass or small nail
• Tape
• Small toy car such as Matchbox or Hot Wheels - one per group (a toy truck with a flat bed or a van works best for attaching the dropper)

For the Procedure:

• 1 - 2 meter board to be used as a ramp
• Blocks or books to support the ramp
• Narrow track made from one of the following: 2 - 4 meter sticks depending on the length of the ramp; Darda or Hot Wheels car track
• Narrow adding machine tape (1 5/8 inch)
• Tape
• Small toy car with dropper attached (prepared in advance)
• Protractor
• Small cup of water with food coloring added
• Paper towels
• Metric ruler or meter stick
• Graph paper

For the Assessment:

• Stopwatches

#### Procedures

Engagement

Perform the demonstration in the Engagement section of the Student Activity sheet. When discussing the difficulty in making measurements as the car falls, you may wish to introduce the idea that around 1600, the Italian mathematician Galileo Galilei was faced with the same dilemma when he wanted to study the behavior of falling objects. Using a "slow fall" of an object down an inclined plane, Galileo was able to study the behavior of falling bodies.

Your discussion should give you an idea of the strength of the physics backgrounds of the participants in your group.

Assessment: Encourage all participants to take part in the discussion. The evaluation here is informal.

Exploration

To save time, have a dropper car made in advance for each group. However, you may wish to have participants make their own dropper if time allows. They will be more likely to use dropper cars in their own classrooms if they have experience making them.

You will want to follow the general activity directions in the Procedure section of the Student Activity sheet.

Divide your participants into groups of four. You may choose to use the same division of tasks as in the student activity directions or allow each group member to choose his/her own role.

An angle of about 5 to 10o will probably work well for the ramp. The longer the ramp, the more data points you will be able to collect.

It is important to have the paper adding machine tape positioned in the center of the car track. The colored water will leave a more permanent record of where the drops first strike the surface. Since the drops are on the ramp, they tend to run downhill.

Since drops are being made on the paper as the car moves forward, some of the drops may be elongated. Drops should be measured consistently from the end closest to the "zero" position. Due to the narrow end of the dropper, tiny air bubbles may temporarily block the flow of the water drops. Absorbent paper will attract the drops due to capillary action casing the drops to flow again.

The drops will form a pattern on the paper as the car moves down the ramp. The participants will observe the drops getting further and further apart. When the distances are graphed verses the time interval (tu) a curve, rather than a straight line, is formed. This curve indicates that the car must be moving at a changing velocity. A change in velocity is acceleration. Due to irregularities in the cars, friction between the car and the track, or small irregularities in the dropper rates, data points will not be perfectly aligned along the curve. However, an overall curve pattern should be evident.

It is not important here to know the exact value of "tu". The droppers will release their drops at a relatively constant rate. If the participants wish to know the exact rate, they can hold the filled dropper in the air and count the number of drops leaving the dropper in a 30 second time period. Dividing 30 seconds by the number of drops recorded will give you the actual timing of the drops. Repeating this process 2 more time and taking an average of the 3 trials will give a more accurate answer.

If the optional section is preformed, a velocity vs. time graph can be constructed. This graph will have a straight line rather than a curve. The line will have an upward slant. Have participants discuss why the two differing line patterns on the two graphs still represent the same motion.

When computing the slope of the line on the second graph, make sure participants use two points on the line itself. These may not necessarily be actual data points.

Assessment: Monitor the participants' work and discussions. Make sure they are completing the data tables and constructing the graphs. You may want to display each group's graphs in the front of the room. Comparison of the graphs should lead into your discussions in the Explanation section.

Explanation

Both graphs indicate that the car is moving with increasing speed and is therefore accelerating. A curved line on a distance/time graph shows the same motion as a slanted line on the velocity/time graph. Other than the graphs, emphasize that participants had visual proof of the acceleration as they viewed the pattern of drops with increasing distances on the paper.

A more complete explanation of the science involved in this activity can be found in the Student Activity Sheet. However, another way of viewing the data in this experiment is too have the participants cut the paper tape at each dot. If the paper sections are then hung up side by side, in the order in which they originally occurred, a curve at the top edges of the paper strips will be created. This is a method that can be used to demonstrate the motion of the car to students that may have trouble in graphing.

Assessment: Monitor the discussion. Ask participants to give examples from their own teaching experiences relating to the demonstration and understanding of the concept of acceleration in their own classes.

Elaboration

Allow the participants to design their own motion experiments using the materials at hand. They may realize that they also have a pattern of drops across the flat table surface left behind as their car slowed to a stop. They may want to measure the distances between these drops and find the rate of deceleration (negative acceleration) of the car. If so, they should discuss the frictional forces that are causing the car to slow down. They may also wish to experiment with the height of the ramp or the overall distance covered by the car as suggested in the Student Activity.

Have participants discuss how they would apply what they have just learned in their classrooms. Give time for participants to complete the lesson implementation sheet as a first step to moving toward classroom implementation.

Assessment: Check the lesson implementation plans for faithfulness to the lesson just experienced. If possible, visit the participants' classes for onsite assessment.

#### Rationale

Teachers explain to their students that unbalanced forces cause changes in motion or acceleration. However, students have a hard time conceptually moving from the concept of an object moving at a constant velocity to one that is changing in velocity, or accelerating. This activity gives a visual pattern of drops left behind by a toy car accelerating down a ramp. The increasing distances between successive drops indicate that the car is covering more and more distance in comparative time intervals. This leads the observer to conclude that the car must be speeding up or going faster each time interval and helps the observer develop his own understanding of the concept of acceleration.

#### Science Standards

Science as Inquiry:
Content Standard A: As a result of activities in grades 5 - 8, and 9-12, all students should develop:

• Abilities necessary to do scientific inquiry.

Physical Science Standards:
Content Standard B: as a result of their activities in grades 5-8, and 9-12 all students should develop an understanding of:

• Motions and Forces

NSES PROFESSIONAL DEVELOPMENT STANDARD A: Professional development for teachers of science requires learning essential science content through the perspectives and methods of inquiry.

• NSES PROFESSIONAL DEVELOPMENT STANDARD B: Professional development for teachers of science requires integrating knowledge of science, learning, pedagogy, and students; it also requires applying that knowledge to science teaching.

#### Best Teaching Practices

• Discussion
• Inquiry
• Learning Cycle
• Hands-on/Minds-on Learning

#### Time Frame

The opening discussion should take about 5 to 10 minutes. The activities, graphing, and final discussion should take about 90 minutes.

#### Preparation

See the Downhill Racer Worksheet (below) and "Getting Ready" section under the section labeled "Materials".

#### Safety

No special safety precautions and disposal methods are necessary

#### Assessment

Have each group suggest an activity which could be used in their own classrooms to teach the affects of unbalanced forces on motion.

#### Explanation of Science

See the Explanation section of the Downhill Racer Worksheet (below)

#### Handouts

Downhill Racer Worksheet

#### Extensions

Participants may want to tie this activity into Newton's Second Law. If the height of the ramp is increased, the component of gravity which accelerates the car down the ramp increases. The greater the force, the greater the acceleration a=F/m

#### Equity

Form the groups of teachers with diversity in mind. You may want to place participants with weaker physical science backgrounds in groups having participants with stronger backgrounds.

#### Resources

None available for this module

#### References

Physical science activities created for OSCI, summer 2005, in cooperation with Miami University, Center for Chemical Education

OSCI Materials are still available at www.terrificscience.org/osci/physical but it is not known how much longer they will be on the web. The Downhill Racer activity is on pages 99 - 120