Author: Sandy Van Natta
Source: The Motion of a Battery Powered Car - taken from OSCI 7-10 Physical Science Materials developed in 2005
Students use the variables of time and distance traveled to observe the characteristics of a distance vs time graphs for the motion of a battery powered car operating with one battery then again with two batteries. This allows students to explore how the number of batteries used to power the car affects the characteristics of distance vs time graphs. Students then use the time and distance data collected for the motion of the cars in the two trials to calculate the average speed of the car in each case. The average speeds will then be compared to the slopes of the respective distance vs time graphs.
What should students know as a result of this lesson?
What should the students be able to do as a result of this lesson?
For Getting Ready:
For the Engagement:
For the Procedure:
Set a small wind-up toy in a central location where is can be seen by students. Wind the toy up and allow it to move across a smooth level surface. Pick the toy back off the surface before it can begin to slow to a stop. Ask students to describe the motion of the toy. Most students will say that the toy is moving at a constant rate for most of its "run". (The toy does "speed up" as it first starts off but most of its motion will appear to be at a steady rate.) Ask the students how they could prove that the toy was moving at a constant rate. Help your students figure out that they would have to know how far the toy traveled in a given amount of time, then compare equal time periods, in order to accurately describe the motion of the toy.
Assessment: Try to involve all students in the discussion. Ask students to give examples of objects observed in their daily lives that might be moving at a constant rate.
Part A: One Battery Car
Part B: Two Battery Car
Part C: Average Speed
Assessment: See that students are following proper laboratory procedures. Make sure each group is recording all their data and constructing their graphs.
Discuss the student's observations and graphs. Help students gain the following understanding. "How far?", How long?", and "How fast?" are questions asked when motion is studied. For example, if a 100 m dash in a track and field event is watched, the observer wants to know how long the race was? - 100 m; how long it took to run the race? - about 9.8 seconds; and what was the average speed of the runner? - 10.2 m/sec. The motion of a car can be described by measuring the distance and time to calculate the speed.
How Far? - Distance
In order to determine the distance traveled by an object, you first have to establish the car's location (position) as well as its direction. A reference point from which distances can be measured must be selected. In this activity, the first reference point is the starting position which is labeled d=0. This position, along with the other marks made at 100 cm intervals in a straight line, provides a reference frame to be used to determine the car's location. The distance traveled is the change from one position to another relative to our frame of reference. This change in position is also called displacement. Displacement is a distance and the direction of the changing position of an object.
How Long? - Time
When the car is at the starting position of d=0, the time is also 0. The car and the stopwatches were started at the same time. Every time the car passes one of the reference marks at every 100 cm, the time on the stopwatch is recorded. The time for a particular distance traveled is the time recorded for that reference mark at that distance from d=0. To find the time interval between any two reference marks (for example from the 100 cm mark to the 200 cm mark), subtract the time recorded at the 100 cm mark from the time recorded at the 200 cm mark. This will be the amount of time needed to travel the 100 cm from the 100cm to the 200 cm mark.
How Fast? - Speed or Velocity
As the observer watches the car move across the frame of reference, he/she can decide qualitatively whether the car is moving fast or slow. Calculating a numerical value for the speed of the car gives a more precise meaning to the terms fast or slow. The average speed of the car can be found by dividing the distance traveled by the time needed to travel that distance.
vavg = d/t
vavg stands for average speed or velocity. Speed is actually a scalar quantity that only tells how fast an object is moving. Velocity is a vector quantity that tells not only how fast an object is moving but also what direction the object is moving. Once values for vavg have been calculated, it is easy to determine which car is moving faster or slower. The car with the greater numerical value for vavg is moving faster than the car with the lower value.
When graphs of the cars' motion are plotted, the data points will produce nearly a straight line. This indicates that both time and distance are changing at a steady rate. The car is moving at a constant speed. The line with the steepest slope represents the car moving with the higher speed. The numerical value of the slope equals the average speed of the car.
Assessment: Collect and evaluate the students' data sheets and graphs.
Have students design an experiment to determine the velocity of another object moving at a constant speed such as the wind-up toy use in the Engagement section. Or, students may want to predict what would happen to the speed of the battery car if it moved up an inclined plane. If inclined planes are used, they should be at least 1 to 2 meters long so students can record as many data points as possible. However, distances used will need to be measured in much smaller increments than 100 cm. This will depend on the speed of the car and the length on the inclined plane. Distances of 25 cm may be tried. Note: the car will still move up the inclined plane at a constant velocity. However the overall average velocity will be less than that on a car moving on a level surface.
Assessment: Students should write out the procedures, data tables, and data relating to their experiment. They should construct graphs to represent their results. Using their data and graphs as evidence, students should write a conclusion relating to the motion of their car or toy under the circumstances studied.
Students need to be able to make measurements with metersticks and stopwatches. Students should be able to create graphs using x and y coordinates and find slopes of lines.
Common Core Standards:
If time and distance are changing at a steady rate, an object is moving at a constant velocity. Such motion can be plotted on a graph (time on the X axis and distance on the Y axis) and produces a straight line with a positive slope. The slope of the line is the numerical value of the average speed of the object. The steeper the slope, the greater the overall average velocity of the object.
No special safety or disposal procedures are required.
Whenever an object is moving at a constant speed in a given direction it has a constant velocity. A car in cruise control moving on a freeway or a walker moving with a steady even pace covers equal distances in comparative amounts of time. These are examples of objects moving with a constant velocity.
Give student a series of distance vs. time graphs representing the motion of moving objects. Some of the graphs should show straight lines with positive slopes. Others may have curved lines or horizontal lines. Ask students to pick out the objects moving at constant velocities (the graphs showing straight lines with positive slopes) and determine the average speeds of each object from the data presented.
Grouping Suggestions Try to place students with varying backgrounds and abilities in each group. Having a student with strong mathematical skills in each group will help with the groups' calculations.
Pacing/Suggested Time: Getty ready will take about 15 minutes. The remaining activity should take about 90 minutes to complete.