In this activity, students are tasked with measuring the mass of a car using simple tools.
Dividing students into groups of 3-4, the instructor begins with a demonstration using a wheeled chair. The instructor pushes the chair (causing it to accelerate) then releases it (allowing it to decelerate). In their groups, students are to create free body diagrams and equations of motion describing the two stages. The instructor should ensure the students don’t forget friction during the acceleration phase.
The instructor then presents a problem: students are to measure the mass of the car. They will have access to several bathroom scales, a stopwatch, and a measuring device (possibly along with other implements such as string which can serve as red herrings). Students discuss how this can be done using their equations of motion, then plan out how they will take their measurements and what parameters they will need, essentially designing an experiment. They need not use the equations of motion they have come up with if they feel they have a better solution; students should feel free to come up with their own experimental designs and be creative.
In an empty stretch of parking lot, the plans are executed using several trial runs. The actual experiments should be done as a class to save time, with students organizing into groups to take on different roles in the experiment and sharing data. Several methods should be attempted, however the “ideal” method is to push the car using the scales, which measures the applied force. Using a constant force, the acceleration can be measured when the force is applied then again after it is released using kinematics (assuming constant acceleration) or by using a ticker tape-style method in which objects such as sandbags are dropped out of the car window at regular intervals. In the second method the acceleration can be found using the slope of a velocity-time graph. They are then left with two equations and two unknowns (the coefficient of friction and the mass).
Working in their original groups, students then use the data they have collected to calculate the mass of the car, then write up a lab report describing their experiment and result.
Note that the downloadable package includes a description of the activity and a student handout, but they differ somewhat from that described here. The original version has been modified by Kevin Lenton for use in his own mechanics course.
Students learn to apply their knowledge of kinematics and dynamics to real world problems.
Students learn to distinguish inertial mass from gravitational mass, and to solve systems of equations.
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