Weighing a Car with Bathroom Scales
This lab will be assessed on all the assessment criteria.
Introduction:
In this lab the entire lab group will work together to design and execute an experiment to determine the mass of a car. The usual method for weighing objects is to place them on some type of scale or balance. However this method will not work when weighing a car with bathroom scales. Instead we will make use of Newton’s Second Law of motion:
Fnet=ma
If a net force acts on the car and the car's subsequent acceleration is measured then the mass of the car can be determined.
Procedure:
Many of the details of the experimental design will be left to the student group. Here is a broad outline.
The measurement of applied force will be made using the bathroom scales, which will be secured to the back of the car. The tricky part will be to apply a constant force for the duration of each run. This will require practice. The car pushers will need to decide the appropriate values of the applied force so that a measurable acceleration can be obtained.
The measurement of acceleration will necessarily be indirect. Here are
two approaches:
2. The velocity of the car should increase uniformly with time according to the equation v=at. Velocity timers can determine the velocity of the car at each distance mark by timing how long it takes the car to pass by a small interval on either side of the distance mark. Graphical analysis of velocity as a function of time should give a value of acceleration.
Tasks
What follows is an outline of the different tasks that should be carried out. The number of people required for each task is indicated in parenthesis. Think about your talents and where you would best serve the group.
Car Pushers (2)
We will need two or three car pushers. The car must be pushed with a constant force. This will need practice ahead of time. Before the experiment begins the car pushers must establish the appropriate value of the applied force that will yield a measurable acceleration. What is measurable depends on the length of the track, the number of distance markers, and the size of the intervals for measuring velocity. Liase with the track designers.
Car Driver (1)
We will need three car riders: one to steer, one to drop cones, and one to time. As the car riders will contribute to the mass of the car they must weigh themselves ahead of time. car riders will need to choose a suitable time interval for cone dropping so that sufficient cones are dropped to yield a reasonable graph. This will depend on the time for each run and consequently will depend on the acceleration. Liase with car Pushers.
Distance timers (6)
There should be six different distances marked over the length of the
track so that a graph with sufficient data points can be obtained.. Inform
the record keeper of your time before making the next measurement. Distance
timers should also design the track. You will work with all the other teams
to design a track that meets the requirement of the experiment. After the
track is designed you will be responsible for informing the record keepers
of all distance and interval measurements.
Velocity timers (6)
Velocity timers will be responsible for measuring the time it takes the car to pass a small interval around each distance marker. Inform the record keepers of each of your times at the end of each run. The velocity is actually changing continuously through out the run, so to obtain an estimate of the velocity at each distance marker the intervals should be as small as possible. It is not really possible to time an event that takes less than 1 second, and even timing a 1 second event reliably takes some practice. Velocity timers should practice timing an event that takes about a second before the experiment is conducted. Try timing a pendulum as it completes one period. When all the timers are obtaining accurate and consistent measurements they can join the track designers to choose appropriate intervals about the distance markers. Notice these intervals do not need to be the same length at each marker.
Record Keeper. (1)
This is probably the most important job of the experiment. You will
keep track of all the data generated in the experiment. You should set
up a data table ahead of time with one column for the distance from the
start line to each of the distance marks, one column for the interval distance
around each distance mark and then six more columns for the various times
that are recorded at each distance mark. As there will be several runs
you should have several copies of these data tables ready ahead of time.
You will be responsible for copying completed and well-labelled tables
for the rest of the class.
Chief Organizer (1)
You will make sure everybody is talking to everybody else and will liase with the various teams to make sure they are getting the job done. Make sure the car pushers can push with constant force. Make sure the velocity timers can reliably time intervals of 1 second. Your job is to try and get the first run underway in less than an hour. You should allow for one or two practice runs, since their will be communication issues that need to be resolved before actual useful data is recorded. You will blow the whistle to start each run.
Materials
1 car
2 or 3 rolls of duct tape
15 stopwatches
2 bathroom scales.
2 100 m tapes
Pendulum bobs