With little or no friction to stop it, a moving object will keep moving at a constant velocity. This experiment explores a few simple ways you can take friction out of the picture.
What You Need
- Hover Puck
- tape measure
- 5 stopwatches
- masking tape
- several people to serve as timers
- Set up a course that is horizontal and free of obstructions. Do a trial run to make sure the Hover Puck does not move unless it's pushed and that it follows a reasonably straight line. (If you don't have a Hover Puck, a basketball or other similar object will do.)
- Place distance markers, such as masking tape labels, at regular intervals. (Typically in physics, meters are used for distance. However, for this project any convenient unit can work as long as you're consistent throughout.)
- Each of the timers should be assigned to measure the time at a specific distance along the path.
- Timers should set their stopwatches to read zero and be prepared to start measuring the time as soon as the object starts moving.
- Push the puck (or basketball) in the designated direction. Start with a medium push. See Figure 1-1.
- As the puck (or basketball) passes each mark, each timer should stop the stopwatch and note the time.
- Repeat with a slow push. A slow push is defined as slower than the medium push, but fast enough not to be pulled off course or stopped by friction.
- Repeat with a medium push
- Repeat with a fast push. This may be the most challenging one to time, especially for the first couple of timers.
- The velocity for each of the runners will be the slope of the graph where distance is on the y-axis and time is on the x-axis.
- Place the people with the timers on the 10, 20, 30, 40, and 50 yard lines of a football field.
- Use a runner or several runners to run from the goal line to the 50 yard line.
- As in number 2, get the time that each runner passes the designated distance marker, and then plot and interpret the results.
With constant velocity, each of the graphs should be linear (a straight line). The fastest runner has the highest slope, followed by the medium runner, with the slowest runner bringing up the rear.
If, for some reason, the motion was not perfectly constant, the points that differed will not be on the line. For instance, if the assumption that friction can be ignored is not completely valid, you may see some deceleration. In that case, the overall linear curve may be seen to taper off with a lower slope than the earlier points. If these data come from runners, it can be used to determine how steady the runners actually are. Also, if the runners start from zero, the first 10 yards will show an upward curve indicating acceleration.
Figure 1-2 shows expected results for three runs of 0.5, 1.0, and 1.5 meters per second (m/s).
Why It Works
Average velocity can be thought of as the distance you go divided by the amount of time it took to get there. More specifically, we can say average velocity is the change in distance divided by the change in time. v =Δd/Δt is the slope of the distance versus time graph. (Δ is the Greek letter delta, which means "change in.")
Other Things to Try
This experiment can be done using a person riding in style in a Hovercraft, as pictured in Figure 1-3. This can be done as an interesting way to do the previous experiment or just simply for the fun of doing it.
Because of the nearly frictionless motion, the person moves at constant velocity and makes a perfect object to measure at various speeds. You can purchase a Hovercraft (PASCO, part number ME 9838).
A Hovercraft can also be built by following these basic steps:
- Drill a hole in the center of a 3-to-4 foot diameter piece of plywood.
- Cut a hole halfway between the center and the edge just large enough to fit the end of a leaf blower.
- Staple a plastic sheet to the bottom of the Hovercraft. Trim off the excess plastic.
- Insert a bolt from the underside of the Hovercraft, through a plastic spacer (made from a plastic coffee-can lid). Attach the bolt through washers on the top and bottom, and then secure it with a nut.
- Tape all the seals between the leaf blower and the plywood, and the plastic sheet and the plywood, to make them as airtight as possible.
- Cut several approximately 2-inch diameter vent holes in the plastic sheet a few inches from the outer circumference of the plastic spacer.
- With the leaf blower turned on, a cushion of air should enable a person to move with a minimum of friction.
Detailed plans can be found at http://amasci.com/amateur/hovercft.html.
The tendency of a moving object to keep moving is called inertia, which is addressed in Newton's first law. This is the subject of experiments that follow.
Constant velocity is represented by a straight line on a distance versus time graph. The slope of the line is equal to the average velocity.