Measuring Forces on an Inclined PlaneJames Lincoln
The Forces on an Inclined Plane Demonstrator is a new piece of physics equipment that can help make the abstract concepts of vector components of forces a tangible reality. The innovation of the device is that it can be manipulated at will. The angles can be set and reset quickly and the forces measured fairly quickly.
The device breaks the weight of an object into its component forces and allows for accurate data to be taken without having to set up clumsy and cumbersome ramps.
Each module comes with a built in scale (that measures how the Normal Force varies with the angle of inclination) and a parallel spring scale (that measures how the Parallel Force increases with the angle of inclination).
The module contains three unique features. Built in scale, protractor, and spring scale mount.
The measurements rely heavily on Balanced Forces. Balanced Forces result in zero acceleration. The action of gravity pulling the cart downhill is balanced by the equal and opposite action of the spring scale pulling the cart uphill. Similarly, the component of the weight that is wasted in the hill is balanced by a reaction force which is perpendicular to the hill. This is called the Normal Force (normal meaning perpendicular).
The sine and cosine relationships will come naturally out of well-calibrated data.
Create Graphs of Sine & Cosine: The two forces measured by the device will trace out the sine and cosine curves (with an amplitude mg) as the device is rotated through angle.
Verify Specific Predictions: Test out the special triangles: 45 45 90, 30 60 90, 3 4 5, to reinforce the behavior of the forces as the vary with tilt angle. For example, 5N tilted to an angle of 37 degrees will have a normal force of 4N and a downhill force of 3N. But what will happen for 53 degrees?
In an open-ended lab the students invent their own procedures and hypothesize the relationships without formal instruction.
Open-Ended Lab: Have students try to invent the formulas for themselves. Taking data from the digital balance and from the spring scale to determine the relationships from scratch. This style of lab is consistent with the NGSS Standards and the AP Physics 1 curriculum.
Tips for Success
While taking measurements the user will have to “tare” the scale every time. This is because the plate that sits on the scale is itself an object with weight. Once the angle is selected, simply lift the cart and tare then reweigh.
It is also important to recalibrate the spring scale when making a measurement of the component downhill.
How it looks to correctly set 45 degrees.
Be careful not to confuse the screw that holds the up the incline plane with the angle indicator. The angle is measured best by the lower edge of the plane being in line with the angle in question.
James Lincoln is an experienced physics teacher with graduate degrees in education and applied physics. He has become known nationally as a physics education expert specializing in original demonstrations, the history of physics, and innovative hands-on instruction.
The American Association of Physics Teachers and the Brown Foundation have funded his prior physics film series and SCAAPT’s New Physics Teacher Workshops.
Lincoln currently serves as the Chair of AAPT’s Committee on Apparatus and has served as President of the Southern California Chapter of the AAPT, as a member of the California State Advisory for the Next Generation Science Standards, and as an AP Physics Exam Reader. He has also produced Videos Series for UCLA’s Physics Demos Project, Arbor Scientific, eHow.com, About.com, and edX.org.