




The information below is intended to provide introductory material
for elementary students and further material for high school students.
Please keep in mind that not all demonstrations are presented
at each show, and each topic may not be covered.






A stationary object will not move unless it is acted on by an unbalanced force.
This statement is half of Newton's First Law, or the Law of Inertia. Newton created his "laws" to describe how objects interact.
They have been verified by several centuries of testing and observations.
An unbalanced force is a push or a pull which is not balanced by an equal and opposite push or pull.
For instance, two people pulling on opposite ends of a rope with the same force are each exerting a force on the rope.
Since each person is pulling with an equal amount, and the directions of the pulls are opposite, the two forces are balanced.
If one person were to let go, however, the force on the rope would become unbalanced. This unbalanced force would cause the rope to move.




An object at rest will remain at rest unless it is acted on by a net, external force.
This statement is half of Newton's First Law, or the Law of Inertia. Newton created his "laws" to describe how objects interact.
They have been verified by several centuries of testing and observations.
Force is a vector. This means that a force entails both a value and a direction.
To determine the total net, external force acting on an object, it is necessary to determine all forces acting on the object, and then perform vector addition.
For instance, a person sitting on a chair is experiencing a downward force due to gravity.
He is also experiencing, however, an upward force due to the chair. These two forces, when added using vector addition, cancel each other out, giving no net, external force.
Since there is no net, external force, the person does not move.
Pulling the chair out from underneath him, however, changes the situation.
The person is still experiencing the force downward due to gravity, but there is no longer a force upward due to the chair.
There is a total net, external force acting on the person. This force causes him to fall to the ground.




The following demonstrations illustrate this physics topic:




Sponsored by the Physics Department and the Center for Science, Mathematics, and Engineering Education  University of Virginia
