Image 1: Contrived Image of Door Opening
Above is a picture of a door taken by the author in his living room. By definition, doors are a hinged, sliding, or revolving barrier at the entrance to a building, room, vehicle, or other enclosed areas (1). Doors can be considered a lever, which is a rigid bar resting on a pivot. Specifically, they are considered second class levers because the force is usually applied on the bar at the end opposite to the pivot and the weight lies in the center of the bar. Doors are an excellent example of the physics principle of torque in our everyday lives. Torque is an influence, or force, which when applied to an object can change its rotational motion. Torque is to rotational physics what force is to dynamics. Torque can be calculated by multiplying the force applied by sin of theta, where theta is the angle between the level arm and the applied, by the distance from the axis of rotation (pivot) where said force was applied. On the door, the force applied was perpendicular to the lever arm because the knob is attached perpendicularly and thus, sin of theta where theta is 90 degrees is equal to zero. Because the knob is on the end of the door that is away from the axis of rotation, or the hinges, relatively meager force has to be applied in order to open a door. The distance between the knob and the hinge is so large that even if a small force is applied to open the door, a large torque will be applied because torque is equal to force times sin of theta times distance. The knob was placed on the end of doors by design to make opening and closing doors more convenient. Doors are also an excellent example of the physics principle of action-reaction forces. Newton’s third law states that for every action, there is an equal (in magnitude) but opposite (in direction) reaction. When the door handle is pulled with force x to open a door, the door handle applies a force similar in magnitude to force x but opposite in direction, on the door opener. The force applied on the door handle and by extension, the door, does more work than the force applied on you by the door handle because your mass is much greater than that of the door. Force equals mass times acceleration and given that your mass is much greater than the doors, the same force applied to a door of lesser mass will cause for the door to accelerate much more than if the same force was applied to you. Furthermore, the force applied on the door also applies a torque on the door (previously explained) which because of the distance between the door handle and the hinge, causes for a greater rotational acceleration. In summary, a door is a great example of the physics principles of torque and action-reaction forces.
[1] “Types of Lever - Simple Machines - Physics.” Quatr.us Study Guides, 15 Jan. 2019, quatr.us/physics/types-lever-simple-machines-physics.htm.
[1] “Types of Lever - Simple Machines - Physics.” Quatr.us Study Guides, 15 Jan. 2019, quatr.us/physics/types-lever-simple-machines-physics.htm.