0.1 Forces

An object in motion, or at rest, will not change its state of motion unless a force is applied. This resistance to changes in motion is called inertia . To be clear, a change in motion is not just beginning to move from astop. Slowing down, speeding up, and changing direction are all changes in motion. The only way to change a object'smotion is to apply a force to that object. A book slid across a table only comes to a stop because of the frictional forcesacting on it. Inertia is proportional to mass, so a more massive object is more difficult to move or stop than alighter one, even on a frictionless surface. This module will consider forces and friction, which both act on an object's inertia.

Just as a book slides until a force opposes its motion, a disc will spin until its rotation is opposedby some force. This property is aptly named rotational inertia . One of the most common applications of rotational inertia is shown in . Many children's toys use rotational inertia. In friction-drivecars, the child pushes the car forward several times to set an internal flywheel in motion. When the car is put down,the flywheel is still spinning and the car moves. This is an interesting way to store energy -- in kinetic, rather thanpotential form. A flywheel could conceivably be used to store energy to keep smallrobot operating after its motors were required to be shut off. Rotational inertia is also used to avoidchanges in motion for such objects as record players, where it is important to maintain rotation at a constant speed.

Force

Whether a force is the push of a motor or the pull of gravity, the important characteristics arethe magnitude and direction of the force, and the mass and previous state of motion of the object being affected. Bypushing on a moving car, one can either cause it to gain speed or come to a stop, depending on which direction the force isapplied, and that same force applied to a feather would be expected to more drastically affect the motion of thefeather.

It is common practice to determine the expected changes in motion that an object will experience due to aparticular force with the aid of a free body diagram . A diagram can tell us at a glance in which direction we would expect an object to accelerate ordecelerate. A free body diagram shows all of the forces acting on an object, even if their effects are balanced out by anotherforce. We will use free body diagrams to consider different situations involving the lamp that you find at your lab station( ).

One force that always acts on the lamp is gravity. This familiar force would accelerate the lamp downwardtoward the center of the earth if left unchallenged. However, when the lamp is placed on a table itdoes not move downward because the table holds it up. The lamp is pushing down on the table and the table is pushing up on thelamp. This pair of forces is an action-reaction pair: equal and opposite forces acting on two different objects in contact. Thereaction force from the table is called the normal force because this force is oriented normal (perpendicular) to the surface of the table. The arrowsrepresenting the forces are labeled. The symbols over the labels remind us that the forces are vector quantities and that thedirection in which the force is applied is important. The length of the force vector should be proportional to theirmagnitudes.