acceleration: a change in the speed and/or direction of motion of an object. A car that is speeding up or slowing down is accelerating. A car that is turning is also accelerating.
air resistance: a kind of friction in which air molecules rub against a moving object, creating a force that opposes the motion of the object. Air resistance makes it hard for airplanes to fly fast or for you to throw a whiffle ball very far.
collision: striking of two or more moving objects against one another.
elastic collision: a collision in which the sum of the kinetic energies of the colliding objects remains exactly the same after the collision (the kinetic energy is conserved).
inelastic collision: collision in which the sum of all of the kinetic energies of the colliding objects are changed into another form of energy, such as heat or sound.
conserved: kept whole or complete. Energy is always conserved because a system of objects neither loses nor gains energy. Often, energy can be changed from one form to another (like potential energy being changed to kinetic energy or kinetic energy being changed to heat), but it is always conserved.
energy: the ability to do work or cause change. Energy can come in the form of kinetic energy, potential energy, heat, electrical energy, or even chemical energy.
kinetic energy: the energy that an object has because of its speed.
potential energy: the energy that an object has because of its position.
equilibrium: when all forces (including reaction forces) acting on an object are in balance so that there is no net force on the object. An object in equilibrium will not accelerate.
force: a push or pull on an object. Forces result from interactions between objects, either by the objects physically touching each other, or gravity and other forces that act over long distances. Usually, you exert a force on an object by pushing on it.
net force: the combined force of all the separate forces acting on an object.
reaction force: a force that occurs in opposition to and that is equal to another force.
frequency: how many times per second an oscillating object goes through one cycle. Frequency also describes the pitch of a sound and the color of light, since both sound and light are produced by things that oscillate.
friction: a force that appears when two substances are in contact with each other and that tends to oppose motion between those objects. Friction causes a rolling ball to come to a stop eventually and it keeps you from slipping as you walk. See also air resistance.
g: g is the acceleration of gravity at the Earth's surface, which is 9.8 m/s². g is used to calculate the weight of an object (weight=mg) or the potential energy of an object (potential energy=mgh).
gravity: a force that pulls all objects together. Gravity acts over any distance, but is stronger the closer together the objects are.
inertia: the tendency of an object to resist changes in speed or direction. The effect of inertia is explained by Newton's First Law. Objects with more mass have more inertia.
magnitude: a measure of the amount of something. Magnitude can refer to how much mass an object has, how fast it's going, how much energy it has, etc.
mass: the amount of matter in an object. Objects with greater mass will weigh more, although they won't necessarily be larger. For instance, a cup of water might weigh more than a whole bag of popcorn even though it is smaller. The cup of water therefore has more mass. The greater an object's mass, the more inertia it has.
momentum: the mass of an object multiplied by its velocity. In a collision between two or more objects, the sum of the momentums of all the objects is always conserved. See also collision.
Newton's First Law: an object in motion tends to stay in motion at the same speed and in the same direction. An object at rest tends to stay at rest. See also inertia.
Newton's Second Law: the acceleration of an object is proportional to the net force on the object and inversely proportional to the mass of the object. Newton's Second Law produces the equation f=ma.
Newton's Third Law: every force has an equal and opposite force. If one object pushes a second object towards the north, then the second object pushes on the first object just as hard towards the south.
oscillate: to go back and forth. A pendulum oscillates.
period: the amount of time it takes an oscillating object to go through one cycle. See also frequency.
simple harmonic motion: the motion of a pendulum or any one of several other objects that oscillate at a constant frequency.
simple machine: a device, such as a lever, inclined plane, or screw, that changes the direction or magnitude of an applied force.
speed: how fast an object is moving. Speed is not the same as velocity, because speed has nothing to do with what direction an object is moving in, while velocity does.
torque: a type of force that turns an object around a central point or axis. Torque is calculated by multiplying the force applied to the object by the distance between the center (or axis) and where the force is applied. When you turn a doorknob, you are applying a torque.
velocity: a measurement of the speed and direction of an object. In physics, the velocity of an object is described by two numbers: the magnitude (how fast it's going) and the direction (where it's going -- usually measure in degrees, like on a compass).
weight: an object's mass times the acceleration of gravity. An object weighs more on the Earth than it does on the Moon because, even though it has the same mass, the acceleration of gravity is greater on the Earth than it is on the Moon.