Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.


Force, in physics, any action or influence that accelerates an object. Force is a vector, which means that it has both direction and magnitude. When several forces act on an object, the forces can be combined to give a net force. The net force acting on an object, the object's mass, and the acceleration of the object are all related to each other by Newton's second law of motion, named after English physicist and mathematician Isaac Newton. The second law of motion states that the acceleration an object experiences multiplied by the mass of the object is equal to the net force acting on an object. Thus, if a given force acts on two objects of different mass, the object with a larger mass will have a lower acceleration. See Mechanics.

An object experiences a force when it is pushed or pulled by another object. For example, shoving a stationary shopping cart applies a force that causes the shopping cart to accelerate. An object can also experience a force because of the influence of a field. For example, a dropped ball accelerates toward the ground because of the presence of the gravitational field (see Gravitation); electrical charges attract or repel each other because of the presence of an electric field (see Electricity).

Usually, several forces act on an object at once. If multiple forces combine to give a net force that is zero, then the object will not accelerate; the object will either remain motionless or continue moving at a constant velocity. For example, if a person pushes a shopping cart with a force equal in magnitude to the force of friction that opposes the cart's motion, the forces will cancel, giving a net force of zero. As a result, the cart will move down the aisle with a constant velocity. If the person suddenly stops pushing the cart, the only force acting on the cart is the frictional force. Since the net force is no longer zero, the cart accelerates: its velocity drops to zero.

In the international system of units, the unit of force is the newton, which is the force that imparts to an object with a mass of 1 kg an acceleration of 1 m/sec2. In English units, the unit of force is the poundal, which is the amount of force that accelerates a 1-lb object 1 ft/sec2.

Forces acting at the molecular and atomic level are also known as interactions. See also Elementary Particles.