Falling objects accelerate in response to the force exerted on them by Earth’s gravity. Different objects accelerate at the same rate, regardless of their mass. This illustration shows the speed at which a ball and a cat would be moving and the distance each would have fallen at intervals of a tenth of a second during a short fall.

Neglecting air resistance, a ball thrown into the air at an angle will travel in a parabolic path. The velocity of the ball (V) has independent vertical (V) and horizontal (H) components; the horizontal component stays the same the entire time the ball is in the air, while the vertical component, the only component affected by gravity, changes continuously while the ball is aloft.

Often, an object will have many forces acting on it simultaneously. Calculating the effect of each of the forces separately can be extremely complex and difficult. However, forces are vectors, and as such, any number of forces can be combined into a single net force vector (R) from which the object’s behavior can be determined.

Newton’s second law states that the net force on an object is proportional to the acceleration that object undergoes. If there is no net force, then according to Newton’s first law, there can be no acceleration. A book on a table experiences a downward force due to gravity, and an upward force due to the table pushing on the book (called the normal force). The two forces cancel each other out exactly; there is no net force, so the book does not accelerate off the table.

Microscopic bumps on surfaces cause friction. When two surfaces contact each other, tiny bumps on each of the surfaces tend to run into each other, preventing the surfaces from moving past each other smoothly. An effective lubricant forms a layer between two surfaces that prevents the bumps on the surfaces from contacting each other; as a result the surfaces move past each other easily.

A moving pendulum changes potential energy into kinetic energy and back again. When the bob (weight on the end of string) is first released, it has potential energy due to its height, but no kinetic energy since it is not yet moving. As the bob accelerates downward, potential energy is traded for kinetic. At the bottom of its swing, the bob has no potential energy since it cannot fall any further. The bob is moving quickly at this point since all of its former potential energy has been transformed into kinetic energy.