AP Physics C - Unit 2.1: Newton's Laws

free body diagram

A simple diagram that shows all the forces (both magnitude and direction) acting on a single, isolated body.

Newton's 3rd Law

Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force back on the first object.

N3L force pairs

The two equal and opposite forces, described by Newton's 3rd Law, that are always present whenever a force occurs. One force acts on one object and the other force acts on the other object. They never both act on the same object.

force

A push or pull acting on an object that has the ability to cause an object to accelerate (change its state of motion).

applied force

A generic name given to any force that is applied to an object.

normal force (or support force)

The force that is generated when an object comes in contact with a second object and that second object pushes back against it to prevent it from falling through. This force is always perpendicular to the two surfaces in contact.

tension force

The force transmitted through a rope, cable, string, wire, etc. pulled taut by forces acting on both ends. The direction is always parallel to the end of the rope and away from the object being pulled.

spring force (or elastic force)

The force exerted by a spring that is stretched or compressed. Stretched springs pull and compressed springs push. (Many compressible and stretchable objects can act like springs.)

friction force

The force exerted by a surface as an object moves across it (kinetic) or tries to move across it (static). This force always acts opposite to an object's direction of motion.

drag force (or air resistance)

A force that acts upon objects as they travel through a fluid (Air is considered a fluid.) The force always acts opposite to an object's motion. It is frequently neglected when objects are traveling at low speeds but becomes significant at higher speeds.

inertia

The tendency of an object to resist a change in state of motion. (Objects at rest tend to stay at rest; objects in motion tend to stay in motion.)

Newton's 1st Law (also known as The Law of Inertia)

If the sum of the external forces on an object is equal to zero (Fnet = 0), then the object is either at rest or is moving in a straight line at a constant speed.

net force (Fnet)

The vector sum of all the forces acting on an object. We often sum the forces separately for horizontal forces (ΣFx) and vertical forces (ΣFy).

balanced forces

When all the external forces balance (or "cancel out") so that the net force = 0 in both the x and y direction. In this condition, the object does not accelerate.

unbalanced forces

When all the external forces do NOT balance and there is some leftover net force in the x and/or y direction. This condition causes the object to accelerate.

gravity

A physical phenomenon that produces an attractive force between every pair of objects in the universe. This force depends only on the mass of the two objects and the distance between them.

mass

The amount of matter in an object; or the measure of an object's inertia (tendency to resist changes in motion).

weight

The measure of the force of gravity of the Earth (or whichever planet you are on) pulling downward on an object.

Newton's 2nd Law

If the sum of the external forces on an object is NOT equal to zero (Fnet ≠ 0), then the forces are unbalanced and the object will accelerate in the same direction as the net force according to the equation Fnet = m*a.

dynamics

The part of mechanics that describes the motion of bodies under the action of forces applied to them.

gravitational force

The force with which the earth, moon, or other massively large objects attract other objects towards itself. All objects on Earth experience this force which is always directed "downward" towards the center of the earth. On the Earth's surface, this force is equal to the weight of the object.

inertial reference frame

A reference frame in which Newton's Laws of Motion are valid. For this to happen, the reference frame can be moving but must have 0 acceleration.

kinetic friction (or sliding friction)

The force exerted between 2 surfaces sliding against each other (must be moving).

static friction

The force exerted between 2 surfaces trying to slide against each other (but not yet moving).

coefficient of static friction

For an object experiencing friction, the ratio of the maximum static friction force to the normal force.

coefficient of kinetic (sliding) friction

For an object experiencing friction, the ratio of the kinetic friction force to the normal force.

translational equilibrium

An object is said to be in this state when all of the forces in the x-direction and all of the forces is the y-direction are balanced (Fnet = 0). When in this state, the object is either at rest or moving in a straight line at constant speed.

contact forces

A force exerted when an object comes in contact with another object. Examples include applied force, drag force, friction force, normal force, spring force, tension force, etc.

non-contact forces (or action-at-a-distance forces)

A force exerted over a distance where no contact is required. Examples include gravitational force, magnetic force, electric force, etc.

acceleration due to gravity (g)

The acceleration caused by the force of gravity on an object in free fall. On Earth, this value is 9.81 m/s^2.

system

In physics, any group of one or more objects we choose to consider for analysis. Everything outside the system is ignored except for its effects on the system.

external force

A force caused by an interaction with an object other than itself. They are considered to act from outside the current system of interest and are responsible for accelerations to the object if they are unbalanced.

internal force

A force caused by interactions between objects within the system. These forces cancel each other out and do not cause any external forces nor any acceleration to the center of mass of the system. We do not consider these forces when drawing free body diagrams.