Newton’s first law states that a body at rest remains at rest or, if moving, remains in motion in a straight line at a constant speed, unless acted on by a net external force. This law is also known as the law of inertia.
Inertia is the tendency of an object at rest to remain at rest or, if moving, to remain in motion at constant velocity. Inertia is related to an object’s mass.
Friction is a force that opposes motion and causes an object or system to slow down.
Acceleration is a change in velocity, meaning a change in speed, direction, or both.
An external force acts on a system from outside the system, as opposed to internal forces, which act between components within the system.
Newton’s second law of motion states that the acceleration of a system is directly proportional to and in the same direction as the net external force acting on the system, and inversely proportional to the system’s mass.
In equation form, Newton’s second law of motion is or This is sometimes written as or .
The weight of an object of mass m is the force of gravity that acts on it. From Newton’s second law, weight is given by
If the only force acting on an object is its weight, then the object is in freefall.
Newton’s third law of motion states that when one body exerts a force on a second body, the first body experiences a force that is equal in magnitude and opposite in direction to the force that it exerts.
When an object rests on a surface, the surface applies a force on the object that opposes the weight of the object. This force acts perpendicular to the surface and is called the normal force.
The pulling force that acts along a stretched flexible connector, such as a rope or cable, is called tension. When a rope supports the weight of an object at rest, the tension in the rope is equal to the weight of the object.
Thrust is a force that pushes an object forward in response to the backward ejection of mass by the object. Rockets and airplanes are pushed forward by thrust.