Motion can be measured by speed, velocity, and acceleration. Speed is a measure of how fast something is moving. It is a ratio of the distance covered between two locations to the time that elapsed while moving between the two locations. The average speed considers the distance covered during some period of time, while the instantaneous speed is the speed at some specific instant. Velocity is a measure of the speed and direction of a moving object. Acceleration is a change of velocity per unit of time.

A force is a push or a pull that can change the motion of an object. The net force is the sum of all the forces acting on an object.

Galileo determined that a continuously applied force is not necessary for motion and defined the concept of inertia: an object remains in unchanging motion in the absence of a net force. Galileo also determined that falling objects accelerate toward the earth's surface independent of the weight of the object. He found the acceleration due to gravity, g, to be 9.8 m/s² (32 ft/s²), and the distance an object falls is proportional to the square of the time of free fall (d α t²).

Compound motion occurs when an object is projected into the air. Compound motion can be described by splitting the motion into vertical and horizontal parts. The acceleration due to gravity, g, is a constant that is acting at all times and acts independently of any motion that an object has. The path of an object that is projected at some angle to the horizon is therefore a parabola.

Newton's first law of motion is concerned with the motion of an object and the lack of a net force. Also known as the law of inertia, the first law states that an object will retain its state of straight-line motion (or state of rest) unless a net force acts on it.

The second law of motion describes a relationship between net force, mass, and acceleration. A newton of force is the force needed to give a 1.0 kg mass an acceleration of 1.0 m/s².

Weight is the downward force that results from the earth's gravity acting on the mass of an object.Weight is measured in newtons in the metric system and pounds in the English system.

Newton's third law of motion states that forces are produced by the interaction of two different objects. These forces always occur in matched pairs that are equal in size and opposite in direction.

Momentum is the product of the mass of an object and its velocity. In the absence of external forces, the momentum of a group of interacting objects always remains the same. This relationship is the law of conservation of momentum. Impulse is a change of momentum equal to a force times the time of application.

An object moving in a circular path must have a force acting on it, since it does not move in a straight line. The force that pulls an object out of its straight-line path is called a centripetal force. The centripetal force needed to keep an object in a circular path depends on the mass of the object, its velocity, and the radius of the circle.

The universal law of gravitation is a relationship between the masses of two objects, the distance between the objects, and a proportionality constant.Newton was able to use this relationship to show that gravitational attraction provides the centripetal force that keeps the moon in its orbit.

Summary of Equations

2.1

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