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Motion and Force Chapter Four: Forces • 4.1 Forces • 4.2 Friction • 4.3 Forces and Equilibrium Investigation 4A What is a Newton? • What is force and how is it measured? 4.1 The cause of forces • • A force is a push or pull, or any action that has the ability to change motion. The key word here is action, force is an action. 4.1 The cause of forces • Fundamentally, forces come from the interaction between atoms and energy. 4.1 Units of force • • When you measure weight in pounds on a scale, you are measuring the force of gravity acting on the object. The word “pound” comes from the Latin word pondus, which means “weight.” 4.1 Units of force • • The newton (N) is a metric unit of force. A force of 1 newton is the exact amount of force needed to cause a mass of 1 kilogram to speed up by 1 m/s each second. 4.1 Converting newtons and pounds • One pound of force equals 4.48 newtons. 4.1 Gravity and weight • • The force of gravity on an object is called weight. Mass and weight are not the same thing! 4.1 Gravity and weight • • A 10-kilogram rock has a mass of 10 kilograms no matter where it is in the universe. A 10-kilogram rock’s weight however, can vary greatly depending on where it is. 4.1 Calculating weight • The weight equation can be rearranged into three forms to calculate weight, mass, or the strength of gravity. 4.1 The force vector • A force vector has units of newtons, just like all forces, but also includes enough information to tell the direction of the force. • Positive and negative numbers indicate opposite directions. 4.1 Addition of vectors • • It is sometimes helpful to show the strength and direction of a force vector as an arrow. When drawing a force vector, you must choose a scale. If 1 cm = 1 N, how many newtons of force does this line represent? 4.1 Forces in springs and ropes • • Tension is a force that acts in a rope, string, or other object that is pulled. Tension always acts along the direction of the rope. In what direction will each boy go? 4.1 Forces in springs and ropes • • When you apply a compression force to a spring, it collapses to become shorter. The force created by stretching or compressing a spring always acts to return the spring to its natural length.