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.