Motion and Force

Chapter Thirteen: Forces 4.2 Addition of Forces and Equilibrium

Investigation 13A What is a Newton? What is force and how is it measured?

13.1 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.

13.1 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.”

13.1 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.

13.1 Converting newtons and pounds One pound of force equals 4.48 newtons.

13.1 Gravity and weight The force of gravity on an object is called weight. Mass and weight are not the same thing!

13.1 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.

13.1 Calculating weight The weight equation can be rearranged into three forms to calculate weight, mass, or the strength of gravity.

13.1 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.

13.1 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?

13.1 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?

13.1 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.

Investigation 13B Friction How does friction affect motion?

13.1 Friction Friction is a force that resists the motion of objects or surfaces. Many kinds of friction exist.

13.1 Friction

13.1 Friction Friction depends on both of the surfaces in contact. When the hockey puck slides on ice, a thin layer of water between the rubber and the ice allows the puck to slide easily.

13.1 Identifying friction forces Friction is a force, measured in newtons just like any other force. Static friction keeps an object at rest from moving.

13.1 Identifying friction forces Sliding friction is a force that resists the motion of an object moving across a surface.

13.1 A model for friction Friction depends on a material’s properties such as roughness, how clean the surfaces are, and other factors. The greater the force squeezing two surfaces together, the greater the friction force.

13.1 Reducing the force of friction Unless a force is constantly applied, friction will slow all motion to a stop eventually. It is impossible to completely get rid of friction, but it can be reduced.

13.1 Reducing the force of friction The friction between a shaft (the long pole in the picture) and an outer part of a machine produces a lot of heat. Friction can be reduced by placing ball bearings between the shaft and the outer part.

13.1 Useful friction Friction is also important to anyone driving a car. Grooved tire treads allow space for water to be channeled away from the road-tire contac point, allowing for more friction in wet conditions.

13.1 Useful friction Shoes are designed to increase the friction between their soles and the ground. Why do you think these shoes increase friction? Players wearing cleats can apply much greater forces against the ground to help them move and to keep from slipping.

13.1 Friction and energy Friction changes energy of motion into heat energy.

13.1 Friction and energy Friction is always present in any machine with moving parts. If the machine is small, or the forces are low, the amount of heat produced by friction may also be small.

13.1 Friction and energy Each time two moving surfaces touch each other, tiny bits of material are broken off by friction. Breaking off bits of material uses energy. Friction has affected which pile of stones more?