LESSON 13 FEBRUARY 4 TH, 2011 Forces and Free body Diagrams

Forces Force  A push or pull; it is a vector quantity, symbol  The direction can be stated in various ways; up, down, east, west, northeast, etc.

Every day we experience several forces. One of these is the force of gravity. Force of Gravity  The force of attraction between all objects; symbol ; only noticeable if the mass of at least one object is huge  The direction of the force is directed vertically downwards towards the center mass of the huge object. Example: Earth’s core.

The force of gravity is an action-at-a-distance force; this means that contact between the objects is not needed.

Example: If your pen is sitting on your desk, the force of gravity is pulling down on it, but since it is at rest, it must be experiencing an opposite force to balance the force of gravity. The opposing force is the desk pushing upward. This is called the normal force. The force at right angles to objects in contact; symbol Table

Friction Another import force in our lives is friction. The force between objects in contact; it is parallel to the Contact surfaces and acts in a direction opposite to any Motion or attempted motion; symbol Friction is will bring your pen to a stop if you try to slide it across the table.

Types of Friction Static friction  The force that prevents a stationary object from starting to move.

Types of Friction Kinetic Friction The force that acts against an objects motion.

Types of Friction Air resistance  Friction that acts on an object moving through air; it is noticeable at high speeds and is a type of kinetic friction.

Another common force is tension, which is exerted by strings, ropes, fibres, and cables.

Normal force, friction, and tension are all examples of contact forces. The forces exist where objects are in direct contact with each other. The term applied force,, can be used as a general term for any contact force.

Measuring Force The SI unit of force is the newton, symbol N. A Newton is a derived unit from a combination of the base units metres, kilograms, and seconds.

Drawing Force Diagrams The use of diagrams helps us to explain or visualize what factors are acting. There are two types of diagrams.

System diagrams A drawing of all the objects in the situation under analysis

Free-Body diagram (FBD) a drawing of just the object being analyzed, not the entire situation, that shows all the forces acting on the object.

Examples A boat anchor held by its chain. + y

A ball held in a student’s hand. + y

+x A student pushing a text book on a table.

Questions 1. State an everyday example in which a force causes an object to; A. Increase its speed B. Become compressed C. Become stretched 2. You are facing east, in front of a gate that can swing. In what direction is your force if you? A. Pull on the gate? B. Push on the gate?

Engineers say “you can’t push a rope.” What do they mean? State the magnitude of the force to hold each of the forces steady;  1.0 kg of sugar  A stapler of mass 0.20 kg  A student of mass 65 kg

The force exerted by gravity on a book resting on a desk is 8.5 N [down].  State the magnitude and direction of the normal force acting on the book.  If the same book is hanging from a string instead, what is the direction of the tension force acting on the book?

For each of the following situations below, draw a system diagram and an FBD for each object in italics.  Your notebook resting on a desk  A tennis ball is falling through the air from the server’s hand. Neglect air resistance.  A fully loaded dogsled, moving slowly along a flat, snowy trail, is being pulled horizontally by dogs attached to it by a rope.

In the following, state the direction of each force acting on the objects in italics.  A puck experiences friction on rough ice while sliding south  The force of gravity exerts a force on you.  The force of gravity acts on the moon, keeping it in orbit around the Earth.  The force of the wind pushes against a cyclist who is cycling eastward.