LESSON 13
FEBRUARY 4TH, 2011
Forces and Free bodyDiagrams
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 ofthese is the force of gravity.
Force of Gravity
The force of attraction between all objects;symbol ; only noticeable if the mass of at least oneobject is huge
The direction of the force is directed verticallydownwards towards the center mass of the hugeobject. Example: Earth’s core.
The force of gravity is an action-at-a-distanceforce; this means that contact between the objects isnot needed.
Example: If your pen is sitting on your desk, the force ofgravity is pulling down on it, but since it is at rest, itmust be experiencing an opposite force to balance theforce of gravity. The opposing force is the desk pushingupward. 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. Theforce between objects in contact; it is parallel to the
Contact surfaces and acts in a direction opposite toany
Motion or attempted motion; symbol
Friction is will bring your pen to a stop if you try toslide it across the table.
Types of Friction
Static friction
The force that prevents a stationary object from startingto 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 isnoticeable at high speeds and is a type of kinetic friction.
Another common force is tension, which is exertedby strings, ropes, fibres, and cables.
Normal force, friction, and tension are all examplesof contact forces.
The forces exist where objects are in direct contactwith each other.
The term applied force, , can be used as ageneral term for any contact force.
Measuring Force
The SI unit of force is the newton, symbol N. ANewton is a derived unit from a combination of thebase units metres, kilograms, and seconds.
Drawing Force Diagrams
The use of diagrams helps us to explain or visualizewhat factors are acting.
There are two types of diagrams.
System diagrams
A drawing of all the objects in the situation underanalysis
Free-Body diagram (FBD)
a drawing of just the object being analyzed, not theentire situation, that shows all the forces acting onthe object.
Examples
A boat anchor held by its chain.
+ y
A ball held in a student’s hand.
+ y
+y
+x
A student pushing a text book on a table.
Questions
1.State an everyday example in which a forcecauses an object to;
A.Increase its speed
B.Become compressed
C.Become stretched
2.You are facing east, in front of a gate that canswing. 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 dothey mean?
State the magnitude of the force to hold each of theforces 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 adesk is 8.5 N [down].
State the magnitude and direction of the normal forceacting on the book.
If the same book is hanging from a string instead, what isthe direction of the tension force acting on the book?
For each of the following situations below, draw asystem diagram and an FBD for each object initalics.
Your notebook resting on a desk
A tennis ball is falling through the air from the server’shand. Neglect air resistance.
A fully loaded dogsled, moving slowly along a flat, snowytrail, is being pulled horizontally by dogs attached to it bya rope.
In the following, state the direction of each forceacting on the objects in italics.
A puck experiences friction on rough ice while slidingsouth
The force of gravity exerts a force on you.
The force of gravity acts on the moon, keeping it in orbitaround the Earth.
The force of the wind pushes against a cyclist who iscycling eastward.