Bell Work

Consider the following question, select (a) A=Agree, (b) D=Disagree (c) NS=Not Sure

A brick is lying on the bed of a truck, ___ (1) the brick will move forward if the

truck driver hits the break___ (2) the brick will move backward if the

truck driver hits the gas pedal

Why things move?

List things which you think will make things move

Contact and Field Forces

Forces Usually think of a force as a push or pull Vector quantity May be a contact force or a field force

Contact forces result from physical contact between two objects

Field forces act between disconnected objects Also called “action at a distance”

Types of Forces Contact Forces

Frictional Force Tensional Force Normal Force Air Resistance

Force Applied Force Spring Force

Action-at-a-Distance Forces Gravitational Force Electrical Force Magnetic Force

Units of Force

SI unit of force is a Newton (N)

US Customary unit of force is a pound (lb) 1 N = 0.225 lb

See table 4.1

2s

mkg1N1

Force Description

Applied Force An applied force is a force which is

applied to an object by another object or by a person. If a person is pushing a desk across the room, then there is an applied force acting upon the desk. The applied force is the force exerted on the desk by the person.

Force Description Gravity Force The force of gravity is the force with which the

earth, moon, or other massive body attracts an object towards itself. By definition, this is the weight of the object. All objects upon earth experience a force of gravity which is directed "downward" towards the center of the earth. The force of gravity on an object on earth is always equal to the weight of the object as given by the equation:

Force Description Normal Force The normal force is the support force exerted

upon an object which is in contact with another stable object. For example, if a book is resting upon a surface, then the surface is exerting an upward force upon the book in order to support the weight of the book. On occasion, a normal force is exerted horizontally between two objects which are in contact with each other.

Force Description Friction Force The friction force is the force exerted by a

surface as an object moves across it or makes an effort to move across it. The friction force opposes the motion of the object. For example, if a book moves across the surface of a desk, the desk exerts a friction force in the direction opposite to the motion of the book.

Force Description Air Resistance Force Air resistance is a special type of frictional

force which acts upon objects as they travel through the air. Like all frictional forces, the force of air resistance always opposes the motion of the object. This force will frequently be ignored due to its negligible magnitude. It is most noticeable for objects which travel at high speeds (e.g., a skydiver or a downhill skier) or for objects with large surface areas.

Force Description Tensional Force Tension is the force which is transmitted

through a string, rope, or wire when it is pulled tight by forces acting at each end. The tensional force is directed along the wire and pulls equally on the objects on either end of the wire.

Force Description Spring Force The spring force is the force exerted by a

compressed or stretched spring upon any object which is attached to it. This force acts to restores the object, which compresses or stretches a spring, to its rest or equilibrium position. For most springs (specifically, for those said to obey "Hooke's Law"), the magnitude of the force is directly proportional to the amount of stretch or compression.

Properties

List three examples of forces

Force

Make things

Start to moving

Make things to

Change directions

Make things

Stop to moving

Units

What do you know about Force

Sir Isaac Newton 1642 – 1727 Formulated basic

concepts and laws of mechanics

Universal Gravitation

Calculus Light and optics

Newton’s First Law

An object moves with a velocity that is constant in magnitude and direction, unless acted on by a nonzero net force The net force is defined as the vector

sum of all the external forces exerted on the object