SPH3U – Unit 2

Introduction to Forces

Force

What do you think of when you hear the word, “Force”?

Think of 3 sentences that use the word, “Force”.

What is a Force?

A force is a PUSH or a PULL on something.

Four Fundamental Forces

Fundamental Force

Definition Examples

Four Fundamental Forces

Fundamental Force

Definition Examples

Gravitational Force of attraction between all objects

Planets in orbit

Four Fundamental Forces

Fundamental Force

Definition Examples

Gravitational Force of attraction between all objects

Planets in orbit

Electromagnetic Force cased by electric charges

Chemical reactions; electricity

Four Fundamental Forces

Fundamental Force

Definition Examples

Gravitational Force of attraction between all objects

Planets in orbit

Electromagnetic Force cased by electric charges

Chemical reactions; electricity

Weak Nuclear Forces in-between elementary particles

Involved in particle transformations

Four Fundamental Forces

Fundamental Force

Definition Examples

Gravitational Force of attraction between all objects

Planets in orbit

Electromagnetic Force cased by electric charges

Chemical reactions; electricity

Weak Nuclear Forces in-between elementary particles

Involved in particle transformations

Strong Nuclear Attraction between neutrons and protons

Nuclear reactions

Notes - Force

A force is a push or pull upon an object resulting from the object's interaction with another object.

Whenever there is an interaction (two objects affecting each other in some way) between two objects, there is a force upon each of the objects. When the interaction ceases, the two objects no longer experience the force. Forces only exist as a result of an interaction.

Notes - Force

For simplicity sake, all forces (interactions) between objects can be placed into two broad categories:

contact forces, and

forces resulting from action-at-a-distance

Contact forces are types of forces in which the two interacting objects are physically contacting each other. For example: Friction, air resistance, applied forces (when a push a book across the desk).

Action-at-a-distance forces are types of forces in which the two interacting objects are not in physical contact with each other, yet are able to exert a push or pull despite a physical separation. Examples of action-at-a-distance forces include gravitational forces (e.g., the sun and planets exert a gravitational pull on each other despite their large spatial separation); electric forces, and magnetic forces.

Force is a quantity which is measured using the standard metric unit known as the Newton. One Newton is the amount of force required to give a 1-kg mass an acceleration of 1 m/s2. A Newton is abbreviated by a "N."

To say "10.0 N" means 10.0 Newtons of force. Thus, the following unit equivalency can be stated:

1 Newton = 2

1s

mkg

A force is a vector quantity. Recall, that a vector quantity is a quantity which has both magnitude and direction. To fully describe the force acting upon an object, you must describe both the magnitude (size) and the direction.

Thus, 10 Newtons is not a full description of the force acting upon an object.

In contrast, 10 Newtons [downwards] is a complete description of the force acting upon an object; both the magnitude (10 Newtons) and the direction (downwards) are given.

Because a force is a vector which has a direction, it is common to represent forces using diagrams in which a force is represented by an arrow.

The size of the arrow is reflective of the magnitude of the force and the direction of the arrow reveals the direction which the force is acting. Such diagrams are known as free-body diagrams.

Diagrams

Two types of diagrams are important to the study of forces:

System Diagrams (sketch showing all the objects involved)

Free Body Diagram, or FBD. Shows only the forces.

Free Body Diagrams

Free-body diagrams are diagrams used to show the relative magnitude and direction of all forces acting upon an object in a given situation.

The size of the arrow in a free-body diagram is reflective of the magnitude of the force.

Free Body Diagrams

The direction of the arrow reveals the direction which the force is acting.

Each force arrow in the diagram is labeled to indicate the exact type of force.

Free Body Diagrams

It is generally customary in a free-body diagram to represent the object by a box and to draw the force arrow from the center of the box outward in the direction that the force is acting. An example of a free-body diagram is shown at the right.

Free Body Diagrams

In a free-body diagrams is to depict all the forces which exist for that object in the given situation. Thus, to construct free-body diagrams, it is extremely important to know the various types of forces.

Common Abbreviations

= force of gravity

= normal force

= applied force

= force of friction

= force of Tension

gF

NF

TFfF

aF

Common Abbreviations

= force of gravity

= normal force

= applied force

= force of friction

= force of Tension

gF

NF

TFfF

aF

** You will read more about each of these forces on page 115 in your text book.

Net Force

The net force, Fnet (also known as total force or resultant force) is the sum of all the forces acting on an object.

Because forces are vectors, you get the net force by adding vectors together geometrically.

3N

4N

Net Force

The net force, Fnet (also known as total force or resultant force) is the sum of all the forces acting on an object.

Because forces are vectors, you get the net force by adding vectors together geometrically.

3N 4N

Net Force

The net force, Fnet (also known as total force or resultant force) is the sum of all the forces acting on an object.

Because forces are vectors, you get the net force by adding vectors together geometrically.

3N 4N

Net Force

The net force, Fnet (also known as total force or resultant force) is the sum of all the forces acting on an object.

Because forces are vectors, you get the net force by adding vectors together geometrically.

3N

4N

5N

37º

HOMEWORK

Read Section 3.1 (pg. 114-121) Using the Tutorial on pg. 117-118 as a guide,

draw the FBD listed on your handout Complete Questions:

pg. 119 #1-2 (top of page after sample problem 2) Pg. 120 #1-3 (practice at bottom of page)