Force and its Representation

1. Force and its Representation

2. force • dynamics  study of the causes of motion • force  a push or pull on an object resulting from the object’s interaction with another object • force is a vector quantity (has magnitude and direction)

3. newtons • Newton (N)  the SI unit of force • equal to 1 kgm/s2 • “one Newton is the amount of force required to give a 1-kg mass an acceleration of 1 m/s/s”

4. 2 broad categories of force: • contact forces  result when two interacting objects are perceived to be physically contacting each other • ex: friction, tension, air resistance, applied force, normal force • action-at-a-distance forces  result when two interacting objects are able to exert a push or pull despite their physical separation • ex: gravitational, electrical and magnetic forces

5. free-body diagrams • since force is a vector, it can be represented using arrow diagrams • the magnitude is represented by the length of the arrow • the direction of the arrow indicates the direction in which the force is acting

6. free-body diagrams • FBD for short • are simple drawings of an object showing all the forces acting on that object • will not necessarily show all the objects in an entire situation • objects can have any number of forces acting on them

7. constructing FBDs • identify which force types are present in the situation • determine the direction in which each force is acting • draw a box (to represent the object) and add arrows for each existing force in the appropriate direction • label each force arrow according to its type

8. balanced forces • since forces are vectors (directional), opposite forces can cancel the effect of one another example: a 20-Newton upward force acting on a book is cancelled by the effect of a 20-Newton downward force acting upon the book - the two forces balance each other

9. unbalanced forces • forces can also be multi-directional • if one or more of these forces are unbalanced this will change the object’s motion example: the vertical forces cancel out, but the horizontal force does not

10. types of force • applied force (FA) applied to an object by a person or another object (pushing a desk across a room) • force of gravity (Fg) force with which a massively large object attracts other objects towards itself (Earth’s gravitational pull) • normal force (FN) support force exerted on an object that is in contact with another stable object (person leaning against wall)

11. friction force (Ff) exerted by a surface as an object moves across it (sliding and static) • air resistance (Fair) frictional force that acts upon an object as it travels through the air (sky diver) • tension (FT)  force transmitted through a string, rope, cable or wire when it is pulled tight by forces acting on opposite ends (tug of war)

12. weight vs mass • the force of gravity acting upon an object is sometimes referred to as the weight of the object • the mass of an object refers to the amount of matter contained by the object • the weight is the force of gravity acting upon that matter • mass will remain constant, while weight will vary according to g value (Earth = 9.8 N/kg)