1. Mechanical Equilibrium
An object in mechanical equilibrium is stable, without changes in motion

2. Learning hint
Read the chapters quickly but read then more than once!!!
Physics and most other sciences are best learned by going over the same material many, many times!!!

3. Objectives: Distinguish between force and net force (2.1)
Describe the equilibrium rule and give examples (2.2)
Distinguish between support force and weight (2.3)
Give examples of moving objects that are in equilibrium (2.4)
Determine the resultant of a pair of parallel or non-parallel vectors (2.5)

4. Key Terms Force Net force Vector Vector quantity Scalar quantity
a push or a pull
Net force
combination of all pushes and pulls on an object
Vector
An arrow that represents magnitude (how much) and in what direction
Vector quantity
A quantity that needs both magnitude and direction
Scalar quantity
A quantity that that can be described by magnitude only. (time, area, volume)

5. 2.1 Force
A push or pull
A force is needed to change an object’s state of motion
Net force is the sum of forces acting on an object

6. Weight and Tension Is the force of gravity on any object
Fg = mgg
It is measured in newtons
2lbs = 9 newtons
Both are measures of force
If we hang an object by the rope we used earlier there would be several forces acting on the system. Force on:
Object due to gravity
Object due to rope
Rope due to object – this is called tension

7. Force Vectors Vector Vector quantity Scalar quantity
An arrow that represents magnitude (how much) and in what direction
Vector quantity
A quantity that needs both magnitude and direction
Scalar quantity
A quantity that that can be described by magnitude only. (time, area, volume)
Scalar quantities can be manipulated according to the rules of math
Vector quantities have special math rules!!

8. 2.2 Mechanical Equilibrium
An object which has no change in motion
SF = O
This is the Equilibrium rule
Forces acting up must = the forces acting down
S = Sum of
F = force

9. Support Force 2.3
If something is still there must be another force besides gravity in order to have SF=0
This force will be opposite to gravity
Support force = Normal force
Support force is the up force that balances the weight of the object
Support force = +
Force of Gravity = -
A scale gives you your weight when you step on it b/c it is your mass combined with gravity are in the downward direction & the scale & floor forces are in the upward direction
Support force and your weight have the same magnitude

10. 2.4 Equilibrium for Moving Objects
Just b/c an obj. is moving doesn’t mean it is not in equilibrium
Once in motion if SF=O then it is in equilibrium
Hockey puck on ice
Bowling ball rolling at constant velocity
A change in motion means that an obj is not in equilibrium
Friction is a contact force btwn objects that slide or tend to slide against each other

11. 2.4 continued Two types of mechanical equilibrium
Static equilibrium – to have no motion at all
A book on a table
Dynamic equilibrium – unchanging speed in a straight line
Airplane – once in flight
Hockey puck
Parachutist – no wind and once the chute is opened

12. 2.5 Vectors
If you are evenly on two scales then your weight will be divided evenly between the two scales
Combining is easy if parallel (//)
Add if in the same direction
Subtract if opposite
Resultant = sum of 2 or more vectors
Non // vectors use parallelogram rule

13. Parallelogram Rule
Create a parallelogram where the vectors are adjacent side.
The diagonal of the parallelogram gives you the resultant of the two forces (R)
A square is a special case b/c the 2 adjacent sides will have the same magnitude
sq. root of 2 = therefore we multiply the magnitude of one of the adjacent vectors by this number to obtain the resultant.

14. Applying the Parallelogram Rule
Violet is in equilibrium
There are 3 forces acting on her
Tension in Lt rope
Tension in Rt rope
Weight
Tension in each rope is more than half her weight
As the angle btwn the ropes increases the tension increases
The R vector must = the FV downward
Tension
Tension
Weight

15. When ropes are at different angles
There will more tension on the rope that is closer to being vertical. (see figure 2.13)