CHAPTER 2 :: FORCES AND MOTION

2.9 :: ANALYSING FORCES IN EQUILIBRIUM

Newtons Third Law of Motion States that:

FOR ANY ACTION, THERE IS A REACTION WHICH HAS THE SAME MAGNITUDE BUT ACTS IN THE OPPOSITE DIRECTION

Situations involving rd Newtons 3 Law of Motion

1

2A B

A pushes B (action)

B pushes A (reaction)

3

4

5

6

1.Describe situations where forces are in equilibrium.

Forces acting on a balloon..

Forces acting on a person in a lift..

With the aid of diagrams, describe

situations where forces are in equilibrium, e.g. a book at rest on a table, an object at rest on an inclined plane.

2. State what a resultant force is

3. Add two forces to determine the resultant force With the aid of diagrams, discuss

the resolution and addition of forces to determine the resultant force.

4. Resolve a force into the effective component forces.

5. Solve problems involving forces in equilibrium. Solve problems involving forces in

equilibrium (limited to 3 forces).

1. An object is said to be in a state of equilibrium

when the object is : a) At rest (static equilibrium) b) Moving with uniform velocity (dynamic equilibrium)

Examples:

a) At rest (static equilibrium) - A book at rest on the table is in a state of equilibrium because there is no unbalanced force acting on the book. - There are two forces acting on the book : a) The weight of the book b) The normal reaction on the table.- The weight of the book is balanced by the normal reaction of the table. There is no unbalanced force acting on the book. - An object is said to be in the state of equilibrium.

b) Moving with uniform velocity (dynamic

equilibrium) - An air puck moving with uniform velocity. - The forces acting on the air puck are the weight and the normal reaction of the table. - As the track is smooth, there is no frictional force acting against the motion of the air puck. - There is no unbalanced forced acting on the air puck. The weight of the air puck is balanced by the normal reaction of the table. - An object is said to said to be in a state of equilibrium.