2. Chapter Five: Forces 5.1 Forces 5.2 Friction
5.3 Forces and Equilibrium

3. Chapter 5.3 Learning Goals
Determine the net force acting on an object.
Define equilibrium.
Draw free-body diagrams to represent all forces acting on a body.

4. 5.3 Forces and Equilibrium
The sum of all the forces on an object is called the net force.
The word net means total but also means the direction of the forces has been taken into account.
In what direction will this plane go?

5. 5.3 Adding forces
To figure out if or how an object will move, we look at ALL of the forces acting on it.
Four forces act on a plane:
weight
drag (air friction)
the thrust of the engines, and
the lift force caused by the flow of air over the wings.

7. 5.3 Equilibrium When several forces act on the same object:
The net force is zero, or
The net force is NOT zero.

8. 5.3 Normal forces When the forces are balanced, the net force is zero.
When the net force on an object is zero, we say the object is in equilibrium.

9. 5.3 Equilibrium and normal forces
A normal force is created whenever an object is in contact with a surface.
The normal force has equal strength to the force pressing the object into the surface, which is often the object’s weight.
The normal force is sometimes called the support force.

10. 5.3 The free body diagram
How do you keep track of many forces with different directions?
Draw a free-body diagram that contains the objects, like a book on a table.

12. 5.3 Solving equilibrium problems
For an object to be in equilibrium, all the forces acting on the object must add up to zero.
Is this object in equilibrium?

13. Two chains are used to support a small boat weighing 1,500 newtons.
Solving Problems
Two chains are used to support a small boat weighing 1,500 newtons.
One chain has a tension of 600 newtons.
What is the force exerted by the other chain?

14. Looking for: Given Relationships: Solving Problems …tension on chain 2
…weightboat = 1,200N; tension1 = 600 N
Implied: weight and tension are forces
Relationships:
Net force on boat = zero

15. Solution: Solving Problems Draw free body diagram
Upward force of chains = weight of boat
600 N + tension2 = 1,200 N
tension2 = 900 N

16. A The forces are balanced, and the net force is zero.
In the picture, a boy is trying to pull a dog on a leash, but the dog and the boy are moving in the same direction. Which of the following best describes the forces in this situation?
A The forces are balanced, and the net force is zero.
B The forces are unbalanced, and the boy’s force is greater.
C The forces are unbalanced, and the dog’s force is greater.
D The forces are balanced, and the dog is stronger than the boy.

18. What is the net force?

19. What is the net force?