1. Net Force
A vector is a measurement that includes both a quantity and a direction.
Examples of vector quantities are velocity, acceleration, and force.

2. Net Force
A force vector has units of newtons, just like all forces, but also includes enough information to tell the direction of the force.
Positive and negative numbers indicate opposite directions.

3. Net Force
It is sometimes helpful to show the strength and direction of a force vector as an arrow.
When drawing a force vector, you must choose a scale.
If 1 cm = 1 N, how many newtons of force does this line represent?

4. Net Force
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.

6. Net Force
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?

7. Net Force 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.

8. Net Force
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.

9. Net Force
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.

11. Your turn: Net Force Problems
You need a whiteboard and dry erase marker for you and your partner

12. What is the net force?
2,000 N
500 N

13. What is the net force?
1,500 N

14. What is the net force?
250 N
300 N

15. What is the net force?
50 N

16. What is the net force?
10 N
70 N

17. What is the net force?
60 N

18. What is the net force?
20 N
30 N
30 N

19. What is the net force?
20 N

20. What is the net force?
65 N
95 N

21. What is the net force?
30 N

22. What is the net force?
20 N
30 N
20 N

23. What is the net force?
10 N

24. 13.2 Forces in springs and ropes
You are already familiar with weight, normal force, friction, and contact forces.
Some forces have special properties.
If the vectors are the same size is this block moving or at equilibrium?

25. 13.2 Forces in springs and ropes
Tension is a force that acts in a rope, string, or other object that is pulled.
Tension always acts along the direction of the rope.
In what direction will the girls go?

26. 13.2 Forces in springs and ropes
When you apply a compression force to a spring, it collapses to become shorter.
The force created by stretching or compressing a spring always acts to return the spring to its natural length.

27. 13.2 Solving equilibrium problems
For an object to be in equilibrium, all the forces acting on the object must add to zero.
Do not mix forces in the horizontal direction with forces in the vertical direction.
What is the sum of the forces acting on this block?

28. Technology Connection
Defy Gravity? It Can Be Done.
When the force of lift is stronger than the force of an airplane’s weight, it will leave the ground and fly. How do we create the force of lift?