1. A force is a push or a pull.
What is a force?
Measured quantity
Force
SI Unit
Newton
Symbol N
A force is a push or a pull.

2. 2.1 Force
Net Force
The combination of all forces acting on an object is called the net force.

3. An unbalanced force is needed to change an object’s state of motion.
A force is a push or a pull.

4. 2.1 Force
Net Force
The combination of all forces acting on an object is called the net force.
The net force on an object changes its motion (as long as the net force is greater than zero).
Measured quantity
Force
SI Unit
Newton
Symbol N

5. It takes an unbalanced force to change the motion of an object!
Balanced vs. Unbalanced Force
Balanced force occurs when the net force = 0 N
(the object experiences no change in motion)
Unbalanced force occurs when the net force > 0 N
(the object experiences a change in motion)
It takes an unbalanced force to change the motion of an object!

6. Fnorm = Fgrav 2.1 Force Net Force Normal Force
When the girl holds the rock with as much force upward as gravity pulls downward, the net force on the rock is zero.
Fnorm = Fgrav
Gravity

7. The blue arrows are called vectors (they indicate size and direction)
2.1 Force
Net Force
The net force depends on the magnitudes and directions of the applied forces.
The blue arrows are called vectors (they indicate size and direction)

8. 2.1 Force
Net Force
The net force depends on the magnitudes and directions of the applied forces.
The blue arrow in the net force column is a type of vector called the resultant (5 N + 10 N = 15 N)

9. 2.1 Force
Net Force
The net force depends on the magnitudes and directions of the applied forces.

10. 10 N – 5 N = net force of 5 N to the right
The net force depends on the magnitudes and directions of the applied forces.
10 N – 5 N = net force of 5 N to the right

11. 2.1 Force
Net Force
The net force depends on the magnitudes and directions of the applied forces.

12. 5 N – 5 N = net force of 0 N …which is equilibrium!
The net force depends on the magnitudes and directions of the applied forces.
5 N – 5 N = net force of 0 N …which is equilibrium!

13. Force Vectors can be drawn to scale
This vector represents a force of 60 N to the right. N
Vectors show magnitude (size) and direction

14. Resultant The sum of two or more vectors
Can show the magnitude and direction of the net force.

15. Resultant (forces in same direction and in opposite directions)
Net force = 60 N to the right
Resultant?
40 N
20 N
Net force = 20 N to the right
Resultant?

16. Resultant (pulling or pushing at an angle)

17. Resultant (pulling or pushing at an angle)

18. Identify the resultant and the horizontal and vertical forces

19. How can you change an object’s state of motion?
2.1 Force
How can you change an object’s state of motion?
Apply a net force greater than zero.

20. An object in mechanical equilibrium is stable, without changes in motion.

21. Things that are in balance with one another illustrate equilibrium.
Things in mechanical equilibrium are stable, without changes of motion.
The rocks are in mechanical equilibrium.

22.
An object will fall if its center of mass is no longer situated within its base

23. Different Types of Forces
Gravity (also called weight)
Friction
Applied Force
Air Resistance
Tension
Spring Force
Normal Force (also called support force)

24. 2.3 Definition of Forces Gravity Tension Force Spring Force
Force of attraction due to mass of an object
Pulled force applied to a rope, wire or cable
Elastic, compressed or stretched force
Any push or pull on an object
Resistance of two objects in contact moving past each other
Resistance of an object moving through the atmosphere
Force that pushes back on one object resting on another
Gravity
Tension Force
Spring Force
Applied Force
Friction
Air Resistance
Normal force 24

25. 2.3 Support Force
For an object at rest on a horizontal surface, the support force (normal force) must equal the object’s weight (gravity). 25

26. Tension, Weight and Spring
2.1 Force
Tension, Weight and Spring
The upward tension in the string has the same magnitude as the weight of the bag, so the net force on the bag is zero.
The bag of sugar is attracted to Earth with a gravitational force of 2 pounds or 9 newtons.
Spring force (in scale)
Tension (in string)
1 lb = 4.45 N
Weight (of bag)
9 N of weight = 9 N of tension and spring force 26

27. Fnorm = Fgrav 2.3 Support Force
Support Force (also called normal force)
The table pushes up on the book with as much force as the downward weight of the book.
Fnorm = Fgrav
Net force
Weight (also called gravity) 27

28. 2.3 Support Force Physics
The book lying on the table compresses atoms in the table and they squeeze upward on the book. The compressed atoms produce the support force. 28

29. 2.3 Support Force
The upward support force is as much as the downward pull of gravity. 29

30. 2.3 Support Force
The upward support force is as much as the downward pull of gravity. 30

31. 2.3 Support Force
think!
What is the net force on a bathroom scale when a 110-pound person stands on it? 31

32. 2.3 Support Force
think!
What is the net force on a bathroom scale when a 110-pound person stands on it?
Answer: Zero–the scale is at rest. The scale reads the support force, not the net force. 32

33. 2.3 Support Force
think!
Suppose you stand on two bathroom scales with your weight evenly distributed between the two scales. What is the reading on each of the scales? What happens when you stand with more of your weight on one foot than the other? 33

34. 2.3 Support Force
think!
Suppose you stand on two bathroom scales with your weight evenly distributed between the two scales. What is the reading on each of the scales? What happens when you stand with more of your weight on one foot than the other?
Answer: In the first case, the reading on each scale is half your weight. In the second case, if you lean more on one scale than the other, more than half your weight will be read on that scale but less than half on the other. The total support force adds up to your weight. 34

35. The object’s weight (or gravity)!
2.3 Support Force
For an object at rest on a horizontal surface, what is the support force (or normal force) equal to?
The object’s weight (or gravity)! 35

36. 2.1 Force Tension and Weight
A stretched spring is under a “stretching force” called tension.
Pounds and newtons are units of weight, which are also units of force.
Weight is a force. 36