1. Chapter 4.1: Changes in Motion
Introduction to Force
Chapter 4.1: Changes in Motion

2. What is force?
a push or pull upon an object resulting from the object's interaction with another object
For ex., you exert force on a ball when you kick it
You exert a force on the chair you are sitting in right now
Whenever there is an interaction between two objects, there is a force acting on each of the objects. When the interaction ceases, the two objects no longer experience a force. Forces only exist as a result of an interaction.

3. Units (yes, more units to know)
The SI unit of force is the newton (N)
Named after Sir Isaac Newton
Definition:
N = the amount of force, that when acting on a mass, produces an acceleration
1 N = 1 kg * 1 m/s2

4. Two categories of forces
1. Contact
types of forces in which the two interacting objects are physically in contact with each other.
Examples: frictional force, tensional force, air resistance force, normal forces, and applied forces
2. Action at a Distance
types of forces in which the two interacting objects are not in physical contact with each other, but are able to exert a push or pull despite the physical separation.
Examples: gravitational force, magnetic force, and electric force

5. Force Diagrams Force is a vector quantity
A vector quantity is a quantity which has both magnitude and direction. To fully describe the force acting upon an object, you must describe both its magnitude (size) and its direction.
Arrows represent all the forces acting in a situation

6. Free Body Diagrams
Used to show the relative magnitude and direction of all forces acting upon an object in a given situation.
The size of the arrow in a free-body diagram is reflective of the magnitude of the force. The direction of the arrow reveals the direction in which the force acts.
Each force arrow in the diagram is labeled to indicate the type of force.
It is customary in a free-body diagram to represent the object by a box and to draw the force arrow from the center of the box outward in the direction in which the force is acting.
Hint: gravitational force is usually a downward arrow.

7. Examples
Each force arrow in the diagram is labeled to indicate the type of force.
It is customary in a free-body diagram to represent the object by a box and to draw the force arrow from the center of the box outward in the direction in which the force is acting.

8. Practice #1 Draw a free body diagram for the following:
An egg is free-falling from a nest in a tree. Neglect air resistance.

9. Practice #1 Answer
Draw a free body diagram for the following: An egg is free-falling from a nest in a tree. Neglect air resistance.
The only force acting on a free-falling egg is the downward force of gravity.

10. Practice #2 Draw a free body diagram for:
A rightward force is applied to a book in order to move it across a desk at constant velocity. Consider frictional forces.

11. Practice #2 Answer
Draw a free body diagram for: A rightward force is applied to a book in order to move it across a desk at constant velocity. Consider frictional forces
We must draw the applied force which
starts the book in motion as well as the
force of friction which will be opposing
the direction of motion (both in the horizontal
direction). Furthermore, gravity will pull the
book toward the table and the table will
push back up on the book to oppose the
force of gravity (vertical direction).

12. Practice #3 Draw a free body diagram for:
A skydiver is descending with a constant velocity. Consider air resistance.

13. Practice #3 Answer
A skydiver is descending with a constant velocity. Consider air resistance.
Answer: The force pulling the skydiver toward the ground is gravity. Air resistance opposes the skydiver’s fall and opposed the force of gravity. Constant velocity means that there is no acceleration, so there is no net force (the two forces cancel each other out – they’re equal and opposite)