1. Chapter 5 Matter in Motion
Section 2
What Is A Force?

2. The Meaning of Force
A force is a push or pull upon an object resulting from the object's interaction with another object. Whenever there is an interaction between two objects, there is a force upon each of the objects. When the interaction ceases, the two objects no longer experience the force. Forces only exist as a result of an interaction.

3. The Meaning of Force
Knowing what you know about scalar and vector quantities, which is force?
It is a vector quantity because all forces have both size and direction.

4. The Meaning of Force
Force is a quantity which is measured using the standard metric unit known as the Newton. A Newton is abbreviated by a "N." To say "10.0 N" means 10.0 Newtons of force. One Newton is the amount of force required to give a 1-kg mass an acceleration of 1 m/s/s. Thus, the following unit equivalency can be stated:

5. Forces in Combination: Newton’s First Law (Law of Inertia)
“An object at rest tends to stay at rest and
an object in motion tends to stay in motion
with the same speed and in the same
direction unless acted upon by an
unbalanced force.”
In the statement of Newton's first law, the
unbalanced force refers to that force which
does not become completely balanced (or
canceled) by the other individual forces.

6. Forces in Combination: Newton’s First Law

7. Forces in Combination: Newton’s First Law
If either all the vertical forces (up and down)
do not cancel each other and/or all
horizontal forces do not cancel each other,
then an unbalanced force exists.
Let’s look at some examples.

8. Free-body diagrams Free-body diagrams for three situations are
shown below. Note that the actual
magnitude of the individual forces are
indicated on the diagram.
In each of the above situations, there is an unbalanced force.

9. Net Force
It is commonly said that in each situation there is a net force acting upon the object. The net force is the vector sum of all the forces which act upon an object.
The net force is the sum of all the forces, taking into account the fact that a force is a vector and two forces of equal magnitude and opposite direction will cancel each other out.

10. Determine the Net Force

11. Determine the Net Force
A net force (i.e., an unbalanced force) causes an acceleration.
Net Force: Yes or No? (scroll ½ way down)

12. Determine the Net Force

13. Determine the Net Force