1. The Earth’s gravity pulls the atmospheric gases closer to its surface!
Ch.5, Sec.2 – What Is A Force?
force: a push or a pull exerted on an object in order to change the motion of the object; force has size & direction
newton: the SI unit for force and weight (N)
Forces Acting on Objects
- not all forces can be seen and forces can only act on objects with matter
- gravity & magnetism are sometimes hard to see!
The Earth’s gravity pulls the atmospheric gases closer to its surface!

2. Ch.5, Sec.2 – What Is A Force? Determining Net Force
net force: the combination of all the forces acting on an object
- calculating net force is determined by the direction in which the forces are placed
- If one person applies 8 N of force to a book and another person applies 6 N of force (in the same direction), then the book requires 14 N of force to be moved!

3. Ch.5, Sec.2 – What Is A Force?
- If one person applies 20 N of force to a book and another person applies 18 N of force (in the opposite direction), then the book will have 2 N of force pushing it to the left!
- What is the net force of an object when you combine a force of 7 N north with a force of 5 N south?
5 N south =
2 N north
7 N north

4. Ch.5, Sec.2 – What Is A Force?

5. Ch.5, Sec.2 – What Is A Force? Balanced & Unbalanced Forces
- net force is important because it tells us whether the forces on an object are balanced or unbalanced
- when the net force on an object is 0 N, the forces are balanced!
- balanced forces will not cause an object to change its motion
- balanced forces will not cause an unmoving object to start moving

6. Ch.5, Sec.2 – What Is A Force?
- when the net force on an object is not 0 N, the forces are unbalanced!
- unbalanced forces cause a change in motion, such as speed or velocity
- unbalanced forces are necessary to cause an unmoving object to start moving