Physics / Unit 3: Forces and Newton's Laws
3.1 Forces and Free-Body Diagrams
Notes
A force is a push or pull, measured in newtons (N). $1 \text{ N} = 1 \text{ kg·m/s}^2$.
- Common forces: weight (gravity, down), normal force (surface push, perpendicular to the surface), tension (pull through a rope), friction (along the surface, opposing sliding), applied force, air resistance.
- A free-body diagram shows one object as a dot with an arrow for each force acting on it.
- The net force is the vector sum of all forces. If forces are balanced, the net force is zero.
- Example: a book resting on a table has two forces: weight down and normal force up, equal in size.
Practice
A lamp hangs at rest from a ceiling by a cord. Which forces act on the lamp?
The cord pulls up and gravity pulls down; nothing touches the lamp from below.
A box is pushed right with 40 N while friction acts left with 15 N. What is the net force?
$40 - 15 = 25 \text{ N}$ in the direction of the push.
In which direction does the normal force on a box on a ramp point?
"Normal" means perpendicular; the surface pushes out from itself.
Ready for more? Take the Unit 3 test.