Free-Body Diagrams Free body diagrams (FBDs) are simplified representations of an object which includes only the forces acting on the object. The body is free because the diagram shows it without its surroundings (the body is 'free' of its environment). The mass of the object in kilograms should be written inside of the box. FBDs help to eliminate unnecessary information given in a problem, they may include the following forces:
Gravity
The gravitational force acts on all objects. The acceleration due to gravity (on Earth) is constant and acts downwards, it is approximately g = -9.8 m/s, but the force due to gravity depends on the object’s mass. The gravitational force, is Fg = mg (We will also refer to this as the weight of the object, so we can also say W = mg)
Normal
The normal force (FN) is what prevents objects from 'falling' indefinitely. It is always perpendicular (normal) to the surface an object is in contact with. For example, if there is a box on the floor, then we say that the box experiences a normal force by the floor; and because of this force, the box does not fall into the floor. The normal force on the box points upward, perpendicular to the floor. For problems in this course, the normal force opposes gravity, so FN will equal FG
Friction
Friction is a force that opposes motion, it is caused by the rubbing of surfaces. The magnitude of the friction force is related to the normal force, but whereas the normal force was perpendicular to the surface, the frictional force is parallel. More specifically, its vector always pFrroints away from the direction of motion. Some surfaces are “rougher” than others, this is quantified by the coefficient of friction (m). The frictional force is given by the product of the normal force and the coefficient of friction, Ff = mFN.

Applied Force
An object might experience an external force by a physical push or pull. This could be caused by the wind pushing on a ship, a child pulling on a wagon, or in the case of our first example, a person pushing a box on the floor. The applied force is written as FA. In the problems for this course, this will be equal to the frictional force because we are overcoming that force to get the object to move at a steady pace. So, FA = FFr.

Construct free-body diagrams for the situations described below. Use the symbols we discussed in class. Draw force vectors on the circle and label them.
1. A book is at rest on a table top. Diagram the forces acting on the book.

2. A girl is suspended motionless from the ceiling by a rope. Diagram the forces acting on the girl as she holds onto the rope.

3. An egg is free-falling from a nest in a tree. Neglect air resistance. Diagram the forces acting on the egg as it falls.

4. An egg is falling (not freely, do not neglect air resistance) from a nest in a tree. Diagram the forces acting on the egg as it falls.

5. A rightward force is applied to a book in order to move it across a desk with a rightward acceleration. Consider frictional forces. Neglect air resistance. Diagram the forces acting on the book.

6. A rightward force is applied to a book in order to move it across a desk at constant velocity. Consider frictional forces. Neglect air resistance. Diagram the forces acting on the book.

7. A car is stopped at a stop light.

8. A skydiver is descending with a constant velocity. Consider air resistance. Diagram the forces acting upon the skydiver.

9. A car is parked on a sloped street.