Free Body Diagrams A force diagram, which is also known as a free body diagram, is a sketch in which all the force vectors acting on an object are drawn with their initial points at the location of the object.

Free-body diagrams Free-body diagrams are used to show the relative magnitude and direction of all forces acting on an object.

Free Body Diagram Forces Normal Force- F normal Applied perpendicular to the surface Applied perpendicular to the surface Gravitational Force- F g Applied perpendicular to the surface Applied perpendicular to the surface F g = weight = mass x gravity = mg F g = weight = mass x gravity = mg Applied Force- F app Applied parallel to the surface Applied parallel to the surface Force applied to the object Force applied to the object Frictional Force- F f Applied parallel to the surface Applied parallel to the surface Force that counteracts the applied force Force that counteracts the applied force

Opposition to Motion Friction  a force that opposes motion Caused by rough surfaces of all materials

Free Body Diagram

Different Forces Sometimes you do not have all 4 forces acting on an object, sometimes it is only 1, 2 or 3 forces.

1 Force acting on an object An egg is free-falling from a nest in a tree. Neglect air resistance. Draw a free-body diagram showing the forces involved. Gravity is the only force acting on the object

2 Forces acting on an object A book is at rest on a table top. Diagram the forces acting on the book. In this diagram, there are normal and gravitational forces on the book.

3 Forces acting on an object A rightward force is applied to a book in order to move it across a desk. Consider frictional forces. Neglect air resistance. Construct a free-body diagram. Let’s see what this one looks like.

Note the larger applied force arrow pointing to the right since the book is accelerating to the right. Friction force opposes the direction of motion. The force due to gravity and normal forces are balanced.

Practice Problem A man drags a sled across loosely packed snow with a rightward acceleration. Draw a free-body diagram.

The applied force arrow points to the right and is larger than the frictional force since the object is accelerating. Since the sled is on the ground, the normal and gravitational force are balanced.

Practice Problem 2 A car runs out of gas and is coasting down a hill.

The car is coasting down the hill, there is dragging friction of the road (left pointing arrow) as well as gravity and normal forces, but no applied force.

Net Forces Unbalanced forces do not always completely balance (or cancel) other individual forces.Unbalanced forces do not always completely balance (or cancel) other individual forces. An unbalanced forces exists when the vertical and horizontal forces do not cancel each other out.An unbalanced forces exists when the vertical and horizontal forces do not cancel each other out.

Example 1 Notice the upward force of 1200 Newtons (N) is more than gravity (800 N). The net force is 400 N up.Notice the upward force of 1200 Newtons (N) is more than gravity (800 N). The net force is 400 N up. The upward arrow is longer, indicating an unbalanced forceThe upward arrow is longer, indicating an unbalanced force

Example 2 Notice that while the normal force and gravitation forces are balanced (each are 50 N) the force of friction results in unbalanced force on the horizontal axis. The net force is 20 N left.

Another way to look at balances and unbalanced forces

Which objects are moving? How can you tell?