Wize High School Grade 12 Physics Textbook > Dynamics
Free Body Diagrams
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Free Body Diagrams
The study of forces will focus on the application of Newton's Laws and the creation of free-body diagrams (commonly abbreviated as FBD).
What is a Force?
- In the most general possible terms, a force is a push or a pull. (That's it!)
- It's important to remember that force is a vector quantity; that is, it has both magnitude and direction, and all of the operations of vectors can be applied to them. Many problems in dynamics begin by resolving forces into components.
- The unit of force is a Newton [N]. This is a derived unit, with .
What is a Free Body Diagram?
- Free body diagrams are used to show all the forces acting on an object. Drawing a proper free body diagram for each object should be your first instinct when encountering a force-related question.
- Mass (m) is usually represented as a block and the forces acting on the mass are shown as vectors with appropriate relative size and direction.
- Remember to include a coordinate axis to define positive directions
- It’s also a good idea to include an arrow indicating the direction of acceleration so you know which forces add up to ma.
Example:
Watch Out!
A free body diagram accounts for all forces acting ON the object only (not the forces the object may exert on other objects)
A FBD allows you to isolate the object of interest, and observe the forces acting on it.
Exam Tip
For problems involving Forces, you should ALWAYS draw a free body diagram!
Steps to Construct a FBD:
- Decide what you’re interested in, and draw a diagram of it
- Draw and label all forces acting on the body, and indicate angles where necessary
- If you don’t know the direction of a force, assume a direction
- Exaggerate the angles if it helps
3. Define a coordinate system
- This can be the general xyz coordinate system you’re used to
- Or it could be a rotated coordinate system that simplifies the problem
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Example: Free Body Diagram Samples
1. A man is dragging a box of mass m on a horizontal rough floor with a coefficient of kinetic friction of.
2. A box on a rough inclined plane.
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a)
A man pulls 200Kg crate with rope angles 30 o above the horizontal with force 200N. The crate sits on the frictionless surface. Calculate the normal force. Enter answer in N.
Mark Yourself Question
- Grab a piece of paper and try this problem yourself.
- When you're done, check the "I have answered this question" box below.
- View the solution and report whether you got it right or wrong.
Two tennis players are rallying a tennis ball.
Describe the forces (draw a FBD) on the ball when it is:
a) In flight
b) Being hit by a racket. How does the force the ball exerts on the racket compared to that of the racket on the ball (>, < or =).