PHYS 2211 Module 5.7

Drawing Free-Body Diagrams

Recommended Reading

5.7 Drawing Free-Body Diagrams

Learning Objectives

By the end of this section, you will be able to:

  • Explain the rules for drawing a free-body diagram
  • Construct free-body diagrams for different situations

Tactics for Drawing a Free-Body Diagram

  • Identify all forces acting on the object.
  • Draw a coordinate system.  Align one of the axes with the acceleration of the object.
  • Represent the object as a dot at the origin of the coordinate axes. This is the particle model.
  • Draw vectors representing each of the identified forces. Be sure to label each force vector.
  • Draw and label the net force vector Fnet. Draw this vector beside the diagram, not on the particle. Then check that Fnet points in the direction of the acceleration. Or, if appropriate, write Fnet = 0.

Discuss!

Consider how you would answer these questions. Then bring this to class for a group discussion.

When you apply an upward force of magnitude F to a block of mass 2.00 kg, the block accelerates upward at 3.00 m/s2. You can ignore any forces exerted on the block by the air. Rank the following forces in order of their magnitude, from smallest to largest.

  • the force F
  • the net force on the block
  • the gravitational force on the block

Practice!

Practice 5.7.1

A block sits at rest on a frictionless surface. Each red arrow represents a force. Which of the following sketches most closely resembles the correct free body diagram for all forces acting on the block?  
(a) sketch A
(b) sketch B
(c) sketch C
(d) sketch D
(e) none of the above
Practice 5.7.2

Now, the same block moves with a constant velocity to the right on the frictionless surface. Which of the following most closely resembles the correct free body diagram for all forces acting on the block?
(a) sketch A
(b) sketch B
(c) sketch C
(d) sketch D
(e) sketch E
(f) none of these
Practice 5.7.3

Now, the block moves with a constant velocity to the right on a surface that has friction.  Which of the following most closely resembles the correct free body diagram for all forces acting on the block? 
(a) sketch A
(b) sketch B
(c) sketch C
(d) sketch D
(e) none of these
Practice 5.7.4
Imagine standing in an elevator that is moving upward. Which of the following is the most complete list of the forces that belong on your Free-Body Diagram (FBD) in that moment?
(a) The normal force on you by the elevator floor and the force of gravity on you by the earth
(b) The normal force on you by the elevator floor, the force of gravity on you by the earth, and the force of acceleration on you by the motor
(c) The force of gravity on you by the earth and the force of acceleration on you by the motor
(d) The force of gravity by the earth on you and the force of you on the elevator floor
Practice 5.7.5
Imagine a car sitting empty in the parking lot. Which of the following is the most complete list of the forces that belong on a Free-Body Diagram (FBD) of the car?
(a) The force on your car by its engine and the force of you pushing down on your car
(b) The normal force of your car on the pavement it rests on and the force of gravity pulling on the earth
(c) The normal force on your car by the pavement it rests on and the force of gravity on your car by the earth
(d) The force on your car by you and the force of your car on the air around it
Practice 5.7.6

An object rests on a ramp that is angled at 20° above the horizontal. What direction (relative to the vertical) will the normal force from the ramp point?
(a) The normal force will be directed at 70° away from the vertical.
(b) The normal force will be directed at 20° away from the vertical.
(c) The normal force will be directed exactly upward, along the vertical.
(d) The normal force will be directed horizontally, 90° away from the vertical.