It is often difficult for physics students to know how to start when doing physics homework problems. Students should always start by drawing a diagram to help visualize the physical situation in a problem. For physics force problems, physicists start with a free body diagram. The free body diagram helps the problem solver see how to proceed.
What Is a Free Body Diagram?
A free body diagram shows the forces involved in a physics problem. Physics students need to understand that free body diagrams do not show all the forces in a problem. They only show all the forces acting on the object of interest in the problem. Free body diagrams show the forces acting on the object of interest. They ignore the forces exerted by the object of interest.
In a free body diagram, accurately drawing the object of interest is unnecessary. Represent the object as a point and concentrate on visualizing the forces at work in the problem.
Physics students often have difficulty learning how to identify forces and draw a free body diagram, so two examples of free body diagrams for specific situations are provided.
Free Body Diagram of a Tightrope Walker
When a tightrope walker is standing on a stretched rope, it bends sharply at the point in the rope supporting the tightrope walker. The tension in the rope depends on the weight of the tightrope walker and on the angle the rope makes with the horizontal axis. The smaller this angle, the greater the tension in the rope.
To find the tension in the rope, draw the free body diagram of the point on the rope that is supporting the tight rope walker. The forces acting on this point on the rope are the tightrope walker's weight, and the tensions of the rope in each direction away from the point. Because this is a two dimensional problem, the forces must be divided into x and y components. If the tightrope walker is in the middle of the rope, the rope's angles on either side will be equal. If the walker is not at the center, the angles will have different values.
Free Body Diagram of a Skier on a Slope
A skier sliding down a slope is an example of an inclined plane problem in physics. The principles for drawing the skier's free body diagram apply to other inclined plane problems.
The forces acting on the skier include the gravitational force from Earth, the normal force from the slope, and the frictional force. Air resistance is usually ignored in beginning physics classes.
The gravitational force on the skier acts straight down, towards the Earth's center. The frictional force is parallel to the slope and opposes the direction of motion. If the skier is sliding down the slope, the friction acts up the slope.
Physics students need to be careful about the normal force. Many beginning physics students draw all normal forces straight up. Remember, however, that the word normal in mathematics means perpendicular. So the normal force is perpendicular to the slope not straight up.
This situation is a two dimensional problem, so the problem solver must divide the forces into x and y components. Most physics students naturally define the x and y axes horizontally and vertically. However for inclined plane problems, it is almost always easier to define the x axis parallel to the slope and the y axis perpendicular to the slope.
Learning to correctly draw free body diagrams is an essential first step for physics students learning to solve physics force problems.
Paul A. Heckert - I have a Ph.D. in astrophysics, over 30 years experience teaching physics and astronomy, and over 60 published research articles.
