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Free Body Diagram Illustrates Newton's First Law

Free Body Diagrams show the relative magnitude and direction of forces acting on an object. Force vectors are represented as arrows, pointing outward from (usually) or inward toward the center of mass of an object. Drawing free body diagrams is the first step in solving problems involving more than one force on an object.

Newton’s First Law states that if no net force acts on an object, it maintains its state of rest or its constant speed in a straight line. We will be looking at an object maintaining its state of rest.


Newton’s First Law regarding an object maintaining its state of rest, means that if an object is placed at rest on a horizontal surface, with no external unbalanced forces acting upon it, then it will never move. A free body diagram help us to visually analyze the forces acting on an object and assess their effects.

Free body diagrams show the forces acting on a beanbag.

Free body diagrams help us to understand Newton's First Law.

A group of students are playing a game of hackysack with a beanbag. One of the students kicks the beanbag on top of the roof of their flattop garage. Being physics class students, they know that according to Newton’s First Law, the beanbag will not come down unless it is acted upon by an external unbalanced force. They came up with the following ways that they could get their beanbag down. Examples of forces that would move the beanbag are: (See Free Body Diagram Pictures)
1) If a strong wind pushed the beanbag to them, - Figure 1
2) If the roof of the garage collapsed and the beanbag dropped down to them, - Figure 2
3) If one of the students climbed up onto the garage to get the beanbag. - Figure 3

All of the methods involve an external unbalanced force to act on the beanbag. Newton's first law informs us that the bean bag will start to move in the direction of the unbalanced force. The free body diagrams to the left provide a visual clue as to what direction the unbalanced force acts in each situation. If a strong wind pushed the beanbag back to them, this would be an applied force, but it would have to be greater than friction to move the bean bag.. If the roof of the garage collapsed, the normal force on the beanbag would be greatly decreased and a larger gravitational force would be applied on the beanbag, causing it to move in a downward motion. The last idea they came up with was for one of them to climb up onto the garage, pick up the beanbag, and toss it down to the others. Applied force would be exerted, as the beanbag would be lifted up off the horizontal surface, and then tossed down to the other students. If no external unbalanced forces were applied to the beanbag to make it move or to retrieve it, the beanbag could possibly be stuck there forever until an unbalanced force was exerted upon it.


With your beanbag sitting on top of your flat roof garage, which one of the stated methods is the probably the most effective method of applying Newton's First Law?
A) Wait until a strong wind blows the bag back to you. – Free Body Diagram 1
B) Wait for the roof to collapse to get the beanbag. – Free Body Diagram 2
C) Climb up, pickup the beanbag, and throw it back. – Free Body Diagram 3

Answer: The most probable way to get your beanbag back would be to climb up the garage, pickup the beanbag, and throw it back to the other students. All of the ways could work, but you would be waiting for a long time for the wind to blow you the beanbag back, or for the roof to collapse.

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