Interactive Lecture Demonstration 5 – Force Diagrams

Force diagrams (sometimes called free body diagrams) are the key to solving problems involving Newton's Laws. A good force diagram will help you visualize the forces that act on an object and will also help you determine the net force on the object.

In this activity, you will draw force diagrams for a series of situations involving everyday objects. The first step in drawing every force diagram is to clearly define the object for which you are drawing the force diagram. Only forces that act ON the object may appear on the force diagram; forces that the object exerts on another object do not appear (they only appear on the force diagram for that other object !). To keep things simple for now, the name we will give every force will just be the name of the object that is exerting the force. For example, if we are drawing a force diagram for a box that a person is pushing, the name of the push force would just be "Person."

In drawing force diagrams, remember the following rules (we will learn more rules later when we study more complicated situations):

1)      Make sure you include on your force diagram a force from every object that is touching the object you are considering. Any object that touches another object exerts a force on it. The force could be a push or a pull, depending on how the objects are touching.  

2)      Every object that has mass experiences a force from the Earth's gravity, called the object's weight. Only ONE weight force (the weight of the object in question) may appear on any force diagram !

3)      Once you have drawn a force diagram, make sure that it correctly shows the net force on the object. If the object is not accelerating, there must be forces with opposite components that could conceivably balance so that there is no net force. If the object is accelerating, there must be an unbalanced (net) force in the direction of the object's acceleration.

Situation 1: A person stands on a skateboard, not moving.

Forces on person:

Situation 2: A person standing on a skateboard pushes against a wall.

Forces on person:

Situation 3: A book rests on a table.

Forces on book:                                                                       Forces on table:

Circle the forces that form an action/reaction pair according to Newton's Third Law in the diagrams for Situation 3.

Situation 4: A person leans with his hand against a wall.

Forces on person:

Situation 5: A car driven by Mr. Apple (who wears no seat belt) undergoes a rear-end collision with a stationary car driven by Mr. Flower.

Forces on Mr. Apple's car:                                                       Forces on Mr. Apple: