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SPH4C: Physics, Grade 12, College Preparation

# Unit 1: Motion and Its Applications

## Activity 4: Force, Free-Body Diagrams, and Net Force

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Free-Body Diagrams

To help visualize and solve problems involving forces, it is useful and often necessary to draw free-body diagrams.

Free-body diagrams are diagrams in which the forces acting on an object in a given situation are drawn on the object, free of its surroundings.

Points to remember when drawing a free-body diagram:

• Sketch the object, free of its surroundings. This may be as simple as a circle, square, or dot.
• Draw force vectors such that their tails originate at the center of the object and the tip points away from this point.
• Draw force vectors to be representative of the quantities given. (There is no need to construct scale diagrams unless specifically requested.)
• Label all forces. All forces will come from the list of forces you learned on the previous content page.

#### Example

As you are sitting on a chair, there are two forces acting on you. There is the force of gravity acting downward and the force of the chair upward on you, e.g., the normal force. These forces are shown acting on the lady sitting on a chair below. The free-body diagram (FBD) is shown to the right.

Draw free-body diagrams for the red book in each of the following situations. Check your answers by holding your mouse over the image to see the correct FBD.

1. a falling book

2. a book sitting on a desk (FBD is for the book)

3. a book being pushed (and accelerating) across a desk

#### Resources

Unless otherwise indicated, all images in this activity are from the public domain or are © clipart.com or Microsoft clipart and are used with permission.
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