Four Forces

​Forces and Free-Body Diagrams

Force - any influence that tends to accelerate an object; a push or a pull; measured in Newtons (N)

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Net Force - combination of all the forces acting on an object.​

Net Forces Recap

Support Force/Normal Force - the force pushing back on an object at rest

• A book sits on the table:   What forces act on it?

  • Force of book on table (due to gravity)

  • Force of table on book (support force - normal force)

• Bathroom Scale:  What forces act on it?

  • Force of you on scale (due to gravity)

  • Force of scale on you (support force - normal force)

• Gymnast on Rings:  What forces act on it?

  • Rope undergoes “stretching” force when hanging by it

  • Two vertical ropes share the load (think trapeze or pull ups)​

Friction Recap

• Friction results from relative motion between objects

• Frictional forces are forces that resist or oppose motion

  • Depends on . . . 

    • 2 surfaces in contact (silk, sand paper, . . .)

    • Normal Force (more weight on top of the book)

Static Friction

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Sliding Friction

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Rolling Friction​

​Types of Friction

Free-body diagrams are used to show the relative magnitude and directions of all forces acting on an object.

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This diagram shows four forces acting upon an object. There aren't always four forces​

Free-Body Diagrams

Draw

A book is at rest on a table top.  Diagram the forces acting on the book

In this diagram, there are normal and gravitational forces on the book

Draw

A rightward force is applied to a book in order to move it across a desk.  Consider frictional forces.  Neglect air resistance.  Construct a free-body diagram. 

Draw

An egg is free-falling from a nest in a tree.  Neglect air resistance. Draw a free-body diagram showing the forces involved.

Draw

A flying squirrel is gliding (no wing flaps) from a tree to the ground at constant velocity.  Consider air resistance.  A free body diagram for this situation looks like . . .