Free body diagrams

Free Body Diagramsfor High School PhysicsBy Mr. John M. Burnett II

Newton’s 2nd Law

• In order for an object to change its velocity, even it is at rest, a net force must be acting on it.

ΣF = mΣa

• Which reads, the sum of the forces acting on an object is equal to the mass of the object multiplied by the sum of the accelerations.

Free Body Diagram

• Definition - A diagram that shows every force acting on an object.• Example: A book at rest on a table.

Free Body Diagram Rules

1.Isolate the object.2.Choose a coordinate system.3.Sketch the forces.4.Resolve the forces into

components (If necessary).

5.Find the net force.6.Apply Newton’s Second Law of

Motion.

Isolate the object

• Simplify the model just to include the object being observed.

=

Choose a coordinate system

• For our purposes a Cartesian coordinate system works best.

Sketch the forces

• In this case we chose the y-axis of the coordinate system.

• Forces stem from the center of the object outward.

• Length of the arrow represents magnitude of force.

• Direction of arrow represents direction of force.

Resolve the forces into components.

• This is only necessary if the force is applied at an angle to the x or y-axis.• Used to find amount of force in x and y

dimension.

Find the net force.

• The book is at rest and according to Newton’s 1st law the object is not encountering a net force.• The net force is zero.• The table pushes up on the book with the same force that gravity is pulling down with.• Arrows are equal length and opposite direction.

Find the net force.(continued)

Apply Newton’s Second Law of Motion.

•ΣF = mΣa•ΣF = 0N

•So, Σa = 0 m/s2

Try This

• A 2,200 kg car is accelerating at 1.25 m/s2 rightward on a horizontal road. It is encountering a frictional force of 500 N. Draw a free body diagram for the car.

Isolate the object.

Choose a coordinate system

Sketch the forces

• Note that FA is longer than FF due to the fact the car is accelerating rightward.

Find the net force.

Apply Newton’s Second Law of Motion.

The end

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