Introduction to Free-Body Diagrams

Introduction to
Free-Body Diagrams

Free-Body Diagram (FBD)

• A diagram showing the magnitude and direction of forces acting on an object.

• Ex: A box resting on a table.

  • F_n= force coming from surface

  • F_G= force of gravity

  • Forces are balanced- net force=zero

    • This either means the object is at rest or at constant velocity (no acceleration)

Multiple Choice

Which of the following best describes the purpose of a free-body diagram?

• To show the types, directions, and magnitude of forces acting on an object

• To calculate the speed of an object

• To measure the mass of an object

• To identify the color of an object

Method for Drawing FBDs

1. Identify the types of forces acting on the object.

2. Determine the direction of those forces.

3. Determine the magnitude (size) of opposing forces

4. Draw the force diagram with labeled arrows and proper size of forces

Ex: A student pushes a box to the left across the floor at a constant speed.

Determining Net Force (F_net)

1. Identify all forces acting on the object

2. Right and Up are positive - left and down are negative

3. Add up all forces in each direction separately

   1. F_net(x)= F_right - F_left

   2. F_net(y)= F_up - F_down

4. If forces are balanced (equal in opposite directions), then

   1. F_net=0 -> object is at rest or moving at constant velocity

5. If forces are unbalanced,

   1. F_net points in the direction of the larger force

6. Connect to motion: F_net= mass x acceleration (F_net=ma)

   1. Object accelerates in direction of the net force

Determining Net Force (F_net)

Ex: An object is hanging from a rope.
It has a tension force of 1200 N
It has a gravitational force of 800 N
What is the net force?

F_net= F_tens-F_grav
F_net= 1200 N - 800 N
F_net= 400 N upward

Multiple Select

A book is attached to a string and hanging from the ceiling. Determine all forces acting on the box.

• Gravity

• Normal

• Tension

• Applied

• Static Friction

Draw

A book is attached to a string and hanging from the ceiling. Draw the free-body diagram.

Multiple Select

A person pushes a crate to the right across the floor at a constant speed. Determine all of the forces acting on the crate.

• Gravity

• Normal

• Applied

• Kinetic Friction

• Static Friction

Draw

A person pushes a crate to the right across the floor at a constant speed.

Draw the free-body diagram.

Multiple Select

A skydiver is falling downward and speeding up.

Determine all of the forces acting on the skydiver.

• Gravity

• Normal

• Drag

• Kinetic Friction

• Static Friction

Draw

A skydiver is falling downward and speeding up.

Draw the free-body diagram.

Multiple Select

A rightward-moving car has locked wheels and is skidding to a stop.

Determine all of the forces acting on the car.

• Gravity

• Normal

• Applied

• Kinetic Friction

• Static Friction

Draw

A rightward-moving car has locked wheels and is skidding to a stop.

Draw the free-body diagram.

Multiple Select

A freight elevator is attached by a cable, being pulled upward, and slowing down.

Determine all of the forces acting on the elevator.

• Gravity

• Normal

• Applied

• Tension

• Spring

Draw

A freight elevator is attached by a cable, being pulled upward, and slowing down.

Draw the free-body diagram.

Multiple Choice

Choose the diagram that is consistent with the physical situation:

A leftward moving baseball player slows to a stop while sliding across the infield dirt.

Multiple Choice

Choose the diagram that is consistent with the physical situation:

A book is suspended motionless from the end of a string

Multiple Choice

Choose the diagram that is consistent with the physical situation:

A rightward moving sledder is slowing down as the glides across a patch of loosely-packed snow.

Multiple Choice

Choose the diagram that is consistent with the physical situation:

A custodian pushes a crate to the right across the gym floor at a constant speed.

Multiple Choice

Choose the diagram that is consistent with the physical situation:

A downward moving skydiver maneuvers into a spread eagle position and slows down.

Multiple Choice

Choose the diagram that is consistent with the physical situation:

A downward moving skydiver (with an unopened parachute) is falling through the air at a constant speed.

Multiple Choice

Choose the diagram that is consistent with the physical situation:

A book is at rest on a table

Multiple Choice

Choose the diagram that is consistent with the physical situation:

A baseball is moving through the air, upward and rightward towards the peak of its trajectory. (Negligible air resistance)