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### 401-A: Free-Body Diagrams 1

• Topic Cluster: Dynamics
• Topic: Identifying Forces (Newton's Third Law)
• Objective: Recognize the forces in the most common free-body diagrams.
• Content: In free-body diagrams, all forces are represented as arrows showing the direction that force acts; the most common forces in our lives are gravity, normal forces, applied forces, friction, and tension.
• Level: 1

#### Very Common Forces

Any force caused by actually hitting something. Examples: pushes, punches, pulls, kicks, head butt. When a contact force is exerted by a person, it is sometimes called an applied force.

The force of all masses attracting all other masses.
When on earth, the force of gravity is also called weight an always points downward.

A force that acts perpendicular to a surface. Usually, this is a force that is opposite the force of gravity and holds you up.

A force that affects anything sliding on a surface. Always resists motion.

The force of air pushing back on something as it moves. Air resistance affects things that have a large surface area and move very fast. It is also called drag.

For each question, pick a force.

1. What type of force is a punch to the stomach?
2. What kind of force makes you fall down? [2 answers]
3. What is another name for gravity, when you are earth.
4. What force keeps your books from sliding off the table?
5. What force do you feel when you put your hand out the window of a speeding car?
6. What force is very low when you are on ice?
7. What force holds you up when you sit in a chair?
8. What is another name for air resistance?

#### Important Free-Body Diagrams

A free-body diagram shows all of the forces acting on an object, each represented as an arrow.

Drawing good free-body diagrams is the first step to mastering dynamics. Forces cause all changes in motion, and a free-body diagrams shows the forces acting on some object. In this pod, we will begin to draw qualitative free-body diagrams on an object.

A person sitting in a chair.
A weight being lifted by a weightlifter
A block being pushed by someone across the floor.
Someone falling down at a low velocity.
Someone falling down with a high velocity.

#### Questions

For each situation, draw a relevant free-body diagram.

1. A person standing still
2. Free-Body Diagram of the person (2 forces)
3. Someone holding up a box
4. Free-Body Diagram of the box (2 forces)
5. An inanimate object falling down at a low velocity
6. Free-Body Diagram of the object (1 force)
7. A piano falling down at a very high velocity.
8. Free-Body Diagram of the paino
9. Someone is pushing a couch across the floor.
10. Free-Body Diagram of the couch

#### More Forces

The force of a rope pulling.

When you stretch something our, the elastic force is the force that pulls back.

The force of an engine pushing something forward.

The force of air pressing on everything around it. It exists constantly in your life, but you never notice it.

A force that pushes upward when air pressure is reduced by an airplane wing. Allows an airplane to fly.

The force of water pressing upward on something. This is the force that allows a boat to float.

For each question or statement, pick a force.

1. If this force is too strong inside of a balloon, the balloon will pop.
2. This force pushes an airplane forward.
3. This force acts on each group in a game of tug-o-war.
4. This force allows an airplane to stay in the sky, pushing it upward.
5. When you pull a long slinky outward, this force pulls it back together.

#### Examples of Free-Body Diagrams

Someone pulling back an arrow in a bow:
Free body diagram of arrow:
(Omitting gravity and an upward force.)
A person hanging from a rope.
An airplane flying
A boat floating
A boat being pushed forward with an engine.

### Video Resources

##### Flipping Physics Video

Note that this same video appears in another pod.