AP Physics 1: Forces and Free Body Diagrams

Scenarios Presented:
- A rolling ball comes to a stop.
- A person pushes off the ground to get moving on roller skates.
- An apple falls from a tree.
Common Element: All scenarios involve interactions that can be described by forces.

Definition of a Force:
- A force is a quantity that describes the interactions between objects or systems.
Key Points About Forces:
- An object cannot exert a force on itself, indicating that forces must come from external sources.
- When discussing systems, one can consider multiple objects together; however, only external interactions impact changes in the system.

Contact Forces:
- Require direct contact between interacting objects.
Field Forces:
- Can be exerted at a distance; relevant field force in this context is gravitational force, which is the interaction between an object and the gravitational field it is located in.

Case Study: A rolling ball
- Interacting with the floor (contact force) and the Earth (gravitational force).
- Air interaction is generally ignored in AP Physics 1.
Representation:
- Two arrows used to represent the interaction:
- One arrow from the ball to the Earth (force exerted by the ball on the Earth).
- One arrow from the Earth to the ball (force exerted by the Earth on the ball).
- Key Concept: All interactions are bi-directional; both objects exert a force on each other.

Scenario: A person lifts an object above their head.
Identified Interactions:
- Person and the object (contact).
- Person and the ground (contact).
- Object and Earth (field interaction).
Representation of Interactions:
- Draw arrows representing:
- Force of the object on the person (downward).
- Gravitational force on the person from the Earth (downward).
- Normal force from the floor on the person (upward).
- Note: No interaction between the object and the floor as they are not in contact.

Purpose: Focus on one object or system at a time to represent all interactions with its surroundings.
Creating a Free Body Diagram:
1. Define the system: Choose the boundaries and identify the object or set of objects.
2. Identify interactions: Determine all surrounding forces acting on that object or system.
3. Field interactions: Include any relevant field forces (e.g., gravity).
4. Representation: Use a dot to denote the center of mass and arrows to depict interactions, pointed away from the object in their respective directions.

Main Example: Ball rolling across the floor.
- Isolate the ball and determine its interactions:
- Force exerted by the Earth (downward).
- Force exerted by the floor (upward).
- Diagram Elements:
- Center of mass represented by a dot.
- Arrows indicating forces:
  - Downward arrow for gravitational force.
  - Upward arrow for normal force from the floor.
Another Example: Person lifting an object.
- Identify interactions:
- Object’s downward force on the person.
- Earth’s downward gravitational force on the person.
- Floor’s upward push on the person.
- Draw arrows accordingly in the free body diagram with correct labels.

Task: Draw a free body diagram for a suitcase being pulled across the floor.
Identified Interactions:
- Ground (contact force), person pulling the suitcase (contact force), Earth (gravitational pull).
Arrow Directions:
- Arrow from the person (up and to the right) indicating the force of the person on the suitcase.
- Downward arrow for the gravitational force from the Earth.
- Upward arrow for the normal force from the ground.

Key Takeaways:
- A force is a vector quantity describing the interactions between objects or systems.
- An object cannot exert a force on itself; all forces are external.
- Free body diagrams are essential for visualizing and analyzing forces acting on an object or system.
Closing Remarks: Thank you for engaging in this physics topic.