Understanding External Forces

• External forces act on objects from the outside due to the environment pushing or pulling.
• These forces can be contact or non-contact forces.

Non-Contact Forces

• A non-contact force acts on an object without touching it.
• Examples include gravitational, electrostatic, and magnetic forces.
• Gravity is a pulling force that keeps objects on the ground.
• Magnets use magnetic fields to push or pull objects.
• Electric forces, like static electricity, are non-contact forces.

Contact Forces

• A contact force requires physical contact to move an object.
• Examples: wind moving a tree branch, someone kicking a soccer ball, etc.

Types of Friction

• Friction is a force that slows the movement of objects.
• Static friction: Keeps an object still (e.g., a desk on the floor).
• Sliding friction: Slows movement between two surfaces sliding against each other.
• Rolling friction: Slows movement when an object rolls over a surface (e.g., a wheel).

Applied Forces

• Applied forces are contact forces.
• They occur when an external object or person pushes or pulls another object.
• Examples: kicking a soccer ball, a table supporting a book.

Spring Force

• Spring force is created by a compressed or stretched spring on an attached object.
• Compressed springs have potential energy that creates force upon release.
• Examples: pogo stick, trampoline, keys on a keyboard.
• Springs have more force when compressed further due to increased potential energy.
• Releasing a compressed spring converts potential energy into spring force.

Internal Forces

• Internal forces resist external forces acting on a structure from the outside.
• External forces cause motion, while internal forces resist it.
• If the external force is stronger, the structure may move, fail, or break.

Compression

• Compression is a force that squeezes or pushes an object together.
• Example: Your weight compressing a chair.

Tension

• Tension is a force that pulls materials apart or stretches them.
• Example: Pulling on a popsicle stick, stretching a rubber band.

Torsion

• Torsion is a twisting force.
• Example: Twisting the lid of a jar.

Shear

• Shear is a force that causes parts of a material to slide past one another in opposite directions.
• Example: Scissors cutting paper.

Structural Design

• Structures must be designed to withstand various forces.
• Engineers use steel beams and trusses to create stable structures for bridges, houses, etc.
• I-beams/H-beams are used for horizontal and vertical support in heavy structures.
• The design and structure of the I-beam make it capable of handling a variety of loads.
• Trusses use triangles, the strongest shape, for stability.
• Triangles distribute force through all three sides.
• Struts are beams under compression.
• Ties are beams under tension.

Plasticity vs. Elasticity

• Plasticity: An object changes shape and does not return to its original form.
• Elasticity: An object changes shape but returns to its original form when the force is removed.

Action and Reaction Forces

• Action force: A force applied to an object in a particular direction.
• Reaction force: A force that acts in the opposite direction of an action force.
• Forces always come in pairs.
• Newton's Third Law of Motion: "For every action, there is an equal and opposite reaction."
• When you jump off a board, you push down (action force), and the board pushes you up (reaction force).
• When you sit on a chair, you apply an action force on the chair, and the chair exerts a reaction force supporting your weight.