Forces - Definition and Examples

A force is a push or a pull. It is measured in Newtons (N), with SI units of $kg <br />\neq m/s^2$ . Forces are vectors, possessing both magnitude and direction, typically represented as arrows indicating the direction of application.
Forces always involve the interaction of two bodies, even if the interaction is not immediately obvious (e.g., gravity).

A reaction or surface force that balances another force, often gravity, to prevent motion. It is perpendicular to the surface of contact.
The normal force adjusts its magnitude based on the opposing force. For instance, if a person's weight decreases, the normal force from the floor also decreases.
Example: A book on a table experiences a normal force equal to its gravitational force. If an additional force is applied (e.g., pushing a spring on the table), the normal force increases to balance the gravitational force of the spring plus the applied force.

A force that opposes motion. Its direction is always opposite to the direction of the object's movement.
Friction arises from microscopic imperfections on surfaces that create resistance as they rub against each other.
Example: A box moving on a floor experiences friction, making it harder to move.
Air Resistance: A type of friction caused by air molecules. Without air resistance, a skydiver would accelerate indefinitely due to gravity ( $9.8 m/s^2$ ).
Terminal Velocity: The point at which air resistance equals the force of gravity, resulting in no acceleration. Opening a parachute increases air resistance, slowing the skydiver.
Lift and Drag: Air resistance is crucial for avionics. Drag slows down the aircraft, while lift (another form of air resistance) keeps it airborne.

An invisible, non-contact attractive force between objects with mass.
Keeps Earth in orbit around the sun, objects on Earth's surface, and fuels nuclear fusion in the sun.
The force acts along the line connecting the centers of two objects and is always attractive, pulling the objects closer.
We often treat objects as point masses, with gravity acting between these points. The gravitational force on you or a ball points directly towards the Earth's center.

Forces can be thought of as a push or pull that can cause an object to move and are interactions between two bodies.
Forces discussed include frictional, applied, normal, and gravitational forces.
These examples will be used to define and utilize Newton's Laws of Motion in the next lecture.