Glossary
Air resistance (drag)
A resistive force exerted by a fluid (like air) on an object moving through it. It opposes the object's motion.
Example:
A skydiver experiences significant air resistance as they fall, which eventually limits their speed.
Applied forces
External forces that are intentionally exerted on an object, typically through a push or a pull. These are often the forces initiating or influencing motion.
Example:
A student pushing a desk across the classroom is exerting an applied force on the desk.
Applied forces
External pushes or pulls exerted on an object by an agent. These are often explicitly stated in problem descriptions.
Example:
If you push a shopping cart, the force you exert on the cart is an applied force.
Center of mass
The unique point where the weighted average of all the mass of a system is located. For free-body diagrams, all forces are typically drawn as originating from this point.
Example:
When a gymnast performs a flip, they rotate around their center of mass, which might be outside their body during certain maneuvers.
Center of mass
The unique point where the weighted average of all the masses of a system is located. In free-body diagrams, the object is often represented as a dot at this point.
Example:
For a uniformly dense sphere, its center of mass is exactly at its geometric center.
Contact forces
Forces that arise when two objects or systems are physically touching each other. These forces are macroscopic manifestations of interatomic electric forces.
Example:
The push you exert to open a door is a contact force because your hand is directly touching the door.
Contact forces
Forces that arise when two objects are physically touching each other. These are macroscopic manifestations of interatomic electromagnetic interactions.
Example:
When you push a door open, the force you exert on the door is a contact force.
Coordinate system
A framework used to define the position and direction of forces and motion, typically consisting of perpendicular axes (e.g., x and y). Choosing an appropriate one simplifies problem-solving.
Example:
When analyzing a block sliding down an inclined plane, aligning the x-axis of your coordinate system parallel to the incline simplifies the force components.
Direction
The orientation or line along which a vector quantity acts. It specifies where the force is pushing or pulling.
Example:
When a rocket launches, the thrust force has a specific direction upwards, opposite to gravity.
Free-body diagrams
Visual representations used to analyze the forces acting on a single object or system. They simplify complex physical situations by isolating the object and showing all external forces as vectors.
Example:
Before solving a problem about a sled being pulled across snow, a student draws a free-body diagram to clearly see the gravitational force, normal force, tension, and friction acting on the sled.
Free-body diagrams
Visual representations that show all external forces acting on a single isolated object or system. They are crucial for applying Newton's laws.
Example:
Before solving for the acceleration of a block on an incline, drawing a free-body diagram helps identify all forces like gravity, normal force, and friction.
Friction force
A contact force that opposes the relative motion or tendency of motion between two surfaces in contact. It acts parallel to the surfaces.
Example:
When a book slides across a table, the friction force acts to slow it down, opposing its motion.
Gravitational force
The attractive force between any two objects with mass, commonly referring to the force exerted by Earth on an object, pulling it downwards.
Example:
An apple falling from a tree is pulled towards the Earth by the gravitational force.
Gravitational force (weight)
The attractive force exerted by a massive body (like Earth) on an object, directed towards the center of the massive body. It is commonly referred to as an object's weight.
Example:
When an apple falls from a tree, it accelerates downwards due to the Earth's gravitational force pulling it towards the ground.
Magnitude
The numerical value or size of a vector quantity, indicating its strength or extent. For a force, it's how strong the push or pull is.
Example:
If you push a box with a force of 50 Newtons, 50 Newtons is the magnitude of the force.
Net force
The vector sum of all individual forces acting on an object. It determines the object's acceleration according to Newton's Second Law.
Example:
If a car is accelerating, the net force acting on it is non-zero and points in the direction of acceleration.
Normal force
A contact force exerted by a surface on an object resting on it, acting perpendicular to the surface. It prevents the object from passing through the surface.
Example:
A book resting on a table experiences an upward normal force from the table, counteracting the book's weight and preventing it from falling through.
Normal force
A contact force exerted by a surface on an object that is perpendicular to the surface. It prevents the object from passing through the surface.
Example:
A book resting on a table experiences an upward normal force from the table, balancing its weight.
Tension
A pulling force transmitted axially along a string, rope, cable, or similar one-dimensional continuous object. It acts in the direction of the string.
Example:
When a rock climber hangs from a rope, the tension in the rope is the upward force supporting their weight.
Tension force
A pulling force transmitted axially through a string, rope, cable, or similar one-dimensional continuous object. It acts along the length of the rope.
Example:
When a chandelier hangs from the ceiling, the cable supporting it exerts a tension force upwards.
Vector quantities
Physical quantities that possess both magnitude (size) and direction. Forces are a prime example, requiring both how much and in what way they act.
Example:
When a soccer player kicks a ball, the force applied is a vector quantity because it has a specific strength (magnitude) and is directed towards the goal (direction).
Vector quantities
Physical quantities that possess both magnitude and direction. Forces are a prime example, requiring an arrow to represent them fully.
Example:
When a car accelerates, its velocity is a vector quantity because it has both a speed (magnitude) and a specific direction of travel.