Glossary
Action-Reaction Pairs
These are the two forces described by Newton's Third Law: a force exerted by object A on object B (action) and an equal and opposite force exerted by object B on object A (reaction). They always act simultaneously on different objects.
Example:
As a rocket expels hot gases downward (action), the gases exert an equal and opposite force upward on the rocket, causing it to accelerate. These are action-reaction pairs.
Action-Reaction Pairs
These are two forces that are equal in magnitude and opposite in direction, acting on different objects involved in an interaction. They never cancel each other out because they apply to separate bodies.
Example:
When you jump, your feet exert a downward force on the Earth (action), and the Earth exerts an equal upward force on your feet (reaction), propelling you into the air. These are action-reaction pairs.
Center of Mass (COM)
The unique point where the weighted average of all the masses in a system is located. For translational motion, the entire mass of the system can be considered concentrated at this point.
Example:
A gymnast performing a flip tries to keep their body compact, effectively rotating around their center of mass to maintain balance and control.
External Forces
Forces exerted on a system by objects or agents outside of that system. These are the only forces that can change the motion of the system's center of mass.
Example:
When a car brakes, the friction force from the road acting on the tires is an external force that slows the car down.
External Forces
Forces that originate from outside a defined system and act upon objects within that system. These forces are responsible for changing the motion of the system's center of mass.
Example:
When a car accelerates, the friction force from the road pushing on the tires is an external force that changes the car's overall motion.
Free-Body Diagrams
A visual representation used to analyze forces acting on a single object or a system of objects. It shows all external forces acting on the body, represented by vectors originating from the object's center.
Example:
To solve a problem involving a block sliding down an inclined plane, drawing a free-body diagram helps identify and resolve the gravitational, normal, and frictional forces.
Ideal Pulleys
A theoretical pulley model assumed to have negligible mass and no friction. For an ideal pulley, the tension in the string passing over it remains the same on both sides.
Example:
When analyzing a simple machine like an Atwood machine, the pulley is typically assumed to be an ideal pulley to simplify calculations, meaning it only changes the direction of the force.
Ideal Pulleys
A theoretical pulley model assumed to have negligible mass, no rotational inertia, and negligible friction at its axle. Its primary function is to change the direction of a tension force without altering its magnitude.
Example:
When lifting a heavy object with a single pulley, an ideal pulley ensures that the force you pull with is equal to the weight of the object, just in a different direction.
Ideal String
A theoretical string model used in physics problems that is assumed to have negligible mass, be inelastic (not stretch), and be perfectly flexible. Tension is constant throughout an ideal string.
Example:
In a simple Atwood machine problem, the string connecting the two masses is often assumed to be an ideal string to simplify calculations.
Ideal Strings
A theoretical string model used in physics problems that is assumed to have negligible mass and to be inextensible (does not stretch). In such strings, tension is uniform throughout its length.
Example:
In many pulley problems, the string connecting the masses is considered an ideal string, meaning its mass is ignored and the tension is the same everywhere.
Internal Forces
Forces that exist between objects within a defined system. These forces always come in action-reaction pairs and do not affect the overall motion of the system's center of mass.
Example:
In a system consisting of two connected train cars, the tension in the coupling between them is an internal force.
Internal Forces
Forces that act between objects or parts of objects within a defined system. These forces do not affect the motion of the system's center of mass.
Example:
In a tug-of-war, the forces between the team members pulling on the rope are internal forces to the team-rope system; they don't move the system's center of mass.
Newton's Third Law
For every action, there is an equal and opposite reaction. This fundamental principle states that forces always occur in pairs, acting on two different objects.
Example:
When you push off a wall to swim, the wall exerts an equal and opposite force back on you, propelling you forward. This is an example of Newton's Third Law in action.
Newton's Third Law
For every action, there is an equal and opposite reaction. This fundamental principle states that forces always occur in pairs acting on different objects.
Example:
When a rocket expels hot gas downwards, the gas exerts an equal and opposite force upwards on the rocket, causing it to accelerate.
Non-Ideal Strings
A more realistic string model that possesses mass and can stretch. In non-ideal strings, the tension is not uniform along its length, often being greater at the top if supporting its own weight.
Example:
A long, heavy chain hanging from a crane is a non-ideal string; the tension at the top supporting the entire chain's weight is greater than the tension near the bottom.
Tension
The pulling force transmitted axially along the length of a string, cable, chain, or similar one-dimensional continuous object. It arises from the internal forces within the object resisting stretching.
Example:
When you hang a heavy backpack from a hook, the force pulling upward on the hook through the strap is the tension in the strap.
Tension
The pulling force transmitted axially through a string, cable, chain, or similar one-dimensional continuous object when it is pulled taut. It is a force that acts along the length of the string.
Example:
A chandelier hanging from the ceiling is supported by the tension in the chain connecting it to the roof.