Glossary
Angle (θ)
The rotational equivalent of linear position, representing the angular displacement of an object, measured in radians (rad).
Example:
If a door opens 90 degrees, its angle of rotation is π/2 radians.
Centripetal Acceleration (ac)
The acceleration directed towards the center of a circular path, responsible for changing the direction of an object's velocity in circular motion.
Example:
As a car rounds a curve, the friction between the tires and the road provides the centripetal acceleration needed to keep it on the curved path.
Frame of Reference
A coordinate system or set of axes used to describe the position, velocity, and acceleration of an object.
Example:
When you describe the motion of a car, you might use the road as your frame of reference.
Inertial Frame of Reference
A frame of reference in which Newton's laws of motion are valid, meaning it is either at rest or moving at a constant velocity.
Example:
A spaceship cruising at a constant velocity through deep space is an excellent example of an inertial frame of reference.
Net Force
The vector sum of all individual forces acting on an object, determining its acceleration according to Newton's Second Law.
Example:
If you push a box with 10 N to the right and your friend pushes with 5 N to the left, the net force on the box is 5 N to the right.
Newton's Third Law
States that for every action, there is an equal and opposite reaction, meaning forces always occur in pairs acting on different objects.
Example:
When a rocket expels hot gas downwards, the gas exerts an equal and opposite upward force on the rocket, illustrating Newton's Third Law.
Non-inertial Frame of Reference
A frame of reference that is accelerating, where Newton's laws of motion do not directly apply without the introduction of fictitious forces.
Example:
Riding a roller coaster as it speeds up and turns is a non-inertial frame of reference because you feel pushed by 'forces' that aren't real interactions.
Period (T)
The time it takes for an object to complete one full rotation or revolution.
Example:
The period of Earth's rotation is approximately 24 hours.
Rotational Acceleration (α)
The rate at which an object's rotational velocity changes, measured in radians per second squared (rad/s²).
Example:
When a bicycle wheel starts from rest and speeds up, it experiences a positive rotational acceleration.
Rotational Velocity (ω)
The rate at which an object rotates or revolves around an axis, measured in radians per second (rad/s).
Example:
A spinning figure skater increases her rotational velocity as she pulls her arms in.
Scalar
A physical quantity that has only magnitude (size) and no direction.
Example:
Mass is a scalar quantity; a 5 kg object is just 5 kg, regardless of direction.
Vector
A physical quantity that has both magnitude (size) and direction.
Example:
Velocity is a vector because it tells you both how fast an object is moving and in what direction.