Glossary
Acceleration
The rate at which an object's velocity changes over time. It can involve a change in speed, direction, or both.
Example:
When a car speeds up from a stoplight, it experiences positive acceleration.
Action-Reaction (Newton's Third Law)
For every action, there is an equal and opposite reaction. Forces always occur in pairs acting on different objects.
Example:
When a rocket expels gas downward, the gas exerts an equal and opposite action-reaction force upward on the rocket, propelling it.
Circular Motion
The movement of an object along the circumference of a circle or rotation along a circular path. It requires a centripetal force directed towards the center.
Example:
A satellite orbiting Earth is an example of an object undergoing circular motion.
Coefficient of Friction (μ)
A dimensionless constant that represents the ratio of the frictional force to the normal force between two surfaces. There are separate coefficients for static ($\mu_s$) and kinetic ($\mu_k$) friction.
Example:
A high coefficient of friction for rubber on asphalt explains why car tires have good grip on roads.
Components (of forces)
The projections of a vector onto the coordinate axes (typically x and y), used to simplify vector addition and analysis. Forces at angles are often broken into their horizontal and vertical components.
Example:
When pulling a sled with a rope at an angle, you break the pulling force into horizontal and vertical components to analyze its effect on motion and normal force.
Equilibrium
A state where the net force acting on an object is zero, resulting in no acceleration. The object is either at rest or moving at a constant velocity.
Example:
A car cruising at a constant speed on a straight highway is in equilibrium because the engine's thrust balances air resistance and friction.
Force
A push or a pull that can cause an object to accelerate or deform. Forces describe how objects or systems interact with each other.
Example:
When you kick a soccer ball, the force from your foot causes the ball to speed up.
Free Body Diagram (FBD)
A visual representation used to analyze forces, showing all external forces acting on a single isolated object as vectors originating from the object's center.
Example:
To solve a problem involving a block on an inclined plane, drawing a Free Body Diagram is the first crucial step.
Friction Force
A force that opposes relative motion or attempted motion between two surfaces in contact. It acts parallel to the surfaces.
Example:
When you slide a box across the floor, the friction force acts against its motion, slowing it down.
Gravitational Force (Weight)
The force of attraction between any two objects with mass, commonly referring to the Earth's pull on an object. It is calculated as Fg = mg.
Example:
The gravitational force on a person on Earth is their weight, pulling them downwards.
Inertia
The tendency of an object to resist changes in its state of motion. Objects with greater mass have greater inertia.
Example:
When a car suddenly brakes, your body continues to move forward due to inertia.
Kinetic Friction
The friction force that opposes the motion of an object that is already sliding over a surface. Its magnitude is generally constant, $f_k = \mu_k F_n$.
Example:
When a car skids on ice, the kinetic friction between the tires and ice helps to slow it down, though it's much less than on dry pavement.
Mass
A fundamental property of matter that measures an object's inertia, or its resistance to acceleration. It is a scalar quantity.
Example:
A bowling ball has more mass than a tennis ball, which is why it's harder to accelerate.
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 two people push a box with equal and opposite forces, the net force is zero, and the box doesn't move.
Newton's First Law of Motion
A body at rest will remain at rest, and a body in motion at a constant velocity will remain in motion in a straight line, unless acted upon by a net external force.
Example:
A book resting on a table will remain at rest unless an external Newton's First Law of Motion force, like a push, acts on it.
Newton's Second Law of Motion
If an unbalanced force acts on a body, that body will experience acceleration. It is mathematically expressed as ΣF = ma, where ΣF is the net force, m is mass, and a is acceleration.
Example:
If you apply a larger Newton's Second Law of Motion net force to a cart, it will accelerate more rapidly.
Normal Force
The component of a contact force perpendicular to the surface that an object rests on or is in contact with. It prevents objects from passing through surfaces.
Example:
When a book rests on a table, the table exerts an upward normal force on the book, preventing it from falling through.
Static Friction
The friction force that prevents an object from starting to move when a force is applied. It adjusts its magnitude up to a maximum value, $f_s \le \mu_s F_n$.
Example:
It's harder to push a heavy couch from rest due to static friction than to keep it moving once it's already sliding.
Vectors
Physical quantities that have both magnitude (size) and direction. Forces, velocity, and acceleration are examples of vector quantities.
Example:
To describe the wind, you need to state both its speed (magnitude) and the direction it's blowing, making it a vector quantity.
Velocity
The rate at which an object changes its position, including both its speed and direction. It is a vector quantity.
Example:
A car traveling at 60 mph east has a different velocity than a car traveling at 60 mph west.