Glossary
Accelerating
The state of an object when its velocity is changing, either in magnitude (speeding up or slowing down) or direction. This occurs when a net force acts on the object.
Example:
A car accelerating from a stop sign is increasing its speed.
Acceleration
The rate at which an object's velocity changes over time. It is a vector quantity, having both magnitude and direction.
Example:
When a car speeds up, it experiences positive acceleration; when it slows down, it experiences negative acceleration (deceleration).
Action-Reaction Pairs
Two forces that are equal in magnitude and opposite in direction, acting on different objects, as described by Newton's Third Law.
Example:
The force of your foot pushing on the ground and the force of the ground pushing back on your foot form an action-reaction pair.
Apparent Weight
The sensation of weight an object experiences, which can differ from its actual weight when it is accelerating vertically. It is the magnitude of the normal force supporting the object.
Example:
When an elevator accelerates upwards, you feel an increase in your apparent weight.
Atwood's Machine
A common physics laboratory device consisting of two masses connected by a string over a pulley, used to demonstrate Newton's Second Law and concepts of tension and acceleration.
Example:
Analyzing an Atwood's Machine helps students understand how forces and masses affect the motion of connected systems.
Closed Systems
A system that does not exchange matter with its surroundings, but can exchange energy.
Example:
A sealed thermos bottle containing hot coffee is a closed system because heat can escape, but no coffee can.
Coefficient of Friction (μ)
A dimensionless scalar value that describes the ratio of the frictional force to the normal force between two surfaces. It depends on the nature of the surfaces in contact.
Example:
A high coefficient of friction for car tires on asphalt provides good grip for braking.
Conservative Systems
A system where the total mechanical energy (kinetic + potential) remains constant, meaning no energy is lost to non-conservative forces like friction or air resistance.
Example:
A pendulum swinging without air resistance is an example of a conservative system.
Contact Forces
Forces that occur when two objects are physically touching each other.
Example:
The push you exert on a door to open it is a contact force.
Direction
The orientation or path along which a vector quantity, like force or velocity, acts. It specifies where the force is applied or where the object is moving.
Example:
The direction of the wind determines which way a sailboat will be pushed.
Dynamics
The branch of mechanics concerned with the study of forces and their effect on motion. It explains *why* objects move the way they do.
Example:
Understanding the dynamics of a car crash helps engineers design safer vehicles.
Equilibrium
A state where the net force acting on an object is zero. In this state, the object is either at rest or moving with a constant velocity.
Example:
A book resting motionless on a table is in equilibrium.
External Forces
Forces that act on a system from *outside* the system. These forces can change the total momentum or energy of the system.
Example:
Gravity acting on a falling apple is an external force if the apple is the system.
Field Strength (g)
The gravitational force per unit mass at a particular location within a gravitational field. Near Earth's surface, its value is approximately 9.8 m/s².
Example:
The field strength (g) on Mars is less than on Earth, which is why astronauts would feel lighter there.
Forces
Pushes or pulls that can cause an object to accelerate or deform. They are vector quantities, possessing both magnitude and direction.
Example:
When you kick a soccer ball, the force from your foot causes it to accelerate.
Free-Body Diagrams (FBDs)
A visual tool used to represent all forces acting on a single object. The object is typically drawn as a point, and forces are shown as labeled vectors originating from that point.
Example:
Drawing an FBD for a book on a table helps visualize the normal force and weight acting on it.
Friction
A force that opposes relative motion or attempted motion between two surfaces in contact. It arises from the microscopic irregularities of the surfaces.
Example:
The friction between your shoes and the ground allows you to walk without slipping.
Gravitational Constant (G)
A universal constant that quantifies the strength of the gravitational attraction between masses. Its value is approximately 6.67 x 10^-11 Nm^2/kg^2.
Example:
The gravitational constant (G) is a fundamental constant used in calculating the gravitational force between any two objects in the universe.
Gravitational Field
The region of space around a massive object where another massive object would experience a gravitational force. Its strength is represented by 'g'.
Example:
The Earth creates a gravitational field that pulls objects towards its center.
Gravitational Force
The attractive force between any two objects with mass. Its magnitude is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers.
Example:
The gravitational force between the Earth and the Moon keeps the Moon in orbit.
Hooke's Law
A principle stating that the force needed to extend or compress a spring by some distance is proportional to that distance (F = -kx). The negative sign indicates the force is restorative.
Example:
When you stretch a rubber band, the force it pulls back with is described by Hooke's Law.
Inertia
The inherent property of an object to resist changes in its state of motion. The more mass an object has, the greater its inertia.
Example:
It's harder to push a heavy shopping cart than an empty one because the heavy cart has more inertia.
Inertial Reference Frame
A frame of reference in which Newton's first law of motion holds true; that is, an object not subject to any net force moves at a constant velocity.
Example:
A laboratory on Earth is typically considered an inertial reference frame for most physics experiments.
Internal Forces
Forces that act between objects *within* a defined system. These forces do not change the total momentum of the system.
Example:
The tension in a string connecting two blocks in an Atwood's machine is an internal force if both blocks are part of the system.
Isolated Systems
A system that exchanges neither matter nor energy with its surroundings.
Example:
In an ideal scenario, the entire universe can be considered an isolated system.
Kinetic Friction
The force that opposes the *motion* between two surfaces that are sliding past each other. It is typically constant once motion has started.
Example:
The kinetic friction between a sliding hockey puck and the ice slows the puck down.
Magnitude
The numerical value or size of a vector quantity, such as force or velocity, without regard to its direction.
Example:
A car traveling at 60 mph has a magnitude of speed of 60 mph, regardless of whether it's going north or south.
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, so it requires a greater force to accelerate it.
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 you push a box with 10 N to the right and a friend pushes with 5 N to the left, the net force on the box is 5 N to the right.
Newton's First Law (Law of Inertia)
States that an object at rest stays at rest, and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.
Example:
When a car suddenly stops, your body continues to move forward due to Newton's First Law.
Newton's Laws
Three fundamental laws of classical mechanics that describe the relationship between a body and the forces acting upon it, and its motion in response to those forces.
Example:
Understanding Newton's Laws is essential for predicting the trajectory of a thrown baseball.
Newton's Second Law
States that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass (F = ma).
Example:
Pushing a lighter bicycle with the same force as a heavier motorcycle will result in a greater acceleration for the bicycle, demonstrating Newton's Second Law.
Newton's Third Law
States that for every action, there is an equal and opposite reaction. 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, propelling it, illustrating Newton's Third Law.
Non-conservative Systems
A system where the total mechanical energy is not conserved, typically due to the presence of forces like friction or air resistance that dissipate energy.
Example:
A block sliding to a stop on a rough surface is a non-conservative system because friction converts mechanical energy into thermal energy.
Non-contact Forces
Forces that act on an object without physical contact between the interacting objects.
Example:
The Earth's gravity pulling on a satellite is a non-contact force.
Normal Force
The component of a contact force that is perpendicular to the surface of contact. It prevents objects from passing through surfaces.
Example:
When you stand on the floor, the floor exerts an upward normal force on you, preventing you from falling through.
Open Systems
A system that can exchange both matter and energy with its surroundings.
Example:
A boiling pot of water on a stove is an open system as both steam (matter) and heat (energy) are exchanged with the environment.
Spring Force
The restorative force exerted by a spring when it is compressed or stretched from its equilibrium position, proportional to the displacement.
Example:
A car's suspension system uses spring force to absorb shocks from bumps in the road.
Static Friction
The force that opposes the *initiation* of motion between two surfaces in contact. It can vary in magnitude up to a maximum value before motion begins.
Example:
When you try to push a heavy couch, static friction initially prevents it from moving.
Systems
A defined collection of objects or particles chosen for study in physics. Defining a system helps distinguish between internal and external forces.
Example:
When analyzing a car's engine, the engine itself is the system being studied.
Tension
The pulling force transmitted axially by means of a string, cable, chain, or similar one-dimensional continuous object. It always acts along the length of the string.
Example:
The tension in a rope supports a rock climber as they ascend a cliff.
Weight (Force of Gravity)
The force exerted by gravity on an object, calculated as the product of the object's mass and the acceleration due to gravity (W = mg). It always acts downwards.
Example:
Your weight on Earth is different from your weight on the Moon because of the difference in gravitational acceleration.