Forces & Motion — Newton's Laws Flashcards

What is Newton's First Law of Motion?

An object at rest stays at rest, and an object in motion stays in motion at constant velocity, unless acted on by a net external force. This is the law of inertia: things keep doing what they're doing until a force changes it. Example: a book on a table won't slide until you push it, and a hockey puck on frictionless ice would glide forever.

What is inertia?

Inertia is the tendency of an object to resist changes in its state of motion. The more mass an object has, the more inertia it has. Example: it's much harder to start pushing (or to stop) a loaded shopping cart than an empty one, because the loaded cart has more mass and therefore more inertia.

What is Newton's Second Law of Motion?

The net force on an object equals its mass times its acceleration: F = ma (force in newtons, mass in kilograms, acceleration in m/s²). It tells you that a bigger force gives more acceleration, while a bigger mass gives less. Example: a 2 kg ball pushed with 10 N accelerates at a = F/m = 10/2 = 5 m/s².

What is Newton's Third Law of Motion?

For every action there is an equal and opposite reaction: when object A pushes on object B, B pushes back on A with a force equal in size but opposite in direction. The two forces act on different objects. Example: when you jump, your feet push down on the Earth and the Earth pushes up on you, launching you into the air.

What is a force? (and its unit)

A force is a push or a pull that can change an object's motion or shape. It is a vector, meaning it has both size and direction. Force is measured in newtons (N), where 1 N = 1 kg·m/s². Example: lifting a small apple takes about 1 N of upward force.

What is the newton (N)?

The newton is the SI unit of force. One newton is the force needed to accelerate a 1 kg mass at 1 m/s²: 1 N = 1 kg·m/s². Example: holding up a 100 g apple against gravity takes roughly 1 N.

What is the difference between mass and weight?

Mass is the amount of matter in an object, measured in kilograms (kg), and it stays the same everywhere. Weight is the gravitational force on that mass, measured in newtons (N), given by W = mg. Example: a 10 kg bag has the same 10 kg mass on Earth and the Moon, but it weighs about 98 N on Earth and only about 16 N on the Moon.

How do you calculate weight?

Weight is the force of gravity on an object: W = mg, where m is mass in kilograms and g is the gravitational field strength (about 9.8 m/s² on Earth's surface). Example: a 5 kg object weighs W = 5 × 9.8 = 49 N on Earth.

What is gravity?

Gravity is the attractive force between any two objects that have mass; the more massive they are and the closer they are, the stronger the pull. Near Earth's surface it gives falling objects an acceleration of about 9.8 m/s². Example: gravity is what pulls a dropped pencil straight down to the floor and keeps the Moon in orbit around Earth.

What is friction?

Friction is a force that opposes the relative motion (or attempted motion) between two surfaces in contact. It acts opposite to the direction of motion and can produce heat. Example: friction between your shoes and the ground lets you walk without slipping, and it's what slows a sliding book to a stop.

What is net force?

Net force is the single overall force you get by combining all the individual forces acting on an object, accounting for their directions. If forces cancel, the net force is zero; if they don't, the object accelerates in the direction of the net force. Example: if you pull a box right with 8 N while friction pulls left with 3 N, the net force is 5 N to the right.

What is the difference between balanced and unbalanced forces?

Forces are balanced when they cancel out to a net force of zero, so the object stays at rest or keeps moving at constant velocity. Forces are unbalanced when there is a nonzero net force, which causes acceleration. Example: a tug-of-war where both teams pull equally is balanced (no movement); if one team pulls harder, the forces are unbalanced and the rope moves.

What is the difference between speed and velocity?

Speed is how fast an object moves (a scalar with only magnitude, e.g., 30 m/s). Velocity is speed with a direction (a vector, e.g., 30 m/s north). Example: a car going 60 km/h has a speed of 60 km/h, but its velocity is 60 km/h heading east; if it rounds a curve at steady speed, its velocity still changes because the direction changes.

What is acceleration? (and its formula)

Acceleration is the rate of change of velocity over time: a = Δv / Δt, measured in m/s². An object accelerates when it speeds up, slows down, or changes direction. Example: a car going from 0 to 20 m/s in 5 s has an acceleration of a = 20/5 = 4 m/s².

What is momentum? (and its formula)

Momentum is the quantity of motion an object has, equal to its mass times its velocity: p = mv, measured in kg·m/s. It is a vector pointing in the direction of motion, and heavier or faster objects have more of it. Example: a 2 kg ball moving at 3 m/s has a momentum of p = 2 × 3 = 6 kg·m/s.

What is free fall and the acceleration due to gravity?

Free fall is motion under the influence of gravity alone, with no air resistance. Near Earth's surface all objects in free fall accelerate downward at g ≈ 9.8 m/s², regardless of their mass. Example: ignoring air resistance, a dropped hammer and feather fall together and gain about 9.8 m/s of speed for each second they fall.