What is the difference between mass and weight?

Mass is an intrinsic property representing the amount of matter in an object (measured in kg), while weight is the force of gravity acting on that mass (measured in N) and varies with gravitational field strength.

How does Newton's First Law differ from the Second Law in terms of resultant force?

The First Law describes the behavior of objects when the resultant force is zero (constant motion), while the Second Law quantifies the change in motion when the resultant force is non-zero ($F=ma$).

What is the difference between 'constant velocity' and 'acceleration'?

Constant velocity means speed and direction are unchanging because forces are balanced. Acceleration is the rate of change of velocity, which only occurs when an unbalanced resultant force acts on an object.

Why is it a mistake to say a force is required to keep an object moving?

According to Newton's First Law, an object in motion stays in motion at a constant velocity unless a force acts to stop or change it. Force is required to *change* motion, not to *maintain* it.

What happens if you use mass in grams (g) instead of kilograms (kg) in the formula $F=ma$?

The resulting force value will be 1000 times larger than the true value in Newtons, as the Newton is defined specifically using the SI base unit of kilograms.

Does a negative acceleration always mean an object is moving backwards?

No, a negative acceleration means the acceleration is in the opposite direction to the chosen positive axis. It often indicates the object is slowing down (decelerating) while still moving forward.

Define the 'Newton' (N) as a unit.

One Newton is the amount of force required to accelerate a mass of $1 \text{ kg}$ at a rate of $1 \text{ m s}^{-2}$.

What is 'Terminal Velocity'?

Terminal velocity is the constant maximum speed reached by a falling object when the upward drag force equals the downward weight, resulting in zero acceleration.

What is 'Resultant Force'?

The resultant force is the single vector sum of all individual forces acting on an object, determining the object's overall acceleration.

What is 'Gravitational Field Strength' ($g$)?

It is the force per unit mass exerted by a gravitational field, measured in $\text{N kg}^{-1}$. On Earth, it is approximately $9.81 \text{ N kg}^{-1}$.

Library Podcasts

Courses

Referral & Rewards

1. Mechanics & Materials

Force & Acceleration

Summary

The study of force and acceleration explores how interactions between objects change their state of motion. Governed primarily by Newton's first two laws, this domain establishes that resultant forces are the sole cause of acceleration, while balanced forces lead to equilibrium. Understanding these principles allows for the prediction of an object's behavior, from the constant velocity of a car to the terminal velocity of a falling skydiver.

1. Newton's First Law: The Principle of Inertia

Core Definition: Newton's First Law states that an object will remain at rest or continue to move at a constant velocity unless acted upon by a non-zero resultant force.
Inertia and Equilibrium: This law introduces the concept of inertia—the tendency of objects to resist changes in motion. When the sum of all forces is zero ( $\Sigma F = 0$ ), the object is in a state of equilibrium, meaning its speed and direction remain unchanged.
Motion without Force: A critical insight is that motion itself does not require a force; only a change in motion (acceleration) does. If an object is moving at a steady speed in a straight line, the forces acting on it must be perfectly balanced.

2. Newton's Second Law: Quantifying Motion

A free body diagram of an object at terminal velocity where the downward weight force is exactly balanced by the upward drag force, resulting in zero acceleration.

3. Mass, Weight, and Gravitational Field Strength

4. Terminal Velocity: The Dynamics of Falling

The Process of Acceleration: When an object begins to fall, its weight is the only significant force, causing it to accelerate at $g$ . As its velocity increases, air resistance (drag) also increases.
Reaching the Limit: Eventually, the upward drag force grows until it equals the downward weight. At this point, the resultant force becomes zero, and the object stops accelerating.
Terminal Velocity Definition: This constant maximum velocity is called terminal velocity. It is a state of dynamic equilibrium where the object continues to move at a high speed but can no longer speed up.

5. Key Distinctions: Balanced vs. Unbalanced Forces

6. Exam Strategy & Common Pitfalls