What causes objects to move or stay still

Overview

The movement or stillness of objects in our everyday lives is governed by fundamental principles of physics, primarily described by Isaac Newton's laws of motion. These laws explain why a ball remains on the ground until kicked, why a car continues to roll after the engine is turned off (for a while), and why pushing a heavy box requires more effort than pushing a light one. Essentially, it's all about forces and how they interact with mass.

What are Forces?

A force is a push or a pull that can cause an object to change its state of motion. Forces are vector quantities, meaning they have both magnitude (how strong the push or pull is) and direction. Common examples of forces include gravity, friction, applied force (like pushing or pulling), tension, and air resistance.

Newton's First Law of Motion: The Law of Inertia

Often called the law of inertia, Newton's First Law states that an object will remain at rest or in uniform motion in a straight line unless acted upon by a net external force. This means that:

• An object at rest will stay at rest. Think of a book sitting on a table. It won't move unless you push it or something else acts upon it (like an earthquake).
• An object in motion will stay in motion with the same speed and in the same direction. This is less intuitive in our everyday experience because friction and air resistance are almost always present, acting as opposing forces that cause moving objects to slow down and stop. However, in the vacuum of space, an astronaut could push a tool, and it would travel in a straight line indefinitely at a constant speed until it hit something or another force acted upon it.

Inertia is the tendency of an object to resist changes in its state of motion. The more massive an object is, the greater its inertia, and the harder it is to get it moving or to stop it once it is moving.

Newton's Second Law of Motion: Force, Mass, and Acceleration

This law provides a quantitative relationship between force, mass, and acceleration. It states that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. Mathematically, this is expressed as:

F = ma

Where:

• F is the net force acting on the object (measured in Newtons, N)
• m is the mass of the object (measured in kilograms, kg)
• a is the acceleration of the object (measured in meters per second squared, m/s²)

This equation tells us several important things:

• If the net force (F) increases, the acceleration (a) increases, assuming the mass (m) stays the same. If you push a shopping cart harder, it accelerates faster.
• If the mass (m) increases, the acceleration (a) decreases, assuming the net force (F) stays the same. Pushing a full shopping cart requires more force to achieve the same acceleration as an empty one.
• If the net force is zero (F = 0), then the acceleration is zero (a = 0). This brings us back to the First Law: zero acceleration means either the object is at rest or moving at a constant velocity.

The 'net force' is crucial here. It's the vector sum of all individual forces acting on an object. If multiple forces are acting on an object, we must add them up (considering their directions) to find the net force. If these forces balance each other out, the net force is zero, and the object's motion will not change.

Newton's Third Law of Motion: Action and Reaction

This law states that for every action, there is an equal and opposite reaction. This means that forces always occur in pairs. If object A exerts a force on object B, then object B simultaneously exerts a force of equal magnitude and opposite direction on object A.

• Example: Walking When you walk, your foot pushes backward on the ground (action). The ground, in turn, pushes forward on your foot with an equal and opposite force (reaction), propelling you forward.
• Example: Rocket Propulsion A rocket expels hot gas downwards (action). The expelled gas pushes the rocket upwards with an equal and opposite force (reaction), causing the rocket to accelerate into space.

This law explains how forces are transmitted between objects and why simply applying a force doesn't always result in observable motion for the object applying the force (e.g., trying to push a wall doesn't make the wall move because the wall pushes back on you with equal force).

Common Forces in Everyday Life

Gravity

Gravity is the force of attraction between any two objects with mass. On Earth, we primarily experience the gravitational pull of the planet, which pulls everything towards its center. This is why objects fall to the ground when dropped.

Friction

Friction is a force that opposes relative motion between surfaces in contact. It arises from the microscopic irregularities of the surfaces rubbing against each other. There are different types of friction:

• Static Friction: The force that prevents an object from starting to move. It's the force you need to overcome to get a stationary object moving.
• Kinetic (or Sliding) Friction: The force that opposes the motion of an object that is already sliding across a surface.
• Rolling Friction: The friction experienced by a rolling object, generally much less than sliding friction.

Friction is often responsible for bringing moving objects to a stop, like a sliding puck on ice or a car braking.

Air Resistance (Drag)

Similar to friction, air resistance is a force that opposes the motion of an object through the air. Its magnitude depends on the object's speed, shape, and the density of the air.

Applied Force

This is the force exerted by a person or another object directly on the object in question, such as pushing a door open or pulling a wagon.

Putting It All Together

An object remains still when all the forces acting on it are balanced, resulting in a net force of zero. If the forces are unbalanced, there is a net force, and the object will accelerate according to Newton's Second Law. The direction of the acceleration will be the same as the direction of the net force.

Consider a book on a table: Gravity pulls it down, and the table pushes it up with an equal and opposite force (the normal force). These forces are balanced, so the net force is zero, and the book stays still. If you push the book horizontally, you apply an additional force. If this applied force is greater than the force of static friction, the book will move. Once moving, kinetic friction opposes the motion, and the net force will determine if it speeds up, slows down, or moves at a constant speed.