Physics and Newton's Laws

Physics is the study of how the world moves, pushes, pulls, speeds up, slows down, and transfers energy. This page builds from basic ideas about motion into the main concepts behind force and Newton's Laws.

What physics is trying to explain

Physics asks questions like:

• Why does a ball keep rolling for a while after you push it?
• Why does a heavier object feel harder to start moving?
• Why do you feel pushed backward when a car speeds up?
• Why does a skateboard roll forward when you push backward on the ground?

At a basic level, physics is about patterns in how the world behaves.

Objects, position, and motion

To talk about motion, it helps to start with an object and a reference point.

Examples of objects:

• a soccer ball
• a skateboard
• a book on a table
• a toy car

An object is in motion if its position changes compared with something else.

Examples:

• A book sitting on a desk is not moving compared with the desk
• A car moving down the road is changing position compared with the road
• A person sitting on a moving bus is not moving much compared with the seat, but is moving compared with the ground

That last example shows an important idea: motion always depends on what you compare it to.

Distance and displacement

Two ideas that sound similar are:

• distance
• displacement

Distance is how much ground an object covers.

Displacement is the change in position from the starting point to the ending point.

Example:

• If you walk 5 ft forward and then 5 ft backward, your distance is 10 ft
• Your displacement is 0 ft because you ended where you started

Distance tells you how much path was traveled. Displacement tells you where you ended up compared with where you began.

Speed and velocity

Speed tells you how fast something is moving.

Formula:

speed = distance / time

Examples:

• walking 30 ft in 10 s means a speed of 3 ft/s
• riding a bike at 12 miles per hour means you are covering 12 miles in 1 hour

Velocity is like speed, but it also includes direction.

Examples:

• 3 ft/s east
• 12 ft/s to the left

So:

• speed tells you how fast
• velocity tells you how fast and in what direction

Acceleration

Acceleration is the rate at which velocity changes.

That can mean:

• speeding up
• slowing down
• changing direction

Formula:

acceleration = change in velocity / time

Examples:

• A toy car going from 0 ft/s to 12 ft/s in 4 s has an acceleration of 3 ft/s^2
• A bike slowing down from 15 ft/s to 0 ft/s is also accelerating, because its velocity is changing

Even turning in a circle counts as acceleration, because the direction keeps changing.

Mass

Mass is the amount of matter in an object.

In basic physics, mass also tells you something very important: how hard it is to change an object's motion.

Examples:

• An empty shopping cart is easier to start moving than a full one
• A tennis ball is easier to throw than a bowling ball

The more mass an object has, the more it resists changes in motion.

Force

A force is a push or a pull.

Forces can:

• start motion
• stop motion
• speed something up
• slow something down
• change direction

Examples:

• your hand pushing a door
• gravity pulling a dropped object downward
• friction slowing a sliding box
• a rope pulling a wagon

On this page, examples use common U.S. measurements such as inches, feet, miles, and pounds.

Balanced and unbalanced forces

Sometimes more than one force acts on an object at once.

If the forces cancel out, they are balanced.

If they do not cancel out, they are unbalanced.

Balanced forces:

• do not change the object's motion
• can keep an object at rest
• can also let an object keep moving at a steady speed in a straight line

Unbalanced forces:

• change the object's motion
• can make it speed up, slow down, or turn

Example:

• A book on a table has gravity pulling down and the table pushing up
• Those forces balance, so the book does not accelerate downward

Inertia

Inertia is the tendency of an object to resist changes in its motion.

That means:

• an object at rest tends to stay at rest
• an object in motion tends to keep moving the same way unless something changes it

Mass is closely tied to inertia. More mass means more inertia.

Real-world examples:

• You feel your body lean backward when a car starts moving suddenly
• A heavy box is harder to get moving and harder to stop than a light box

Inertia is the big idea behind Newton's First Law.

Newton's First Law

Newton's First Law says:

An object will stay at rest, or keep moving at a constant velocity, unless acted on by an unbalanced force.

In other words:

• things do not start changing motion for no reason
• something has to push, pull, or otherwise apply a net force

Examples:

• A soccer ball stays still until someone kicks it
• A hockey puck can glide for a long time because there is not much friction
• A moving skateboard eventually slows because friction and air resistance act on it

A common beginner mistake is thinking motion always needs a force to keep going. A more accurate way to think about it is:

• motion changes because of unbalanced forces
• constant motion does not need a constant force in the ideal case

Friction and air resistance

In everyday life, moving objects often slow down even when nobody touches them.

That is usually because of forces like:

• friction
• air resistance

Friction happens when surfaces rub against each other.

Air resistance happens when moving objects push through air.

These forces matter because they often hide the cleaner version of Newton's First Law. Without them, objects would continue moving more easily.

Newton's Second Law

Newton's Second Law connects force, mass, and acceleration.

Formula:

F = m x a

Where:

• F is force
• m is mass
• a is acceleration

The key relationship is:

• more force causes more acceleration
• more mass means less acceleration for the same force

In everyday U.S. units, people often describe motion with feet, inches, miles, seconds, and pounds.

Examples:

• If you push two carts with the same force, the lighter cart speeds up more
• If you push the same cart harder, it speeds up more

You can also rearrange the formula:

• a = F / m
• m = F / a

Example:

If one cart has twice as much mass as another cart, the heavier cart needs about twice as much force to speed up the same way.

Net force

The force in Newton's Second Law is the net force, which means the overall force after all the pushes and pulls are combined.

Examples:

• 10 lb right and 10 lb left gives a net force of 0 lb
• 12 lb right and 4 lb left gives a net force of 8 lb right

Net force is what determines whether motion changes.

That means:

• balanced forces give a net force of 0
• unbalanced forces give a nonzero net force

Newton's Third Law

Newton's Third Law says:

For every action, there is an equal and opposite reaction.

That does not mean the forces cancel each other on one object. It means forces come in pairs between two interacting objects.

Examples:

• Your foot pushes backward on the ground, and the ground pushes you forward
• A swimmer pushes water backward, and the water pushes the swimmer forward
• A rocket pushes gas downward, and the gas pushes the rocket upward

The paired forces are:

• equal in size
• opposite in direction
• acting on different objects

That last point is very important.

Gravity

Gravity is the force that pulls masses toward each other.

Near Earth, gravity pulls objects downward toward the ground.

Examples:

• a dropped pencil falls
• a ball thrown upward comes back down
• your body has weight because Earth pulls on it

Weight is related to gravity. Your mass stays the same, but your weight depends on the gravitational force acting on you.

Momentum

Momentum is the amount of motion an object has.

Formula:

momentum = mass x velocity

Objects with more mass or more velocity have more momentum.

Examples:

• A fast bike has more momentum than a slow bike
• A heavy cart moving at the same speed as a light cart has more momentum

Momentum is useful for thinking about collisions and why heavier or faster objects are harder to stop.

Energy in motion

Physics is also connected to energy.

When an object is moving, it has kinetic energy.

When something has energy stored because of its position, shape, or condition, it can have potential energy.

Examples:

• A moving skateboard has kinetic energy
• A book held high above the floor has gravitational potential energy
• A stretched rubber band has elastic potential energy

Energy and force are different ideas, but they often appear in the same situations.

Putting the ideas together

A simple way to connect the main terms is:

• motion means position changes over time
• velocity describes motion with direction
• acceleration means velocity is changing
• force is a push or pull
• mass measures how much matter an object has and how much it resists changes in motion
• inertia is the tendency to resist changes in motion

Then Newton's Laws tie those ideas together:

• First Law: motion does not change without an unbalanced force
• Second Law: F = m x a
• Third Law: forces come in equal and opposite pairs on different objects

Final takeaway

At the most basic level:

• Physics helps explain how objects move and interact
• Motion can be described with position, speed, velocity, and acceleration
• Forces are pushes and pulls that can change motion
• Mass and inertia help explain why some objects are harder to move than others
• Newton's Laws describe the main rules behind motion and forces

Once these ideas make sense, experiments with ramps, carts, balls, and moving builds become much easier to understand.

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